Fundamentals of
UV-visible
spectroscopy
A Workbook
Fundamentals of
UV-visible spectroscopy
A Workbook
 Copyright Hewlett-Packard Company,
1998
All rights reserved. Reproduction, adaption,
or translation without prior written
permission is prohibited, except as allowed
under the copyright laws.
The information contained in this
workbook is subject to change without
notice.
Printed in Germany 08/98
Hewlett-Packard Company
Publication Number
12-5967-6357E
5
Preface
UV-visible spectroscopy is a well-established analytical technique with mature methods and
equipment. It is commonly used in both research and science as well as in industry.
Applications are found in classic analytical fields such as in the chemical industry (mainly
petrochemical and dyestuff industry), the pharmaceutical industry or in environmental
analyses. Other fields of application are gaining more and more importance such as
biochemistry and bioscience. UV-visible spectrophotometers are available in almost all
laboratories that do chemical or physical measurements. In general, they are simple to
operate and modern spectrophotometers with built-in or external computer systems
deliver processed results very rapidly. However, the apparent simplicity and speed of the
technique can mean that spectrophotometers are used without sufficient understanding
and erroneous results can be all too common.
This workbook is intended to provide information for practical training and instruction of
school groups or students of engineering or natural sciences. It is also meant for self-study
and independent learning of laboratory personnel and professional users. It is a companion
to the primer "Fundamentals of Modern UV-visible Spectroscopy" (Hewlett-Packard
publication number 12-5965-5123E) which can be used in teaching and learning the theory
of UV-visible spectroscopy and instrumentation. With this workbook users can deepen the
theoretical knowledge they may already have gained and complement it with practical
exercises. The experiments and the results that are achieved help to better understand the
theoretical background of UV-visible spectroscopy. The workbook will enable users to
understand the possibilities, but also the sources of error, pitfalls and the limits when
working with UV-visible spectroscopy. Soon users will develop a better understanding of
their equipment and will be able to appreciate the advantages of diode array
spectrophotometers compared to conventional scanning spectrophotometers.
Little prior knowledge of the equipment and of laboratory work is required before starting
with the experiments. Nevertheless all users are asked to read and thoroughly follow the
safety instructions for laboratory work and especially for the chemical substances they
deal with. Unqualified persons have to be guided and supervised by authorized personnel.
Make yourself familiar with the handbooks and users' guides of your equipment. Only if the
experiments are carried out carefully according to the Good Laboratory Practice (GLP)
regulations best results can be achieved.
We would be very pleased to receive feedback on this workbook, especially with
suggestions for improvements or additional experiments that could be included.
The primer "Fundamentals of modern UV-visible
Spectroscopy", is available from Hewlett-Packard as
publication number 12-5965-5123E.
6
General Equipment
t UV-visible spectrophotometer.
With a few exceptions all experiments described in this workbook were performed on an
HP 8453 diode-array UV-visible spectrophotometer but, in principle, any good quality
UV-visible spectrophotometer may be used. The times given (for the experiments including
evaluation) are based on the use of the HP 8453 which scans full spectra in about 1.5
seconds. If a scanning spectrophotometer is used, the time for experiments that require
spectral measurement will be significantly longer.
t Calculator or personal computer for evaluation of data.
t Analytical balance for accurate preparation of test samples.
t Spatula for handling powders.
t Pipette bulb (to avoid mouth pipetting).
t Weighing papers.
t Cuvettes. When using a single beam spectrophotometer only one cuvette is needed. If a
double-beam spectrophotometer is used two, preferably matched, cuvettes are required.
General Aspects of Sample Handling and Preparation
To minimize possible sources of error always consider the following general aspects of
good sample handling:
* Use a pipette to empty and fill the cells.
* Rinse the cell with the solvent or solution to be measured at least three times before
measurement.
* Take care not to dirty the optical surfaces of the cell with fingerprints or with any other
substances.
* In order to verify the quality and cleanliness of the cell used, measure a reference on air and
a sample on the cell filled with distilled water before sample measurement.
* When using a conventional scanning spectrophotometer, measure the references in the
same wavelength range as the sample measurements.
* Use fresh samples only. Storing the samples, especially in light and warm rooms, may create
multiple sources of error due to unwanted reactions such as decomposition and sample
contamination.
Caution:
Carefully read, understand and follow the safety instructions for the
substances you deal with. Chemical substances must always be handled
according to legal regulations for hazardous materials that are in force in
your country and at your place of work. All substances used must be
handled using exactly the concentrations and compositions described in
this workbook.
3
Contents
Part 1.
Basic Principles and Applications ..............................................................7
1.1. Basic Principles--What is a UV-visible Spectrum?...............................................8
1.2. Chromophores.........................................................................................................13
1.3. Environment of Chromophores ............................................................................17
1.4. The Effect of Solvents on UV-visible Spectra......................................................21
1.5. The Meaning of Color in Spectroscopy ................................................................25
1.6. Quantitative Analysis -- Lambert-Beer's Law
Path Length Dependence of Absorbance Values ................................................29
1.7. Concentration Dependence of Absorbance Values ............................................37
1.8. Possible Sources of Error -- Influence of Impurities ........................................46
1.9. Influence of Temperature -- Potassium Chloride ..............................................52
1.10. Influence of Temperature -- Methyl Orange.......................................................57
1.11. Influence of pH -- Buffered Methyl Orange Solutions.......................................64
1.12. Influence of pH -- Potassium Dichromate Solution ..........................................69
1.13. Effect of Concentration..........................................................................................72
1.14. Principle of Additivity.............................................................................................77
Part 2.
Measuring Instrument Performance .......................................................83
2.1. Wavelength Accuracy .............................................................................................85
2.2. Wavelength Accuracy Using the Deuterium Lines..............................................89
2.3. Photometric Accuracy Using Potassium Dichromate........................................92
2.4. Photometric Accuracy Using Neutral Density Glass Filters..............................96
2.5. Stray Light ................................................................................................................99
2.6. Spectral Resolution...............................................................................................102
2.7. Noise .......................................................................................................................107
2.8. Baseline Flatness...................................................................................................110
2.9. Stability...................................................................................................................113
4
Part 3.
Sample Handling and Measurement......................................................115
3.1. Sample Handling ...................................................................................................117
3.2. Cell Types...............................................................................................................123
3.3. Cleanliness.............................................................................................................127
3.4. Influence of Instrumental Parameters................................................................130
3.5. Properties of Solvents ..........................................................................................137
3.6. Background Absorbance......................................................................................141
3.7. Sample Decomposition.........................................................................................146
Part 4.
Applications..............................................................................................................151
4.1. Single Component Analysis .................................................................................152
4.2. Multicomponent Analysis.....................................................................................162
4.3. Derivative Spectroscopy ......................................................................................178
4.4. Kinetic Analysis -- Studying Melting Temperatures of DNA..........................183
4.5. Isosbestic Points ...................................................................................................189
4.6. Biochemical Spectroscopy...................................................................................208
Appendix ................................................................................................................214
Part 1
7
5 Basic Principles and
Applications
8
Basic Principles and Applications
1.1. Basic Principles--What is a
UV-visible Spectrum?
Introduction
A spectrum is a graphical representation of the amount of light absorbed or transmitted by
matter as a function of the wavelength. A UV-visible spectrophotometer measures
absorbance or transmittance from the UV range to which the human eye is not sensitive to
the visible wavelength range to which the human eye is sensitive.
In the following experiment the spectra of two compounds, a colored one and a colorless
one, are measured. The intention is to demonstrate that in many cases colorless
compounds have a UV absorbance.
Reagents and Equipment
t caffeine (a)
t erythrosine
t distilled water
t two 1.0-l volumetric flasks
t 0.5-ml syringe or pipette
t disposable glass pipettes (minimum 3)
t 10-mm path length quartz cell
Experiment
Time: about 45 min
1 Prepare the following solutions:
a) about 8 mg erythrosine in 1.0 l distilled water
b) about 5.0 mg caffeine in 1.0 l distilled water
2 Measure a reference on distilled water.
3 Measure the absorbance spectra of each solution in the range from 190 to 700 nm. If your
spectrophotometer offers this functionality: overlay the spectra of the two solutions.
I
N
N
CH3O
N
N
CH3
O
(a)
9
Basic Principles and Applications
Evaluation
1 Enter the wavelengths of the main absorbance maxima of caffeine and erythrosine in the
table below.
2 If the color of light and wavelength are related as follows, which colors of light do caffeine
and erythrosine absorb?
Color Wavelength Range [nm]
violet 380­435
blue 435­480
green 480­560
yellow 560­595
orange 595­650
red 650­780
3 Explain the relationship between the color of light absorbed by matter and the color of light
that you can observe when looking at it.
4 The following equation shows the relationship between absorbance and transmittance:
A = 2 ­ log T [%]
Calculate the transmittance values at the main absorbance maximum of each sample and
enter the values in the table below.
Evaluation Table 1.1. Measured Wavelengths and Absorbance Values of the Absorbance Maxima
Compound max [nm] Absorbance [AU]
Caffeine
Erythrosine
Evaluation Table 1.2. Calculated Transmittance Values of the Absorbance Maxima
Compound max [nm] Absorbance [AU] Transmittance [%T]
Caffeine
Erythrosine
10
Basic Principles and Applications
5 Which compound absorbs light of higher energy (per photon)? Use the following equations
to calculate the energy of light in Joule that is absorbed at the maxima of both compounds.
where:
E = energy [J]
h = Planck's constant (6.62 × 10-34 Js)
c = speed of light (3 × 108 ms-1)
v = frequency [s-1]
 = wavelength [m]
E h v=
c v=
Evaluation Table 1.3. Calculated Energies of Light at the Absorbance Maxima
Compound max [nm] Energy [J]
Caffeine
Erythrosine
11
Basic Principles and Applications
Example Results & Discussion
Samples: erythrosine in distilled water (8.0 mg/l)
caffeine in distilled water (5 mg/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 190­600 nm
absorbance range: 0.0­1.0 AU
Diagram: Measured Absorbance Spectra of the Samples
1 The measured wavelengths and the absorbance values at the absorbance maxima are listed
in the table below.
2 Caffeine does not absorb light in the visible range and therefore has no color. erythrosine
absorbs in the visible range so it has color. Its absorbance maximum can be found at 526 nm
so the color it absorbs is green.
3 We usually see matter under sunlight or similar light which is essentially "white" light--a
mixture of all wavelengths. When a sample absorbs a certain color we see the rest of the light
that is not absorbed. This is called the complementary color. When green light is absorbed
what we see is the red light that remains.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
190 290 390 490 590
Wavelength [nm]
Absorbance
[AU]
Caffeine Erythrosine
Results Table 1.1. Measured Wavelengths and Absorbance Values of the Absorbance Maxima
Compound max [nm] Absorbance [AU]
Caffeine 205 0.699
Erythrosine 526 0.798
12
Basic Principles and Applications
4 Transmittance values calculated from the absorbance values are shown in the table below.
An absorbance of 1 is measured when the sample absorbs 90 % of the incident light.
5 The calculated energies of the light at the maxima are shown in the table below.
Caffeine absorbs light of higher energy than erythrosine. The lower the wavelength, the
higher the energy per photon.
Results Table 1.2. Calculated Transmittance Values of the Absorbance Maxima
Compound max [nm] Absorbance [AU] Transmittance [%T]
Caffeine 205 0.699 20.000
Erythrosine 526 0.798 15.922
Results Table 1.3. Calculated Energies of Light at the Absorbance Maxima
Compound max [nm] Energy [J]
Caffeine 205 9.69 10-19
Erythrosine 526 3.78 10-19
13
Basic Principles and Applications
1.2. Chromophores
Introduction
Chromophores are parts of a molecule that have electronic bands with energy differences
comparable to the energies of the UV-visible light which is then absorbed by them.
Chromophores, for example, dienes, nitriles, carbonyl, or carboxyl groups often contain 
bonds. Other types of chromophores are transition metal complexes and their ions.
Molecules without chromophores, such as water, alkanes or alcohols, should be ideal
solvents for UV-visible spectroscopy because they hardly show any absorbance themselves.
The following experiment shows that small variations in the structure of molecules can
lead to significant differences in the resulting absorbance spectra.
Reagents and Equipment
t acetone
t acetaldehyde
t 2-propanol
t distilled water
t three 20-ml volumetric flasks
t 0.1-ml pipette or syringe
t disposable glass pipettes (minimum 4)
t 10-mm path length quartz cell
Experiment
Time: about 45 min
1 Prepare the following solutions:
a) 0.1 ml acetone in 20 ml distilled water
b) 0.2 ml acetaldehyde in 20 ml distilled water
c) 0.1 ml 2-propanol in 20 ml distilled water
2 Measure a reference on distilled water.
3 Measure the spectra of the acetone, acetaldehyde and 2-propanol solutions in the range from
200 to 350 nm.
14
Basic Principles and Applications
Evaluation
1 Complete the following table with the absorbance wavelengths and corresponding values of
the absorbance maxima of each compound:
2 Discuss the differences in the UV spectra of these compounds taking their molecular
structures into consideration.
3 Which of these solvents would be best as a solvent for UV-visible analyses?
Evaluation Table 1.4. Wavelengths and Absorbance Values at the Absorbance Maxima
Compound max [nm] Absorbance [AU]
Acetone
Acetaldehyde
2-Propanol
15
Basic Principles and Applications
Example Results & Discussion
Samples: acetone in distilled water (5 ml/l)
acetaldehyde in distilled water (10 ml/l)
2-propanol in distilled water (5 ml/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 200­350 nm
absorbance range: 0.0­1.5 AU
Diagram: Measured Absorbance Spectra of the Different Chromophores
1 The wavelengths and absorbance values at the maxima for each compound are shown in the
table below.
1
2
3
Absorbance
[AU]
200 225 250 275 300 325 350
0.00
0.25
0.50
0.75
1.00
1.25
1 Acetone
2 Acetaldehyde
3 2-Propanol
Wavelength [nm]
Results Table 1.4. Wavelengths and Absorbance Values at the Absorbance Maxima
Compound max [nm] Absorbance [AU]
Acetone 265 1.205
Acetaldehyde 277 1.052
2-Propanol no maximum found not applicable
16
Basic Principles and Applications
2 The molecular structures of the three compounds are:
The absorbance bands of acetaldehyde and acetone are caused by the presence of the C=O
chromophore. Note that the position and intensity of the absorbance bands are similar. If
either of these solvents has to be used for making sample solutions for UV-visible
measurements, the data in the range from 225 to 325 nm will be affected by the absorbance
of the solvent itself.
3 Propanol has no chromophore and shows no significant absorbance band in the UV-visible
range. It is an almost ideal solvent for UV-visible measurements.
Acetone 2-PropanolAcetaldehyde
C
O
CH3 CH3
C
OH
CH3 CH3
H
C
O
CH3 H
17
Basic Principles and Applications
1.3. Environment of
Chromophores
Introduction
Chromophores give rise to "characteristic" absorbance bands. Changes in their
environment cause changes in their energy levels which then affect the wavelength and the
intensity of absorbance.
The following experiment shows how the molecular environment of a chromophore affects
its absorbance spectrum.
Reagents and Equipment
t naphthalene
t anthracene
t phenanthrene
t ethanol (CH3-CH2-OH)
t three 100-ml volumetric flasks
t disposable glass pipettes (minimum 4)
t 10-mm path length quartz cell
Experiment
Time: about 60 min
1 Prepare the following solutions:
a) about 2.4 mg naphthalene in 100 ml ethanol
b) about 16 mg anthracene in 100 ml ethanol
c) about 5 mg phenanthrene in 100 ml ethanol
2 Measure a reference on ethanol.
3 Measure the spectra of the naphthalene, anthracene and phenanthrene solutions in the
range from 200 to 400 nm.
18
Basic Principles and Applications
Evaluation
1 Discuss the differences in the measured spectra of naphthalene, anthracene and
phenanthrene solution. Take the wavelengths of absorbance maxima and the structure of
bands into consideration.
2 Enter the wavelength ranges of the absorbance maxima of the compounds in the table
below.
Evaluation Table 1.5. Wavelength Ranges of the Absorbance Maxima
Compound max [nm]
Naphthalene
Anthracene
Phenanthrene
19
Basic Principles and Applications
Example Results & Discussion
Samples: naphthalene in 100 ml ethanol (24 mg/l)
anthracene in 100 ml ethanol (16 mg/l)
phenanthrene in 100 ml ethanol (50 mg/l)
Cells: 2-mm path length quartz cell for naphthalene solution
10-mm path length quartz cell for the other two solutions
Instrument
Parameters: wavelength range: 200­400 nm
absorbance range: 0.0­1.2 AU
Diagram: Measured Absorbance Spectra of the Different Chromophores
1 The spectra show that not only the kind of chromophore is important. In our example we
have the benzene-rings that are chromophores with ­*
transitions because of conjugated
double bonds. The environment of the chromophores or the combination with other
chromophores also have a strong influence on the position and values of absorbance bands.
The molecular structures of the three compounds are:
Absorbance
[AU]
200 250 300 350 400
0.00
0.25
0.50
0.75
1.00
1 Naphthalene
2 Anthracene
3 Phenanthrene
Wavelength [nm]
1
2
3
Naphthalene Anthracene Phenanthrene
20
Basic Principles and Applications
From naphthalene to anthracene there is an increasing conjugation of the  bonds. The
spectral region of absorbance changes as well as the intensity of absorbance and the form
of the absorbance bands.
Anthracene and phenanthrene have the same chemical formula but their spectra are
different because their structure and hence the electronic environment of the  bonds are
different.
2 The wavelength ranges of the absorbance maxima are listed in the table below.
Results Table 1.5. Wavelength Ranges of the Absorbance Maxima
Compound max [nm]
Naphthalene 245­295
Anthracene 305­390
Phenanthrene 305­365
21
Basic Principles and Applications
1.4. The Effect of Solvents on
UV-visible Spectra
Introduction
Chromophores give rise to "characteristic" absorbance bands. Changes in their
environment cause changes in their energy levels which then affect the wavelength and the
intensity of absorbance.
The following experiment shows that external factors, in this case the solvent used, can
also influence the chromophore. Because the substance used shows both strong and weak
absorbance bands at different wavelengths, two different sample concentrations will be
examined.
Reagents and Equipment
t benzophenone (a) (alternatively mesityl oxide)
Four or five of the following solution:
t ethanol (CH3-CH2-OH)
t cyclohexane (b)
t n-hexane (CH3(CH2)4CH3)
t acetonitrile (CH3-CN)
t methylene chloride (CH2Cl2)
t five 25-ml volumetric flasks
t five 100-ml volumetric flasks
t 1-ml pipette
t disposable glass pipettes (minimum 15)
t 10-mm path length quartz cell
(a)C
O
(b)
22
Basic Principles and Applications
Experiment
Time: about 90 min
1 Prepare five solutions of about 25 mg benzophenone in 25 ml of the given solvents
(concentration chigh).
2 The following steps have to be repeated for each solution:
a) Dilute the concentration chigh by a factor of 100 to get the concentration clow.
b) Measure a reference on the solvent used.
c) Measure the absorbance spectrum of the chigh-sample in the range from 300 to 400 nm.
d) Measure the absorbance spectrum of the clow-sample in the range from 225 to 300 nm.
Evaluation
1 Enter the wavelengths of the absorbance maxima of benzophenone for the various solvents
in the table below.
2 A measure of the polarity of a solvent is its dielectric constant. Look up the dielectric
constants of the solvents used and enter them in the table below.
3 Is there a relationship between a solvent's polarity and the wavelength of its absorbance
maximum?
4 Can you observe any change in the form of the absorbance bands in the wavelength range
from 300 to 400 nm with changes in solvent polarity?
5 Is it important to use the same solvent to achieve consistent results, for example, between
the measurements made in different laboratories?
Evaluation Table 1.6. Wavelengths of the Measured Absorbance Maxima and Dielectric Constants
Solvent max, Peak 1 [nm] max, Peak 2 [nm] Dielectric Constant
n-Hexane
Cyclohexane
Ethanol
Acetonitrile
Methylene Chloride
23
Basic Principles and Applications
Example Results & Discussion
Samples: a) stock solutions of benzophenone in the following solvents:
ethanol (1 g/l)
cyclohexane (1 g/l)
n-hexane acetonitrile (1 g/l)
methylene chloride (1 g/l)
b) stock solutions diluted by 1:100
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength ranges: a) 300­400 nm
b) 225­300 nm
absorbance range: 0.0­1.4 AU
Diagram: a) Measured Spectra Absorbance of the Sample Solutions at High
Concentrations
Absorbance
[AU]
325 350 375 400
0.00
0.25
0.50
0.75
1 Ethanole
2 Methylenchloride
3 Acetonitrile
4 N-hexane
5 Cyclohexane
Wavelength [nm]
1
2
3
4
5
24
Basic Principles and Applications
b) Measured Spectra of the Sample Solution at Low Concentration
1 The measured wavelengths of the absorbance maxima and the dielectric constants of the
solvents are listed in the table below.
2 The dielectric constants are listed in the table above.
3 There is a clear relationship between the wavelengths of the absorbance maxima and the
polarity of the solvent used.
4 The bands in the range from 300 to 400 nm show that nonpolar solvents such as alkanes
allow considerably fine structures to be preserved in the spectra of many compounds. Polar
solvents such as water and alcohols allow only broad and relatively featureless bands.
5 For comparative analyses the same solvent should be used for all measurements.
Note: Band shifts to higher wavelengths are called bathochromic shifts, shifts to lower
wavelengths are called hypsochromic shifts.
1
2
Absorbance
[AU]
225 250 275 300
0.0
0.4
0.8
1.2 1 Methylenchloride
2 Ethanole
3 Acetonitrile
4 Cyclohexane
5 N-hexane
Wavelength [nm]
3
4
5
Results Table 1.6. Wavelengths of the Measured Absorbance Maxima and Dielectric Constants
Solvent max, Peak 1 [nm] max, Peak 2 [nm] Dielectric Constant
n-Hexane 248 347 1.9
Cyclohexane 249 347 2.0
Ethanol 252 333 24.3
Acetonitrile 251 339 36.2
Methylene chloride 253 339 9.0
25
Basic Principles and Applications
1.5. The Meaning of Color in
Spectroscopy
Introduction
The color of matter is a function of its ability to absorb and to emit light. Light emission can
be either due to surface reflection of non-transparent samples or to transmission of source
light through the samples. Matter can also be an active source of light like gases in lamps.
At room temperature matter usually only reflects or transmits light. Color in its common
meaning refers to daylight as illuminant.
The human eye is capable to differentiate up to 10 million different colors. It is able to
detect light in a range from about 380 to 780 nm, the visible part of a UV-visible spectrum.
In the following experiment food dyes are used to correlate absorbance spectra to color
impressions. Due to their wavelengths of absorbance the partly transparent sample
solutions look yellow, red or blue. What a human observer sees as a yellow sample is the
daylight reduced by the light absorbed by the sample. The color of the sample is therefore
called complementary to the color of the light absorbed by the sample.
Reagents and Equipment
t quinoline yellow (yellow dye)
t erythrosine (red dye)
t indigotin (blue dye)
t distilled water
t three 1.0-l volumetric flasks
t disposable glass pipettes (minimum 4)
t 10-mm path length quartz cell
Experiment
Time: about 30 min
1 Prepare the following solutions:
a) quinoline yellow, about 13 mg in 1.0 ml distilled water
b) erythrosine, about 8 mg in 1.0 l distilled water
c) indigotin, about 18 mg in 1.0 l distilled water
2 Measure a reference on distilled water.
3 Measure the spectra of the three dye solutions in the wavelength range from 350 to 750 nm.
26
Basic Principles and Applications
Evaluation
1 Determine the wavelength of the absorbance maximum of each dye spectrum and enter the
values in the table below.
2 Compare the visible color of the sample with the spectral color of the light absorbed at the
absorbance maximum. Enter both colors in the table below.
3 What does an absorbance spectrum of a sample tell you about its color?
4 How does a sample look that has absorbance values close to zero in the entire visible range?
5 How does a sample look that has very high absorbance values in the entire visible range?
Evaluation Table 1.7. Wavelengths of the Absorbance Maxima
Sample Name max [nm]
Quinoline yellow
Erythrosine
Indigotin
Evaluation Table 1.8. Visible Color Compared to Color of Absorbed Light
Sample Name Visible Color Color of Absorbed Light
Quinoline yellow
Erythrosine
Indigotin
27
Basic Principles and Applications
Example Results & Discussion
Samples: quinoline yellow in distilled water (13 mg/l)
erythrosine in distilled water (8 mg/l)
indigotin in distilled water (18 mg/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 350­700 nm
absorbance range: 0.0­0.8 AU
Diagram: Measured Absorbance Spectra of the Different Dyestuff Solutions
1 The wavelengths of absorbance maxima are listed in the table below.
Absorbance
[AU]
400 500 600 700
0.00
0.25
0.50
0.75 1 Quinoline yellow
2 Erythrosine
3 Indigotin
Wavelength [nm]
1 2
3
Results Table 1.7. Wavelengths of the Absorbance Maxima
Sample Name max [nm]
Quinoline yellow 414
Erythrosine 526
Indigotin 610
28
Basic Principles and Applications
2 The complementary colors of the sample and the spectral color of the light absorbed at the
absorbance maximum are listed in the table below.
3 An absorbance spectrum has all information about the color of a spectrum. For simple dye
spectra with a single absorbance maximum this color is the complementary color of the
spectral color of the light absorbed. Even quantitative color calculations can be done based
on UV-visible spectra.
4 A sample with absorbance values close to zero in the entire visible range is colorless. It is
totally transparent.
5 A sample with very high absorbance values in the entire visible range looks black and is not
transparent.
References
"Measuring Colour", R.W.G. Hunt, Ellis Horwood Limited, Chichester
"Einführung in die Farbmetrik", M. Richter, Walter de Gruyter, Berlin, New York
Results Table 1.8. Visible Color Compared to Color of Absorbed Light
Sample Name Visible Color Color of Absorbed Light
Quinoline yellow yellow blue
Erythrosine red blue-green
Indigotin blue yellow
29
Basic Principles and Applications
1.6. Quantitative Analysis --
Lambert-Beer's Law
Path Length Dependence of
Absorbance Values
Introduction
The amount of light absorbed or transmitted by a sample depends on the path length which
the light has to go through when passing the sample.
In the following experiment we will try to find out how different path lengths affect the
absorbance values. We measure the same sample solution using different cells. We measure
absorbance and transmittance data at three different wavelengths. The measurement data
are evaluated using diagrams and linear regression calculations.
Reagents and Equipment
t quinoline yellow (yellow dye)
t distilled water
t 1.0-ml volumetric flask
t disposable glass pipettes (minimum 2)
t 1-mm path length quartz cell
t 2-mm path length quartz cell
t 5-mm path length quartz cell
t 10-mm path length quartz cell
Experiment
Time: about 30 min
1 Prepare a solution of about 28 mg quinoline yellow in 1.0- ml distilled water (or use quinoline
solution from the previous experiment).
2 For each of the cells with different path lengths:
a) Measure a reference on distilled water.
b) Measure the absorbance values of the quinoline yellow solution at 224, 337 and 414 nm.
c) Measure the transmittance values of the quinoline yellow solution at 224, 337 and
414 nm.
30
Basic Principles and Applications
Evaluation
1 Enter your absorbance and transmittance measurement values in the tables below.
Evaluation Table 1.9. Measured Absorbance Values at Different Wavelengths and at Different Path Lengths
Absorbance [AU]
at Different
Wavelengths
Path Length
1 mm 2 mm 5 mm 10 mm
224 nm
337 nm
414 nm
Evaluation Table 1.10. Measured Transmittance Values at Different Wavelengths and at Different Path Lengths
Transmittance [%T]
at Different
Wavelengths
Path Length
1 mm 2 mm 5 mm 10 mm
224 nm
337 nm
414 nm
31
Basic Principles and Applications
2 Enter your measured data values in the following two diagrams.
Diagram: Absorbance as a Function of Path Length
Absorbance [AU]
2
1.5
0.5
1
51 2 3 4 6 7 8 9 10
Path length [mm]
0
2.5
32
Basic Principles and Applications
Diagram: Transmittance as a Function of Path Length
3 Calculate a linear regression with offset for the above diagrams. Use the calculation
formulae given in the appendix for y = ax + b or the built-in function of a pocket calculator.
Here the x,y pairs are the path length (x) and the corresponding data value (y). Enter the
calculation results for all wavelengths in the following tables.
Evaluation Table 1.11. Calculation Results for the Normalized Absorbance Data
Name Wavelength
224mm 337 mm 414 mm
Slope a
Intercept b
Correlation Coefficient R
Transmittance [%]
80
60
20
40
51 2 3 4 6 7 8 9 10
Path length [mm]
0
100
33
Basic Principles and Applications
4 Which data values have a linear correlation with the path length?
5 Normalize your absorbance measurements to 1-mm path length by dividing your
measurement data by the corresponding path length and enter the values in the table
below.
6 Are the normalized data path length specific at a given wavelength?
Evaluation Table 1.12. Calculation Results for Transmittance Wavelengths
Name Wavelength
224 nm 337 nm 414 nm
Slope a
Intercept b
Correlation Coefficient R
Evaluation Table 1.13. Calculation Results for the Normalized Absorbance Data
Normalized Absorbance
[AU/1 mm] at Different
Wavelengths
Path Length
1 mm 2 mm 5 mm 10 mm
224 nm
337 nm
414 nm
34
Basic Principles and Applications
Example Results & Discussion
Sample: quinoline yellow in distilled water (28.3 g/l)
Cells: 1-mm path length quartz cell
2-mm path length quartz cell
5-mm path length quartz cell
10-mm path length quartz cell
Instrument
Parameters: wavelengths: 224, 337 and 414 nm
absorbance range: 0.0­1.8 AU
transmittance range: 0.0­100 %
1 The following tables show the measured absorbance and transmittance values.
Results Table 1.9. Measured Absorbance Values at Different Wavelengths and at Different Path
Lengths
Absorbance [AU]
at Different Wavelengths
Path Length
1 mm 2 mm 5 mm 10 mm
224 nm 0.178 0.348 0.874 1.692
337 nm 0.007 0.011 0.033 0.065
414 nm 0.159 0.321 0.809 1.610
Results Table 1.10. Measured Transmittance Values at Different Wavelengths and at Different Path
Lengths
Transmittance [%T]
at Different Wavelengths
Path Length
1 mm 2 mm 5 mm 10 mm
224 nm 66.37 44.92 13.37 2.03
337 nm 98.48 97.62 92.63 86.18
414 nm 69.28 47.81 15.52 2.46
35
Basic Principles and Applications
2 The following diagrams show the absorbance and transmittance as a function of path length.
Diagram: Absorbance as a Function of Path Length
Diagram: Transmittance as a Function of Path Length
Absorbance
[AU]
0.0 2 4 6 8 10
0.0
0.5
1.0
1.5
Path length [mm]
A ( = 414 nm)
A ( = 224 nm)
A ( = 337 nm)
Transmittance [%]
0 2 4 6 8 10
Path length [mm]
0
20
40
60
80
T (
T ( = 224 nm)
T ( = 337 nm)
= 414 nm)
100
36
Basic Principles and Applications
3 The following tables show the linear regression calculation results.
4 The absorbance values have a linear correlation with the path length.
5 The following table shows the calculation results for normalized absorbance data.
6 The normalized absorbance values are not path length specific at a given wavelength. The
measured absorbance values are proportional to the path length.
Results Table 1.11. Calculation Results for Absorbance Measurements
Name
Wavelength
224 mm 337 mm 414 mm
Slope a 1.6843 0.0658 1.6124
Intercept b 0.0150 -0.0009 0.0008
Correlation coefficient R 0.99986 0.99863 0.99999
Results Table 1.12. Calculation Results for Transmission Measurements
Name
Wavelength
224 mm 337 mm 414 mm
Slope a 66.68 -13.96 -69.53
Intercept b 61.68 100.01 65.06
Correlation Coefficient R -0.91665 -0.99814 -0.92457
Results Table 1.13. Calculation Results for the Normalized Absorbance Data
Absorbance [AU/1 mm]
at Different Wavelengths
Path Length
1 mm 2 mm 5 mm 10 mm
224 nm 0.178 0.174 0.175 0.169
337 nm 0.007 0.005 0.007 0.007
414 nm 0.159 0.160 0.162 0.161
37
Basic Principles and Applications
1.7. Concentration Dependence
of Absorbance Values
Introduction
This is the basic experiment for quantitative UV-visible measurements: how do different
concentrations affect absorbance and transmittance values?
In the following experiment we measure the absorbance and transmittance of five different
quinoline yellow solutions to examine the effect of different concentrations. We evaluate
our measurement data by using diagrams and by doing linear regression calculations.
Reagents and Equipment
t quinoline yellow (yellow dye)
t distilled water
t 1.0-l volumetric flask
t four 100-ml volumetric flasks
t disposable glass pipettes (minimum 6)
t 10-mm path length quartz cell
Experiment
Time: about 120 min
1 Prepare a quinoline yellow stock solution of about 28 mg in 1.0 l distilled water.
2 Prepare four sample solutions in the 100-ml volumetric flasks by diluting the stock solution:
sample concentrations: 14, 7, 3.5, 1.75 mg/l.
3 Measure a reference on distilled water.
4 Measure the absorbance and transmittance values at 224, 337 and 414 nm of the stock
solution and of each of the other solutions.
38
Basic Principles and Applications
Evaluation
1 Enter the actual concentrations of the five solutions you prepared in the list below.
2 Enter your measured absorbance and transmittance values in the tables below.
c1 = mg/l
c2 = mg/l
c3 = mg/l
c4 = mg/l
c5 = mg/l
Evaluation Table 1.14. Measured Absorbance Values at Different Wavelengths and Concentrations
Absorbance [AU]
at Different Wavelengths
Sample Concentrations:
c1 c2 c3 c4 c5
224 nm
337 nm
414 nm
Evaluation Table 1.15. Measured Transmittance Values at Different Wavelengths and Concentrations
Transmittance [%T]
at Different Wavelengths
Sample Concentrations:
c1 c2 c3 c4 c5
224 nm
337 nm
414 nm
39
Basic Principles and Applications
3 Enter your data values measured in the following two diagrams. Use different colors for
different wavelengths.
Diagram: Absorbance as a Function of Concentration
Absorbance [AU]
2
1.5
0.5
1
153 6 9 12 18 21 24 27 30
Concentration [mg/l]
0
2.5
40
Basic Principles and Applications
Diagram: Transmittance as a Function of Concentration
4 Which data are preferable for quantitative analysis--absorbance data or transmittance
data--and why?
Transmittance [%]
80
60
20
40
153 6 9 12 18 21 24 27 30
Concentration [mg/l]
0
100
0
41
Basic Principles and Applications
5 Calculate a linear regression with offset for the above graphs. Use the calculation formulae
given in the appendix for y = ax + b or the built-in function of a pocket calculator. Here the
x,y pairs are the concentration (x) and the corresponding data value (y). Enter the
calculation results for all wavelengths in the tables below.
6 Which data values have a linear correlation with the concentration?
7 Normalize your absorbance measurements to concentration 1 mg/l by dividing your
measurement data by the corresponding sample concentration and enter the values in the
table below.
8 Are the normalized data sample specific for a given wavelength?
How are these values also called?
Evaluation Table 1.16. Calculation Results for Absorbance Measurements at Different Wavelengths
Name 224 nm 337 nm 414 nm
Slope a
Intercept b
Correlation coefficient R
Evaluation Table 1.17. Calculation Results for Transmittance Measurements at Different Wavelengths
Name 224 nm 337 nm 414 nm
Slope a
Intercept b
Correlation coefficient R
Evaluation Table 1.18. Calculation Results for Normalized Absorbance Data at Different Wavelengths and Concentrations
Normalized Absorbance
[AU l/mg] at Different
Wavelengths
Concentrations
c1 c2 c3 c4 c5
224 nm
337 nm
414 nm
42
Basic Principles and Applications
Example Results & Discussion
Samples: quinoline yellow in distilled water (28 mg/l)
quinoline yellow in distilled water (14 mg/l)
quinoline yellow in distilled water (7 mg/l
quinoline yellow in distilled water (3.5 mg/l)
quinoline yellow in distilled water (1.75 mg/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelengths: 224, 337, 414 nm
absorbance range: 0.0­1.8 AU
transmittance range: 0.0­100 %
Spectra: Measured Absorbance Spectra for Different Sample Concentrations
1 The actual concentrations of the samples are listed in the figure above.
Absorbance
[AU]
200 300 400 500
0.00
0.25
0.50
0.75
1.00
1.25
1.50
2 14
3 7
4 3.5
5 1.75
Wavelength [nm]
1
2
3
5
4
1 28
Concentrations [mg/l]
43
Basic Principles and Applications
2 The following tables show the measured absorbance data.
3 The following diagrams show absorbance and transmission as a function of concentration.
Diagram: Absorbance as a Function of Concentration
Results Table 1.14. Measured Absorbance Values at Different Wavelengths and Concentrations
Absorbance [AU]
at Different Wavelengths
Sample Concentrations:
c1 c2 c3 c4 c5
224 nm 1.70000 0.8700 0.4472 0.2524 0.1175
337 nm 0.06846 0.0441 0.0217 0.0153 0.0072
414 nm 1.64116 0.8249 0.4206 0.2120 0.1064
Results Table 1.15. Measured Transmittance Values at Different Wavelengths and Concentrations
Transmittance [%T]
at Different Wavelengths
Sample Concentrations:
c1 c2 c3 c4 c5
224 nm 2.00 13.49 35.71 55.91 76.29
337 nm 85.42 90.34 95.14 96.53 98.35
414 nm 2.29 14.96 37.97 61.38 78.28
Absorbance
[AU]
0. 5 10 15 20 25 30
0
0.5
1.0
1.5
A ( =414 nm)
A ( = 224 nm)
A ( = 337 nm)
Concentration [mg/l]
44
Basic Principles and Applications
Diagram: Transmittance as a Function of Concentration
4 The following tables show the linear regression calculation results.
Transmittance [%]
Concentration [mg/l]
0
20
40
60
80
T ( = 414 nm)
T ( = 224 nm)
T ( = 337 nm)
0 5 10 15 20 25 30
100
Results Table 1.16. Calculation Results for Absorbance Measurements at Different Wavelengths
Name 224 nm 337 nm 414 nm
Slope a 0.060 0.002 0.058
Intercept b 0.029 0.006 0.008
Correlation coefficient R 0.9997 0.9809 1.0000
Results Table 1.17. Calculation Results for Transmittance Measurements at Different Wavelengths
Name 224 nm 337 nm 414 nm
Slope a -2.5584 -0.4855 -2.5584
Intercept b 64.439 98.424 64.439
Correlation coefficient R -0.8105 -0.9759 -0.8271
45
Basic Principles and Applications
5 Due to the linear relationship with concentration only absorbance values are used in
quantitative analysis.
This linear relationship can be seen in the diagram and it is also indicated by the correlation
coefficients close to 1. The closer this coefficient is to 1, the better is the correlation.
The diagram of absorbance versus concentration is also called a calibration curve. Such a
diagram can be used to get the concentration of a sample by measuring the absorbance
value at the wavelength used for calibration and reading the corresponding concentration
value from the diagram. The availability of pocket calculators and data-handling computer
systems also allow an easy calculation of the results. To get a quick overview of the quality
of the calibration a visual check of the curve is useful.
6 The following table shows the calculation results for the normalized absorbance data.
The normalized absorbance data are specific for a given wavelength. They are called
extinction coefficients. They are usually given in units of [l/(mol  cm]. Note that extinction
coefficients also depend on the path length.
Results Table 1.18. Calculation Results for Normalized Absorbance Data at Different Wavelengths and
Concentrations
Normalized Absorbance
[AU l/mg] at Different
Wavelengths
Concentrations
c1 c2 c3 c4 c5
224 nm 0.067 0.072 0.064 0.062 0.061
337 nm 0.004 0.004 0.003 0.003 0.002
414 nm 0.061 0.061 0.060 0.059 0.059
46
Basic Principles and Applications
1.8. Possible Sources of Error --
Influence of Impurities
Introduction
Usually only data from a single wavelength is used for quantitative analyses. An impurity
present in the sample solution cannot be detected by this method.
In the following experiment we see how impurities affect quantitative results and we apply
a method to detect samples suspect to impurities.
Reagents and Equipment
t quinoline yellow (yellow dye)
t tartrazine (orange dye)
t distilled water
t 1.0-l volumetric flask
t two 25-ml volumetric flasks
t disposable glass pipettes (minimum 3)
t 10-mm path length quartz cell
Experiment
Time: about 30 min
1 Prepare a stock solution of quinoline yellow, about 16 mg in 1.0 l distilled water.
2 Add a small amount of tartrazine (<0.1 mg) to one of the 250 ml volumetric flasks and fill
both of them with the stock solution.
3 Measure a reference on distilled water.
4 Measure the absorbance values at 224, 270, 290, 337 and 414 nm for both solutions.
47
Basic Principles and Applications
Evaluation
1 Enter the absorbance values for the two samples in the table below.
Evaluation Table 1.19. Absorbance Values of Pure and Impure Samples at Different Wavelengths
Absorbance [AU] at Different Wavelengths Pure Sample Impure Sample
224 nm
270 nm
290 nm
337 nm
414 nm
48
Basic Principles and Applications
2 Enter your measured absorbance values in the diagram below.
Diagram: Absorbance of the Impure Sample as a Function of Absorbance of the Pure
Sample
2
1.5
0.5
1
0.5 1 1.5 2 2.50
2.5
Absorbance [AU] (Impure Sample)
Absorbance [AU] (Pure Sample)
49
Basic Principles and Applications
3 Calculate a linear regression with offset for the diagram above. Use the calculation formulae
given in the appendix for y = ax + b or the built-in function of a pocket calculator. Here the
x,y pairs are the absorbance data of the pure sample(x) and the absorbance data of the
impure sample(y) at the corresponding wavelength. Enter the calculation results in the table
below.
4 How can you tell that a sample is suspect to absorbing impurities?
Evaluation Table 1.20. Linear Regression Calculation Results
Coefficient Value
Slope a
Intercept b
Correlation coefficient R
50
Basic Principles and Applications
Example Results & Discussion
Samples: pure quinoline yellow sample in distilled water (28 mg/l)
"impure" quinoline yellow sample in distilled water (28 mg/l) with a small
amount of tartrazine
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelengths: 224, 270, 290, 337 and 414nm
absorbance range: 0.0­2.0 AU
1 The following table shows the absorbance values of the pure and of the impure samples.
2 The following diagram shows the absorbance of the impure sample as a function of the
absorbance of the pure sample.
Diagram: Absorbance of the Impure Sample as a Function of Absorbance of the Pure
Sample
Results Table 1.19. Absorbance Values of Pure and Impure Sample at Different Wavelengths
Absorbance Pure Sample Impure Sample
224 nm 1.700 1.735
270 nm 0.519 0.603
290 nm 1.267 1.293
337 nm 0.069 0.084
414 nm 1.641 1.751
Absorbance [AU]
(Impure sample)
0
1.0
2.0
0 0.5 1 1.5 2
Absorbance [AU]
(Pure sample)
0.5
1.5
51
Basic Principles and Applications
3 The following table shows the linear regression calculation results.
4 If the samples compared are absorbing at the wavelengths used in the diagram and the
samples obey Beer's law, the result must always be a straight line. Any significant deviations
from this straight line indicate the presence of an absorbing impurity.
For pure samples and accurate absorbance values the correlation coefficient should be as
close to 1 as possible. The intercept should be close to zero. An intercept significantly
different from zero indicates a constantly absorbing background.
The slope is a measure for the concentration ratio of the two samples.
Results Table 1.20. Linear Regression Calculation Results
Name Value
Slope a 0.980
Intercept b -0.032
Correlation coefficient R 0.9986
52
Basic Principles and Applications
1.9. Influence of Temperature --
Potassium Chloride
Introduction
The temperature of a sample affects its absorbance spectrum. This is true for all samples
due to volume expansion of the sample solvent with temperature. If the expansion
coefficients of the solvent are known, a mathematical correction for this "dilution" effect
can be applied. Chemical effects of shifting equilibria with temperature, however, can have
a much higher impact on sample absorbance values than the one due to volume expansion.
The following experiment shows the effect of increasing temperatures on a potassium
chloride sample.
Reagents and Equipment
t potassium chloride (KCl)
t distilled water
t 100-ml volumetric flask
t disposable glass pipettes (minimum 2)
t 10-mm path length quartz cell
t thermostattable cell holder
t temperature control unit or water bath
Experiment
Time: about 120­180 min
1 Prepare a solution of about 120 mg potassium chloride in 100 ml distilled water.
2 Measure a reference on distilled water at 20 °C.
3 Measure the absorbance spectra in the overlay mode in the range from 200 to 214 nm. Repeat
the measurements in 2 degree steps in the range from 20 to 70 °C and read the absorbance
values at 208 nm. Make sure that the temperature of the sample is the same as measured
with the temperature sensor/thermometer and constant when measuring the absorbance
spectra. Verify the temperature of the sample solution with a dipping sensor if available.
53
Basic Principles and Applications
Evaluation
1 Enter the absorbance values at 208 nm for the corresponding temperatures in the table
below.
Evaluation Table 1.21. Measured Absorbance at 208 nm for Different Temperatures
Temperature [°C] Absorbance [AU]
at 208 nm
Temperature [°C] Absorbance [AU]
at 208 nm
20 46
22 48
24 50
26 52
28 54
30 56
32 58
34 60
36 62
38 64
40 66
42 68
44 70
54
Basic Principles and Applications
2 Enter your measured absorbance values in the diagram below.
Diagram: Absorbance as a Function of Temperature
3 Calculate the difference between the absorbance value at 208 nm measured at 20 °C and
70 °C.
2
1.5
0.5
1
4525 30 35 40 50 55 60 65 7020
2.5
Temperature [0
C]
Absorbance [AU]
55
Basic Principles and Applications
Example Results & Discussion
Sample: potassium chloride in distilled water (1.2 g /l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: spectrophotometer:
wavelength range: 200­214 nm
absorbance range: 0.0­1.5 AU
temperature controller:
temperature range: 20­70 °C
temperature steps: 2 °C
equilibration time for temperature: 90 s
Diagram: Overlaid Absorbance Spectra for Different Temperatures
Absorbance
[AU]
200 202 204 206 208 210 212 214
0.00
0.25
0.50
0.75
1.00
1.25
Wavelength [nm]
56
Basic Principles and Applications
1 The following table shows the measured absorbance data at 208 nm.
2 The following diagram shows the absorbance as a function of temperature:
Diagram: Absorbance as a Function of Temperature
The effect of the temperature dependence is most likely due to changes of the hydration of
the Cl-ions. The change of absorbance readings as a function of temperature is most
important for quantitative analyses. For quantitative analyses all measurements must be
carried out at known, constant temperatures.
3 The difference in the absorbance values at 208 nm between 20 °C and 70 °C is 0.642.
Results Table 1.21. Measured Absorbance at 208 nm for Different Temperatures
Temperature [°C] Absorbance [AU]
at 208 nm
Temperature [°C] Absorbance [AU]
at 208 nm
20 0.092 46 0.267
22 0.096 48 0.291
24 0.103 50 0.318
26 0.112 52 0.347
28 0.122 54 0.378
30 0.133 56 0.412
32 0.145 58 0.450
34 0.158 60 0.490
36 0.172 62 0.530
38 0.187 64 0.576
40 0.205 66 0.624
42 0.224 68 0.676
44 0.244 70 0.734
Absorbance
[AU]
20 30 40 50 60 70
0.00
0.25
0.50
0.75
A ( = 208 nm)
Temperature [0
C]
57
Basic Principles and Applications
1.10. Influence of Temperature --
Methyl Orange
Introduction
The environment has an influence on the absorbance spectrum of a compound, as seen in
the experiment before. The following experiment demonstrates, how the temperature can
cause changes in absorbance values as well as shifts of the absorbance bands.
The compound used in the following experiment is a dye which is used as a visual pH
indicator of solutions during acid base titrations.
Reagents and Equipment
t methyl orange
t Na2HPO4 (0.2 M)
t citric acid (0.1 M)
t distilled water
t 100-ml volumetric flask
t 50-ml volumetric flask
t 50-ml pipette
t disposable glass pipettes (minimum 2)
t 10-mm path length quartz cell
t thermostattable cell holder
t temperature control unit or water bath
Experiment
Time: about 90 min
1 Prepare a solution of about 0.05 g methyl orange in 100 ml distilled water.
2 Dilute this stock solution by adding 13 ml of 0.2 M Na2HPO4 and 37 ml of 0.1 M citric acid to
1 ml of stock solution, in order to buffer the solution at a pH of 3.4.
3 Measure a reference spectrum on water.
4 Measure fifteen methyl orange spectra in the temperature range from 15 to 50 °C in steps of
2.5 degrees. Use a wavelength range from 250 to 620 nm. Make sure that the temperature of
the sample is the same as measured with the temperature sensor/thermometer and constant
when measuring the absorbance spectra. Check the temperature of the sample solution with
a dipping sensor if available.
58
Basic Principles and Applications
Evaluation
1 Determine the wavelength of the absorbance maximum at 15 °C.
2 What is the effect of temperature changes on the absorbance values at a particular
wavelength? Use the wavelength of the absorbance maximum at 15 °C for this evaluation.
Enter the absorbance values in the table below.
Evaluation Table 1.22. Measured Absorbance Values at max,15°C at Different Temperatures
Temperature [°C] Absorbance(max 15 °C) [AU]
15.0
17.5
20.0
22.5
25.0
27.5
30.0
32.5
35.0
37.5
40.0
42.5
45.0
47.5
50.0
59
Basic Principles and Applications
3 Draw a graph showing the absorbance at the wavelength of the absorbance maximum at
15 °C versus temperature.
Diagram: Absorbance as a Function of Temperature
0.9
0.85
0.75
0.8
255 10 15 20 30 35 40 45 500
Temperature [0
C]
Absorbance [AU]
0.95
1.0
60
Basic Principles and Applications
4 What is the effect of temperature on the wavelength of the absorbance maximum? Enter the
wavelengths of the absorbance maxima in the table below.
5 Determine the change in the absorbance readings, if the temperature changes by 1 °C.
Quantify the error you make if the temperature of the sample changes by 1 °C.
6 What are possible reasons for the shift of the absorbance band?
Evaluation Table 1.23. Wavelengths of the Absorbance Maxima at Different Temperatures
Temperature [°C] max [nm]
15.0
17.5
20.0
22.5
25.0
27.5
30.0
32.5
35.0
37.5
40.0
42.5
45.0
47.5
50.0
61
Basic Principles and Applications
Example Results & Discussion
Sample: a) stock solution: methyl orange in distilled water (0.5 g/l)
b) sample solution: 1 ml of stock solution, 13 ml of 0.2 M Na2HPO4 and
37 ml of 0.1 M citric acid
Cell: 10-mm path length quartz cell
Instrument
Parameters: spectrophotometer:
wavelength range: 250­620 nm
absorbance range: 0.0­1.5 AU
temperature controller:
temperature range: 15­50 °C
temperature steps: 2.5 °C
equilibration time for temperature: 120 s
Diagram: Measured Absorbance Spectra for Different Temperatures
1 The wavelength of the absorbance maximum at 15 °C is 503 nm.
Absorbance
[AU]
300 400 500 600
0.00
0.25
0.50
0.75
1.00
Wavelength [nm]
62
Basic Principles and Applications
2 The following table shows the measured absorbance values at 503 nm for different
temperatures.
3 The following diagram shows the absorbance at 503 nm for different temperatures.
Diagram: Absorbance Versus Temperature at 503 nm
Results Table 1.22. Measured Absorbance Values at max,15°C at Different Temperatures
Temperature [°C] Absorbance [AU] ( = 503 nm)
15.0 1.049
17.5 1.039
20.0 1.024
22.5 1.008
25.0 0.992
27.5 0.977
30.0 0.961
32.5 0.944
35.0 0.929
37.5 0.912
40.0 0.897
42.5 0.882
45.0 0.866
47.5 0.850
50.0 0.835
Absorbance
[AU]
20 30 40 50
0.80
0.85
0.90
0.95
1.00
1.05
A ( = 503 nm)
Temperature [0
C]
63
Basic Principles and Applications
4 The following table shows the shift of the absorbance maximum depending on the
temperatures:
5 The absorbance at this wavelength decreases with increasing temperature, as can be seen in
the table and the graphic representation of the data. The change in absorbance is
approximately -0.00625 AU/°C. Inaccurate thermostatting or too short equilibrating times
cause wrong absorbance readings of about 0.6 % per 1 °C temperature deviation. Inaccurate
temperature control can cause systematically wrong readings, a circumstance that must
always be taken into account.
6 A possible reason for this temperature dependence of the absorbance readings is the
"optical dilution" effect due to the thermal expansion of the solvent. Changing pH values of
the solution cause changes in the chemical equilibrium of the compound. The dye methyl
orange, used for this experiment, is a pH indicator and extremely sensitive to pH changes in
a certain pH-range.
Results Table 1.23. Wavelengths of the Absorbance Maximum at Different Temperatures
Temperature [°C] max [nm] (Interpolated)
15.0 503.4
17.5 503.0
20.0 503.0
22.5 502.7
25.0 502.5
27.5 502.4
30.0 502.2
32.5 501.8
35.0 501.4
37.5 501.2
40.0 501.1
42.5 500.4
45.0 499.3
47.5 499.3
50.0 498.7
64
Basic Principles and Applications
1.11. Influence of pH -- Buffered
Methyl Orange Solutions
Introduction
The absorbance spectrum of a compound is related to its molecular/electronic structure.
Changes of the environmental conditions can cause changes in the molecular/electronic
structure. A change of the pH value, for example, has an influence on chemical equilibria
and can thus change the absorbance spectrum of a solution.
The compound used in the following experiment is a dye which is used as a visual
pH indicator of solutions during acid base titrations.
Reagents and Equipment
t methyl orange
t disodium hydrogen ortho-phosphate (a)
t citric acid (b)
t distilled water
t eight 25-ml flasks
t 100-ml volumetric flask
t 200-ml volumetric flask
t 500-ml volumetric flask
t eight 50-ml beakers
t 0.5-ml pipette
t 1-ml pipette
t 5-ml pipette
t 10-ml pipette
t 25-ml pipette
t disposable glass pipettes (minimum 16)
t 10-mm path length quartz cell
P
Na
O O
O
H
Na
O
(a)
COOHH2C
COOHHOC
COOHH2C (b)
65
Basic Principles and Applications
Experiment
Time: about 3 h
1 Prepare a stock solution of 0.05 g methyl orange in 100 ml distilled water.
2 Prepare the following solutions:
a) a 0.2 M disodium hydrogen orthophosphate solution:
Dissolve 5.68 g Na2HPO4 in 200 ml distilled water. If necessary, use a magnetic stirrer for
complete dissolution.
b) a 0.1 M citric acid solution:
Dissolve 9.61 g citric acid in 500 ml distilled water.
c) The eight McIlvaine's buffer solutions are prepared by mixing aliquots of the citric acid
solution and the disodium hydrogen orthophosphate solution. Mix the buffer solutions
shown in the table below.
3 Divide each mixture into two equal portions of 25 ml.
4 Add 0.5 ml of methyl orange stock solution to one of the portions of each of the buffer
solutions. These are the sample solutions to be measured. The other portions of buffer have
to be used for the reference measurements.
5 The following two steps have to be repeated for each corresponding pair of the buffer and
the sample (same pH values):
* Measure a reference on the buffer itself.
* Measure the spectrum of the corresponding sample solution in the wavelength range
from 300 to 650 nm.
Amounts of Substances for Preparing Buffer Solutions
Approximate pH Volume [ml] of Na2HPO4 Volume [ml] of Citric Acid
2.2 1.0 49.0
2.6 5.5 44.5
3.0 10.0 40.0
3.4 13.0 37.0
3.8 18.0 32.0
4.2 21.0 29.0
4.6 23.0 27.0
5.2 27.0 23.0
66
Basic Principles and Applications
Evaluation
1 Determine the absorbance maximum of all the spectra measured at different pH values of
the solution. Enter the values in the table below.
2 Check for wavelengths, at which the absorbance is independent of the pH value of the
solution.
3 What are these wavelengths called?
4 What is the advantage of these wavelengths?
Evaluation Table 1.24. Wavelengths of the Absorbance Maximum for Different pH-Values of the Samples
Approximate pH max [nm]
2.2
2.6
3.0
3.4
3.8
4.2
4.6
5.2
67
Basic Principles and Applications
Example Results & Discussion
Sample: Stock solution: methyl orange in distilled water (0.5 g/l)
Buffered solution for pH values: 2.2, 2.6, 3.0, 3.4, 3.8, 4.2, 4.6, 5.2
preparation as described above
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 300 - 650 nm
absorbance range: 0.0 - 1.5 AU
Diagram: Measured Absorbance Spectra for Different pH Values
1 The following table shows the absorbance maximum for different pH values of the samples.
Absorbance
[AU]
300 350 400 450 500 550 600 650
0.00
0.25
0.50
0.75
1.00
1.25
1 pH 2.2
2 pH 2.6
3 pH 3.0
4 pH 3.4
5 pH 3.8
6 pH 4.2
7 pH 4.6
8 pH 5.2
Wavelength [nm]
1
2
3
4
5
6
7
8
Results Table 1.24. Wavelengths of the Absorbance Maximum for Different pH-Values of the Samples
Approximate pH max [nm]
2.2 508
2.6 507
3.0 505
3.4 503
3.8 497
4.2 475
4.6 472
5.2 466
68
Basic Principles and Applications
The wavelength of the absorbance maximum is shifted to the blue with an increasing pH
value. This color change arises from a shift in the equilibrium of non-dissociated and
dissociated dye molecules:
2 The wavelengths of constant absorbance values for different pH values can be found at 350
nm and 469 nm.
3 Wavelengths at which the absorbance does not change are called isosbestic points.
Isosbestic points can occur, if the absorbing species of the equilibrium have the same
extinction coefficient at a certain wavelength. In this case the absorbance is independent of
the position of the equilibrium and depends only on the total amount of the compounds.
4 Isosbestic points are useful for measurements in unbuffered solutions or solutions with
unknown pH values because the absorbance reading is independent of the pH.
H-Indicator H
+
Indicator
+(
)
69
Basic Principles and Applications
1.12. Influence of pH --
Potassium Dichromate Solution
Introduction
The effect of the pH on chemical equilibria is well known and often used to influence them.
The potassium chromate/dichromate equilibrium is markedly pH sensitive. The equilibrium
in acid solution forms mainly dichromate ions and the one in basic solution mainly forms
chromate ions.
Reagents and Equipment
t potassium dichromate (K2Cr2O7)
t potassium hydroxide solution 0.1N (KOH)
t hydrochloric acid solution 0.1N (HCl)
t distilled water
t 100-ml volumetric flask
t two 10-ml volumetric flasks
t 10-ml pipette
t 1-ml pipette
t disposable glass pipettes (minimum 3)
t 10-mm path length quartz cell
Experiment
Time: about 60 min
1 Prepare a stock solution of about 6 mg potassium dichromate in 100 ml distilled water.
2 Prepare the sample solutions:
a) Mix 9 ml of the stock solution with 1 ml potassium hydroxide solution 0.1N.
b) Mix 9 ml of the stock solution with1 ml hydrochloric acid solution 0.1N.
3 Measure a reference on distilled water.
4 Measure the spectrum of the potassium "dichromate" solution in diluted potassium
hydroxide solution in the range from 230 to 500 nm.
5 Measure the spectrum of the potassium "dichromate" solution in diluted hydrochloric acid
solution in the range from 230 to 500 nm.
Cr2O7
2H2O+
2CrO4
2-
2H
+
+
70
Basic Principles and Applications
Evaluation
1 Determine the wavelengths of the absorbance maxima of potassium dichromate dissolved
in acid and basic solution (2 each) and enter the values in the following table.
Evaluation Table 1.25. Absorbance Maxima of Potassium Dichromate in Acid and Basic Solution
Potassium Dichromate Dissolved in max1 [nm] max2 [nm]
Basic Solution
Acid Solution
71
Basic Principles and Applications
Example Results & Discussion
Sample: stock solution: potassium dichromate in distilled water (50 mg/l)
a) 9 ml stock solution mixed with 1 ml hydrochloric acid 0.1N
b) 9 ml stock solution mixed with 1 ml potassium hydroxide solution 0.1N
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 230­500 nm
absorbance range: 0.0­1.2 AU
Diagram: Measured Absorbance Spectra for Sample Solutions with Different pH
Values
1 The absorbance maxima corresponding to the pH of the solution are listed in the table
below.
The spectra of dichromate and chromate ions are different and reflect the molecular
differences of the two anions.
The pH dependence of the potassium dichromate/chromate equilibrium is of practical
interest, because potassium dichromate in acid solution is used as a standard solution to
check the photometric accuracy of spectrophotometers.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
240 340 440
Wavelength [nm]
K2Cr2O7, pH 2.0
K2Cr2O7 ,pH 9.0
Absorbance
[AU]
Results Table 1.25. Absorbance Maxima of Potassium Dichromate in Acid and Basic Solution
Potassium Dichromate
Dissolved in:
max1 [nm] max2 [nm]
Acid Solution 257 352
Basic Solution 274 371
72
Basic Principles and Applications
1.13. Effect of Concentration
Introduction
The compounds measured with UV-visible spectroscopy usually have high extinction
coefficients and must be diluted before the measurements. As a consequence the
concentrations of the compounds are very low and interactions between the molecules of
the compounds can be neglected. Increasing the concentration of a compound can also
change the spectrum by changing the environment of the compound.
The following experiment demonstrates the influence of the concentration on the spectrum
by changing the concentration over three decades.
Reagents and Equipment
t methylene blue
t distilled water
t seven 100-ml beakers
t 25-ml pipette
t 10-ml pipette
t 1-ml pipette
t 0.5-ml pipette
t disposable glass pipettes (minimum 9)
t 10-mm, 5-mm, 2-mm, 1-mm and 0.1-mm path length quartz cells
Experiment
Time: about 2.5­3 h
1 Prepare an aqueous methylene blue stock solution with a concentration of
MW (methylene blue) = 319.86 g/mol (dry)
c0 2 10
3­
mol/l
73
Basic Principles and Applications
2 Prepare 7 different dilutions of the stock solution with distilled water covering the
concentration range from 1 x 10-3 mol/l down to 2 x 10-6 mol/l.
c1 = 1:1 (distilled water: c0)
c2 = 5:1
c3 = 10:1
c4 = 25:1
c5 = 50:1
c6 = 100:1
c7 = 1000:1
3 Measure a reference on water using a 10-mm path length cell.
4 Measure the spectra of all solutions using the same cell for the different solutions starting
with the lowest concentrated solution. Whenever the absorbance readings exceed 2 to 2.5
absorbance units use a cell with shorter path length for the reference and the sample
measurements. Use a wavelength range from 400 to 900 nm.
74
Basic Principles and Applications
Evaluation
1 Determine the wavelengths of the absorbance maxima for the different solutions and enter
them in the table below. Also enter the path length of the cell you used.
2 It is useful to normalize the measured spectra at different concentrations and path lengths
for convenient comparison. In this case a point between the two absorption maxima, for
example, 637 nm, is suitable for normalization. Multiply each spectrum with a factor f so that
the absorbance values at norm are identical.
3 Overlay all spectra in a single plot.
4 What effect do you observe concerning the wavelength of the absorbance maxima going
from high concentrations to low ones?
5 Find an explanation for the change of the absorbance band.
Evaluation Table 1.26. Absorbance Maxima Depending on Concentration
Concentration Path Length [mm] max [nm]
Peak 1 Peak 2
c7
c6
c5
c4
c3
c2
c1
c0
75
Basic Principles and Applications
Example Results & Discussion
Samples: solutions of methylene blue in distilled water:
a) c0 = 1.876 x 10-3
mol/l
b) c1 = 9.379 x 10-4
mol/l
c) c2 = 3.127 x 10-4
mol/l
d) c3 = 1.706 x 10-4
mol/l
e) c4 = 7.216 x 10-5
mol/l
f) c5 = 3.678 x 10-5
mol/l
g) c6 = 1.857 x 10-5
mol/l
h) c7 = 1.874 x 10-6
mol/l
Cells 10-mm, 5-mm, 2-mm, 1-mm and 0.1-mm path length quartz cells
Instrument
Parameters: wavelength range: 400­900 nm
absorbance range: 0.0­1.8 AU
1 The table below shows the absorbance maxima depending on the concentration.
2 In this case the spectra were normalized to an absorbance value of 1.0 AU at 637 nm by
dividing the spectra by their readings at this wavelength.
Results Table 1.26. Absorbance Maxima Depending on Concentration
Concentration Path Length [mm] max [nm]
Peak 1 Peak 2
c7 0.1 616 663
c6 0.1 613 663
c5 0.1 613 663
c4 1 613 663
c3 2 610 663
c2 5 608 664
c1 10 605 662
c0 10 604 -
76
Basic Principles and Applications
Diagram: Normalized Absorbance Spectra for Different Sample Concentrations
3 The wavelengths of the absorbance maxima are shifted to higher wavelengths.
4 At low concentrations, methylene blue is dissolved in the form of monomers. With
increasing concentration the molecules start to interact and to form dimers.
The energies of the electronic levels are changed by forming the dimere instead of the
monomers. As a result a significant shift to shorter wavelengths takes place.
Absorbance
[AU]
400 500 600 700 800 900
0.00
0.25
0.50
0.75
1.00
1.25
1.50
Concentrations
Wavelength [nm]
c0
c1
c2
c3
c4
c5
c6
c7
77
Basic Principles and Applications
1.14. Principle of Additivity
Introduction
According to Beer's law the absorbance is proportional to the number of molecules that
absorb radiation at a specified wavelength. If more than one absorbing species are present
the principle of additivity says, that the absorbance at any wavelength of a mixture is equal
to the sum of the absorbances of each component in the mixture at that wavelength. This
principle is the base for a simple approach to quantitative multi-component analysis. For
calibration the absorbance values of standards of known concentration of the pure
components are measured to determine their extinction coefficients (proportionality
factors).
The following experiment shows an easy approach to multi-component analysis. The
spectra of two food dyes are measured.
Reagents and Equipment
t quinoline yellow (yellow dye)
t indigotin (blue dye)
t distilled water
t three volumetric 1.0-ml flasks
t disposable glass pipettes (minimum 4)
t 10-mm path length quartz cell
Experiment
Time: about 90 min
1 Prepare the following standard solutions:
a) about 12 mg quinoline yellow in 1.0 l distilled water.
b) 16 mg indigotin in 1.0 l distilled water.
2 Prepare a mixture of about 6 mg quinoline yellow and 9 mg indigotin in 1.0 l distilled water.
This solution is the sample solution.
3 Measure a reference on distilled water.
4 Measure the spectra of quinoline yellow and indigotin standards in the range from 300 to 700
nm.
5 Measure the spectrum of the sample (mixture of quinoline yellow and indigotin).
78
Basic Principles and Applications
Evaluation
We use the simplest method for multi-component quantification for this experiment. The
method uses only the absorbance values at the maxima of the pure compounds.
1 Determine the wavelength of the absorbance maxima for quinoline yellow and indigotin and
enter them in the table below.
2 Determine the absorbance of the two standards and the sample at these wavelengths and
enter them in the table below.
Evaluation Table 1.27. Wavelengths of the Absorbance Maxima of the Two Pure Dyes
Compound max [nm]
Quinoline yellow standard 1 =
Indigotin standard 2 =
Evaluation Table 1.28. Measured Absorbance Values at the Absorbance Maxima
Compound Absorbance [AU] at 1 = _________nm Absorbance [AU] at 2 =_________nm
Quinoline yellow standard
Indigotin standard
Sample
79
Basic Principles and Applications
3 Calculate the concentration of the components quinoline yellow and indigotin in the
mixture.
For a mixture of two components the absorbance A at the wavelengths 1 and 2, (for
example, the absorbance maxima of the components x and y) is given in the following two
equations:
with:
, = absorbance values of the sample at 1, 2
, = absorbance values of the quinoline yellow standard at 1, 2
, = absorbance values of the indigotin standard at 1, 2
x = fraction of quinoline yellow in the sample
y = fraction of indigotin in the sample
The concentration of the components can be calculated by:
with:
c01 = concentration of quinoline yellow in the standard solution
c02 = concentration of indigotin in the standard solution
c1 = concentration of quinoline yellow in the sample
c2 = concentration of indigotin in the sample
4 Compare the calculated concentrations c1 and c2 of quinoline yellow and indigotin with the
real ones that you have prepared.
5 Judge the certainty/uncertainty of the method described above. To get an estimation for the
reliability of this method simulate noise by adding an error of x % to the absorbance reading
of the sample and calculate the concentrations again.
6 How can this simple method be improved to achieve more reliable results?
A 1( ) x y+( )
xA 1( )x
yA 1( )y
+=
A 2( ) x y+( )
xA 2( )x
yA 2( )y
+=
A 1( ) x y+( )
A 2( ) x y+( )
A 1( )x
A 2( )x
A 1( )y
A 2( )y
c1 xc01=
c2 yc02=
80
Basic Principles and Applications
Example Results & Discussion
Samples: a) standard solution 1: quinoline yellow in distilled water (12.0 mg/l)
b) standard solution 2: indigotin in distilled water (17.5 mg/l)
c) mixture of 5.85 mg quinoline yellow and 8.9 mg indigotin in 1 l distilled
water
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 300­700 nm
absorbance range: 0.0­0.8 AU
Spectra: Measured Absorbance Spectra of Pure and Mixed Sample
1 The wavelengths listed below were chosen for the calculation.
Absorbance
[AU]
400 500 600 700
0.00
0.25
0.50
0.75
1 Quinoline yellow standard
2 Indigotin standard
3 Sample
Wavelength [nm]
1
2
3
Results Table 1.27. Wavelengths of the Absorbance Maxima of the Two Pure Dyes
Compound max [nm]
Quinoline yellow standard 1 = 414 nm
Indigotin standard 2= 610 nm
81
Basic Principles and Applications
2 The measured absorbance values at the absorbance maxima are listed in the table below.
3 Calculation:
The fractions x and y of quinoline yellow and indigotin were calculated as:
The concentrations of quinoline yellow and indigotin in the mixture were calculated as:
4 Comparing the measured concentrations with the calculated concentrations gives a good
correlation with an error of about 1 %.
5 To simulate noise, we add an error of about 1 % of the absorbance measurement of the
sample (0.3948 and 0.3531) results. This changes the calculated concentrations of quinoline
yellow and indigotin in the mixture as follows:
6 Any measurement error such as noise in the absorbance reading influences the calculated
concentrations of the mixture directly. The effect of random noise can be reduced through
the use of additional spectral information. Instead of using only two data points, a series of
data points can be used for quantification. In this case we have a so-called over-determined
system. A least square fit of the standard spectra to the spectrum of the measured sample
yields quantitative results.
Results Table 1.28. Measured Absorbance Values at the Absorbance Maxima
Compound 1 = 414 nm 2 = 610 nm
Quinoline yellow standard 0.772 -0.001
Indigotin standard 0.039 0.680
Sample 0.399 0.350
0.7719 x + 0.0384 y = 0.3988 -0.0007 x + 0.6801 y = 0.3496
x = 0.491 y = 0.514
c1 = 5.89 mg/l c2 = 9.00 mg/l
c1 = 5.83mg/l c2 = 9.10 mg/l
82
Basic Principles and Applications
Part 2
83
29 Measuring Instrument
Performance
84
Measuring Instrument Performance
General
Instrumental performance is one of the main factors that effect the accuracy and
reproducibility of measurements. The following experiments are procedures for measuring
many of the key instrumental performance parameters.
The relevance of the various procedures can best be appreciated if they are made on two
instruments with different overall performance specifications. For example a research
grade instrument and, for comparison, a lower priced instrument with a performance
intended for less demanding routine work can be used simultaneously.
85
Measuring Instrument Performance
2.1. Wavelength Accuracy
Introduction
Wavelength accuracy is an important performance parameter when comparing data
measured on different instruments. Wavelength accuracy is normally checked by using a
calibration standard which has a series of narrow transmittance valleys. A commonly used
standard is holmium perchlorate solution which is prepared by dissolving holmium oxide
in perchloric acid. It has a series of narrow valleys over the wavelength range from 200 to
700 nm allowing a check of wavelength accuracy over the whole UV range and far into the
visible range.
Reagents and Equipment
t holmium oxide (Ho2O3)
t perchloric acid (HClO4)
t distilled water
t 50-ml beaker
t 100-ml volumetric flask
t 10-ml pipette
t disposable glass pipettes (minimum 2)
t 10-mm path length quartz cell
t heater with stirrer
Experiment
Time: about 120­150 min
1 Prepare a solution of about 4.0 g holmium oxide in 100 ml 10 % v/v pure perchloric acid:
a) Weigh 4 g of holmium oxide and add 10 ml of distilled water and 10 ml of pure perchloric
acid.
b) Dissolve the holmium oxide by heating and stirring the mixture at about 80 °C for one
hour.
c) Transfer the clear solution quantitatively to a 100-ml volumetric flask and bring to
volume with distilled water.
2 Measure a reference on 10 % perchloric acid.
3 Measure the transmittance spectrum of the holmium oxide solution in the wavelength range
from 210 to 700 nm. Use a slow scan speed if you are working with a conventional scanning
spectrophotometer. Determine the wavelengths of the transmittance minima using the built
in function, if available in your instrument. Otherwise determine the wavelengths of the
transmittance minima manually.
86
Measuring Instrument Performance
Note: Instead of preparing the holmium oxide solution you can also use a holmium oxide
solution SRM 2034 from NIST (National Institute of Standards and Technology).
Evaluation
1 Enter the measured wavelengths of the transmittance minima in the table below. The
reference values for instruments with 0.5, 1 and 2 nm bandwidth are also shown (taken from
NIST 2034 certificate). Calculate the deviations of the measured values from the specified
values for the bandwidth closest to the instrument you used and enter them in the table
below.
Note: If the optical bandwidth of the instrument you are using is not known you can
estimate it using the procedure from the experiment "Spectral Resolution" on
page 102.
Evaluation Table 2.1. Measured Transmittance Minima Compared to Reference Values
0.5 nm
ref [nm]
1 nm
ref [nm]
2 nm
ref [nm]
Measured
min [nm]
Deviation
 [nm]
241.01 241.13 241.08
249.79 249.87 249.98
278.13 278.10 278.03
287.01 287.18 287.47
333.43 333.44 333.40
345.52 345.47 345.49
361.33 361.31 361.16
385.50 385.66 385.86
416.09 416.28 416.62
--- 451.30 451.30
467.80 467.83 467.94
485.27 485.29 485.33
536.54 536.64 536.97
640.49 640.52 640.84
87
Measuring Instrument Performance
Example Results & Discussion
Sample: holmium oxide in 10 % v/v perchloric acid (40 g/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 210­700 nm
transmittance range: 0.0­100 [%]
Diagram: Transmittance Spectrum of Holmium Perchloride Solution
Wavelength [nm]
0
20
40
60
80
200 300 400 500 600 700
100
Transmittance
[%]
88
Measuring Instrument Performance
1 The following table shows the measured wavelengths of transmittance minima compared to
the reference values.
* These values are interpolated between the 1- and 2-nm reference values to a nominal bandwidth of 1.5 nm
which is the bandwidth of the spectrophotometer used.
References
"Holmium Oxide Solution Wavelength Standard from 240 to 640 nm - SRM 2034" Victor R. Weidner,
Radu Mavrodineanu, Klaus D. Mielenz, Rance A. Velapoldi, Kenneth L. Eckerle and Bradley Adams,
National Bureau of Standards, Gaithersburg, MD 20899
Results Table 2.1. Measured Transmittance Minima Compared to Reference Values
ref* [nm] Measured max [nm] Deviation  [nm]
241.11 241.03 -0.08
249.93 249.90 -0.03
278.07 277.98 -0.09
287.33 287.09 -0.24
333.42 333.22 -0.20
345.48 345.16 -0.32
361.24 361.04 -0.20
385.76 385.47 -0.29
416.45 416.18 -0.27
451.30 451.43 -0.13
467.89 467.65 -0.28
485.31 485.00 -0.31
536.81 536.34 -0.47
640.68 640.27 - 0.41
89
Measuring Instrument Performance
2.2. Wavelength Accuracy Using
the Deuterium Lines
Introduction
Another way to check the wavelength accuracy is to use the emission lines from various
sources such as mercury or deuterium arc lamps. The easiest ones to use are the deuterium
emission lines of the deuterium lamp -- one of the light sources in virtually all UV-visible
spectrophotometers. The advantage of this method compared to the one described above is
that it is quick and easy since no reagents are required. The disadvantage is that only two
calibration wavelengths are available and there are no calibration wavelengths in the UV
region.
Reagents and Equipment
t spectrophotometer capable of measuring intensity spectra
Experiment
Time: about 20 min
1 Measure the intensity spectrum of the deuterium lamp in the wavelength range from
200 to 700 nm. If you work with a conventional scanning spectrophotometer use the slowest
scan speed and scan the wavelength ranges from 480 to 490 nm and from 650 to 660 nm. If
your instrument offers this functionality determine the wavelengths of the intensity maxima
using the built-in function. Otherwise the intensity maxima have to be determined manually.
90
Measuring Instrument Performance
Evaluation
1 Enter the measured values in the following table and calculate the wavelength errors:
Evaluation Table 2.2. Measured Wavelengths of Intensity Maxima Compared to Reference Values
Reference max [nm] Measured max [nm] Deviation  [nm]
486.0
656.1
91
Measuring Instrument Performance
Example Results & Discussion
Sample: no sample required
Cell: no cell required
Instrument
Parameters: conventional dual beam spectrophotometer:
scan speed: 7.5 nm / min
wavelength ranges: 480­490 nm and 650­660 nm
Diagram: Intensity Spectrum of the Deuterium Lamp
1 The following table gives a comparison of the measured and the reference wavelengths of
the intensity maxima.
Intensity
200 300 400 500 600 700 800 900
Wavelength [nm]
400 500 600 700 800 900
484 486 488 654 656 658
1 2
1 2
Results Table 2.2. Measured Wavelengths of Intensity Maxima Compared to Reference Values
Reference max [nm] Measured max [nm] Deviation  [nm]
486.0 486.92 -0.07
656.1 656.36 0.26
92
Measuring Instrument Performance
2.3. Photometric Accuracy Using
Potassium Dichromate
Introduction
Photometric accuracy is the most important criterion for quantitative analysis when
extinction coefficients or factors are used. A potassium dichromate solution (60 mg/l) in
sulphuric acid (0.01 N) is used in the following experiment.
Reagents and Equipment
t potassium dichromate, dried to constant weight at 130 °C (KCr2O4)
t 0.01-N sulphuric acid (H2SO4)
t 100-ml volumetric flask
t disposable glass pipettes (minimum 2)
t 10-mm path length quartz cell
Experiment
Time: about 20 min
1 Prepare a solution of about 6 mg (maximum range 5.7­6.3 mg) potassium dichromate in
100 ml 0.01 N sulphuric acid.
2 Measure a reference on sulphuric acid (0.01 N).
3 Immediately afterwards do a sample measurement of the potassium dichromate solution in
the wavelength range from 220 to 380 nm.
93
Measuring Instrument Performance
Evaluation
1 Determine the absorbance values of the peaks and valleys at the wavelengths 235, 257, 313
and 350 nm. Calculate absorbance values corrected from the actual concentration to a
reference concentration of 60.06 mg/l using the following equation.
2 Enter the measured and the corrected values in the table below. Calculate the deviation
between the corrected absorbances and the reference values.
Corrected Absorbance [AU]
actual weight [mg]
6.006 mg
--------------------------------------------- Measured Absorbance [AU]=
Evaluation Table 2.3. Measured Absorbance Values Compared to Reference Values
Wavelength
[nm]
Measured
Absorbance [AU]
Corrected
Absorbance [AU]
Reference
Absorbance [AU]
Deviation
A [AU]
235 0.742
257 0.861
313 0.291
350 0.639
94
Measuring Instrument Performance
Example Results & Discussion
Sample: potassium dichromate in 0.01 N sulfuric acid (60.06 mg/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 220­380 nm
absorbance range: 0.0­1.0 AU
Diagram: Absorbance Spectrum of the Potassium Dichromate Solution
1 The following table shows a comparison of the measured and of the reference absorbance
values at defined wavelengths.
2 In this example the concentration of potassium dichromate used was exactly the same as
the one that was used for measurement of the reference values. Therefore no correction
needs to be applied.
Absorbance
[AU]
220 240 260 280 300 320 340 360 380
0.0
0.2
0.4
0.6
0.8
Wavelength [nm]
235 257 313 350
Results Table 2.3. Measured Absorbance Values Compared to Reference Values
Wavelength
[nm]
Measured
Absorbance
[AU]
Corrected
Absorbance
[AU]
Reference
Absorbance
[AU]
Deviation
A [AU]
235 0.741 0.741 0.742 0.009
257 0.861 0.861 0.861 0.010
313 0.290 0.290 0.291 0.004
350 0.640 0.640 0.639 0.001
95
Measuring Instrument Performance
One problem with using potassium dichromate as a standard is that it is not only a test of
the instrument accuracy but also a test, for example, of the accuracy of the balance used to
weigh the dichromate. The accuracy of the volumetric flask and the experimental skill of
the chemist preparing the solution also have a strong impact on the accuracy of the results.
The advantage of potassium dichromate is that it absorbs in the UV range. There are only
very few suitable standards available for this part of the spectrum.
References
"European Pharmacopeia" Third Edition 1997, Council of Europe, Strasbourg, ISBN 92-871-2991-6
96
Measuring Instrument Performance
2.4. Photometric Accuracy Using
Neutral Density Glass Filters
Introduction
Another way to check the photometric accuracy is using neutral density glass filters, for
example, NIST 930e or similar. Such standards are supplied with calibration values at
specific wavelengths which have been measured on a highly precise reference
spectrophotometer. The advantage of this method compared to using solutions is that no
sample preparation is required.
Reagents and Equipment
t neutral density filter NIST 930e or similar with 10 % transmittance
Experiment
Time: about 10 min
1 Measure a reference on air (no cell in the cell holder).
2 Measure the absorbance spectrum of the neutral density glass filter in the wavelength range
from 400 to 700 nm.
97
Measuring Instrument Performance
Evaluation
1 Determine the absorbance values at the certified wavelengths and enter them in the table
below. For the NIST filters they are 440.0, 465.0, 546.0, 590.0 and 635.0 nm. Find out the
specified wavelengths from the calibration certificates if another standard is being used.
Enter the reference absorbance values from the calibration certificate of the standard and
calculate the errors.
Evaluation Table 2.4. Measured Absorbance Values Compared to Reference Values
Wavelength
[nm]
Reference Absorbance
[AU]
Measured Absorbance
[AU]
Deviation
A [AU]
98
Measuring Instrument Performance
Example Results & Discussion
Sample: NIST 930e neutral density glass filter, 10 % transmittance
Cell: no cell required
Instrument
Parameters: wavelength range: 400­700 nm
absorbance range: 0.8­1.2 AU
Diagram: Absorbance Spectrum of NIST 930e Neutral Density Glass Filter
1 The following table shows the absorbance values of the NIST 930e neutral density glass filter
at defined wavelengths and compares the measured values to the reference values.
Absorbance
[AU]
400 450 500 550 600 650 700
0.9
1.0
1.1
Wavelength [nm]
440 465 546 590 635
Results Table 2.4. Measured Absorbance Values Compared to Reference Values
Certified
Wavelength [nm]
Reference Absorbance
[AU]
Measured Absorbance
[AU]
Deviation
A [AU]
440.0 1.1051 1.1067 0.0016
465.0 1.0278 1.0290 0.0012
546.0 1.0570 1.0569 0.0001
590.0 1.0996 1.0997 0.0001
635.0 1.0487 1.0484 0.0003
99
Measuring Instrument Performance
2.5. Stray Light
Introduction
Stray light is the factor that most strongly affects the linear relationship between
absorbance and concentration at high absorbance values. It introduces a systematic bias to
lower absorbances at increasing concentrations. Stray light is also the primary influence on
the upper limit of the linear dynamic range for an analysis.
To detect stray light at given wavelengths solutions of sodium nitrite (340 nm), sodium
iodide (220 nm) and potassium chloride (200 nm) in water are used as cut-off filters.
Reagents and Equipment
t sodium nitrite (NaNO2)
t sodium iodide (NaI)
t potassium chloride (KCl)
t distilled water
t three 100-ml volumetric flasks
t disposable glass pipettes (minimum 3)
t 10-mm path length quartz cell
Experiment
Time: about 60­75 min
1 Prepare the following sample solutions:
a) 5 g sodium nitrite in 100 ml distilled water,
b) 1 g sodium iodide in 100 ml distilled water and
c) 1.2 g potassium chloride in 100 ml distilled water
2 Measure a reference on distilled water.
3 Measure the transmittance spectra of the stray light sample solutions in the wavelength
range from 200 to 500 nm.
100
Measuring Instrument Performance
Evaluation
1 Determine the transmittance at the specified wavelengths for each solution and enter it in
the table below.
Evaluation Table 2.5. Measured Transmittance of Different Stray Light Filters at Different Wavelengths
Stray Light Filter Wavelength
[nm]
Measured Transmittance
[% T]
NaNO2 340
NaI 220
KCl 200
101
Measuring Instrument Performance
Example Results & Discussion
Samples: a) sodium nitrite in distilled water (50 g/l)
b) sodium iodide in distilled water (10 g/l)
c) potassium chloride in distilled water (12 g/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 200­500 nm
transmittance range: 0­100 %
Diagram: Transmittance Spectra of the Tested Stray Light Filters
1 The following table shows the measured transmission values compared to the instrument
specifications.
Transmittance [%]
200 250 300 350 400 450 500
0
20
40
60
80
100
Wavelength [nm]
1 2 3
1 KCl
2 Nal
3 NaNO2
Results Table 2.5. Measured Transmittance of Different Stray Light Filters at Different Wavelengths
Stray Light Filter Wavelength [nm] Measured Transmission [% T]
NaNO2 340 0.016
NaI 220 0.010
KCl 200 0.497
102
Measuring Instrument Performance
2.6. Spectral Resolution
Introduction
Resolution is a critical factor in determining the shape of measured peaks. In general the
instrumental resolution should be at least 10 times as high as the natural bandwidth of the
peak measured. If the instrumental resolution is not sufficient, the absorbance value will be
lower than the true value.
Resolution can be estimated by measuring the ratio of the absorbance of the maximum at
ca. 269 nm to that of the minimum at ca. 266 nm of a toluene solution in hexane.
Reagents and Equipment
t toluene (a)
t hexane (CH3(CH2)4CH3)
t 100-ml volumetric flask
t 20-l pipette
t disposable glass pipettes (minimum 2)
t 10-mm path length quartz cell
Experiment
Time: about 20­60 min
1 Prepare a solution of 0.02 % v/v toluene in n-hexane in the 100-ml volumetric flask.
2 Measure a reference on hexane.
3 Measure the spectrum of the toluene in hexane solution in the wavelength range from
265 to 272 nm.
4 If your spectrophotometer allows you to vary the slit width, repeat the measurement of the
toluene sample with different slit widths from 0.25 to 4 nm.
CH3
(a)
103
Measuring Instrument Performance
Evaluation
1 Calculate the ratio absorbance of the peak Ap(269) /absorbance of the valley Av(266) and
enter it together with the instrument slit width in the table below.
The following table shows the relationship between instrumental slit width and the
269/266 nm ratio. Use the table below to estimate the actual instrumental slit width and
compare it to the nominal value.
Evaluation Table 2.6. Measured Absorbances at Different Wavelengths for Different Slit Widths and Their Ratios
Nominal Instrument Slit
Width [nm]
Absorbance [AU]
at 269 nm
Absorbance [AU]
at 266 nm
Measured
Ratio
Reference Ratio of Maxima to Minima Depending on the Slit With
Instrument Slit Width [nm] Reference Ratio
0.25 2.3
0.50 2.2
1.00 2.0
2.00 1.4
3.00 1.1
4.00 1.0
104
Measuring Instrument Performance
2 Create a plot showing the calculated absorbance ratios versus slit width.
Diagram: Calculated Absorbance Ratios Versus Slit Width
Ratio
2
1.5
0.5
1
1 2 3 4
Slit width [mm]
0
2.5
5
105
Measuring Instrument Performance
Example Results & Discussion
Sample: toluene in n-hexane (0.02 % v/v)
Cell: 10-mm path length quartz cell
Instrument
Parameters: conventional scanning spectrophotometer with variable slit width:
slit widths: 0.25­4.0 nm
wavelength range: 265­72 nm
absorbance range: 0.0­0.7 AU
Diagram: Spectra for Different Slit Widths
1 The following table shows a comparison between the maxima to minima ratio calculated
from the measurements and the reference ratios.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
265 266 267 268 269 270 271 272
Wavelength [nm]
Absorbance
[AU]
Slit width
0.25 nm
0.50 nm
1.00 nm
2.00 nm
3.00 nm
4.00 nm
Results Table 2.6. Measured Absorbances at Different Wavelengths for Different Slit Widths and
Their Ratios
Instrument Slit Width
[nm]
Absorbance [AU]
at about 269 nm
Absorbance [AU]
at about 266 nm
Measured Ratio Reference Ratio
0.25 0.54 0.29 1.902 2.3
0.50 0.53 0.29 1.85 2.2
1.00 0.50 0.30 1.65 2.0
2.00 0.43 0.34 1.26 1.4
3.00 0.38 0.37 1.05 1.1
4.00 0.40 0.36 0.9 1.0
106
Measuring Instrument Performance
2 A plot of slit width versus 269/266 ratio is shown below.
Decreasing slit width improves resolution but it reduces the amount of light that reaches
the detector. This results in higher noise and poorer precision.
References
"Fundamentals of Modern UV-visible Spectroscopy", Tony Owen, Hewlett-Packard primer 1996,
publication number 12-5965-5123E
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0 1 2 3 4
Slit width [nm]
Ratio
Ratio of peak/valley
around 269 and 266 nm
107
Measuring Instrument Performance
2.7. Noise
Introduction
Noise is the major factor affecting the precision of the absorbance measurements. It is the
limiting factor at low absorbances. Noise is typically measured at zero absorbance, that is,
with no sample in the light path. Sometimes noise is quoted peak-to-peak but a more
common method is to calculate the standard deviation value which has more statistical
significance.
Reagents and Equipment
t no equipment and reagents required
Experiment
Time: about 25 min
1 Measure a reference on air (no cell in the cell holder).
2 Make sixty consecutive measurements on air (no cell in the cell holder) at 1 second intervals
with 0.5 s integration time* and at 500 nm.
* This is the standard integration time for the HP 8453 spectrophotometer. If using
another kind of spectrophotometer use the standard response settings.
3 Repeat the experiment using shorter (0.1 s) and longer (5.0 s) integration times.
108
Measuring Instrument Performance
Evaluation
1 Calculate the noise and mean of absorbance, using the following equations:
where: x = measured absorbance values,
n = number of points
2 How does the noise value change with shorter and with longer integration times?
Enter the calculated values of mean and standard deviation at different integration times in
the table below.
3 Would you expect the same noise level at other wavelengths?
Noise (SD)
nx
2
x( )
2
n
n 1­( )
---------------------------------= ; Mean Absorbance
x
n
------=
Evaluation Table 2.7. Calculated Means and Standard Deviations
Integration Time [s] Mean Absorbance [AU] Noise Standard Deviation [AU]
0.1
0.5
5.0
109
Measuring Instrument Performance
Example Results & Discussion
Sample: no reagents required
Cell: no cell required
Instrument
Parameters: fixed wavelength: 500 nm
absorbance range: -0.3­0.7 mAU
Diagram: Plot of Measured Absorbance Values Versus Time
1 The following values were calculated from the measured values:
2 Noise increases with shorter integration times and decreases with longer integration times.
3 Noise varies from wavelength to wavelength. This is due to the fact that the intensity of the
light sources and detector characteristics vary with wavelength.
-0.0003
-0.0001
0.0001
0.0003
0.0005
0.0007
0 20 40 60 80 100
Time [s]
Absorbance
[AU]
0.1
0.5
5.0
Integration times [s]
Results Table 2.7. Calculated Means and Standard Deviations
Integration Time [s] Mean Absorbance [AU] Noise Standard Deviation [AU]
0.1 1.10 10-4
2.35 10-4
0.5 3.187 10-5 1.14 10-4
5.0 -7.23 10-6
4.79 10-5
110
Measuring Instrument Performance
2.8. Baseline Flatness
Introduction
In the previous experiment we measured noise at a specific wavelength. Ideally, for a
proper characterization of an instrument its noise characteristics should be measured at all
wavelengths. However, this would be extremely time consuming. One way to get an
overview of the relative noise level at all wavelengths is to measure the baseline flatness. It
also reveals wavelengths with instrumental problems resulting from switching filters or
light source exchanges. Baseline flatness is typically measured at zero absorbance with no
sample in the light path.
Reagents and Equipment
t no equipment and reagents required
Experiment
Time: about 30 min
1 Measure a reference on air (no cell in the cell holder).
2 Measure a spectrum on air over the full range of the spectrophotometer with 0.5 s
integration time.
3 Vary the integration times to 0.1 s and to 5 s to see the effect of varying integration times.
111
Measuring Instrument Performance
Evaluation
1 Calculate the baseline flatness and mean absorbance using the following equation:
where: x = measured absorbance value
n = number of points
How does the noise value change with shorter and longer integration times. Enter the
calculated values in the table below.
Baseline (SD)
nx
2
x( )
2
n
n 1­( )
---------------------------------= ; Mean Absorbance
x
n
------=
Evaluation Table 2.8. Calculated Mean Absorbance and Standard Deviation
Integration Time [s] Mean Absorbance [AU] Baseline Standard Deviation [AU]
0.1
0.5
5
112
Measuring Instrument Performance
Example Results & Discussion
Sample: no reagents required
Cell: no cell required
Instrument
Parameters: wavelength range: 190­900 nm
absorbance range: -0.5­0.5 mAU
Diagram: Absorbance Spectra of Baselines
1 Calculated mean absorbance and standard deviation is listed in the table below.
-0.0005
-0.0003
-0.0001
0.0001
0.0003
0.0005
Wavelength [nm]
Absorbance [AU]
-0.0005
-0.0003
-0.0001
0.0001
0.0003
0.0005
Integration Time = 5 s
-0.0005
-0.0003
-0.0001
0.0001
0.0003
0.0005
190 290 390 490 590 690 790 890
Integration Time = 0.5 s
Integration Time = 0.1 s
Results Table 2.8. Calculated Mean Absorbances and Standard Deviation
Integration Time [s] Mean Absorbance [AU] Baseline Standard Deviation
[AU]
0.1 3.25 10-5 1.65 10-4
0.5 7.67 10-6 6.47 10-5
5 4.76 10-6 3.65 10-5
113
Measuring Instrument Performance
2.9. Stability
Introduction
Stability affects the accuracy of absorbance measurements as a function of time. Drift in
absorbance measurements introduces systematic errors in photometric accuracy. Stability
is typically measured at zero absorbance with no sample in the light path.
Reagents and Equipment
t no equipment and reagents required
Experiment
Time: about 75 min
1 Be sure that the instrument has warmed up and thermally equilibrated sufficiently (for
example, one hour).
2 Measure a reference on air (no cell in the cell holder).
3 Measure the absorbance of air (no cell in the cell holder) at the wavelength of 340 nm.
Repeat this measurement 60 times every 60 s with an integration time of 0.5 s.
Note: Be sure that the ambient temperature is constant during measuring time.
Evaluation
1 Calculate the difference between maximum and minimum values of measured absorbance.
Evaluation Table 2.9.
Maximum Absorbance [AU] Minimum Absorbance [AU] Stability [AU/h]
Amax = Amin = A Amax Amin­= =
114
Measuring Instrument Performance
Example Results & Discussion
Sample: no reagents required
Cell: no cell required
Instrument
Parameters: fixed wavelength: 340 nm
absorbance range: -1.5­2.0 mAU
integration time: 0.5 s
measuring time: 60 minutes
cycle time: 60 s
Diagram: Measured Changes of Absorbance Versus Time
1 The following table shows the measured absorbances and the figured stability.
Absorbance
[mAU]
0 10 20 30 40 50 60
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Time [min]
Results Table 2.9. Measured Absorbance and Figured Stability
Maximum Absorbance [AU] Minimum Absorbance [AU] Stability [AU/h]
Amax 1.699 10
4=
Amin 1.299­ 10
4=
A Amax Amin­ 2.999 10
4=
=
Part 3
115
10 Sample Handling and
Measurement
116
Sample Handling and Measurement
General
Every chemist, laboratory staff, or user of spectrophotometric instruments must be aware,
to which extent inaccurate sample or cell handling and nonreproducible measurement
conditions can influence the accuracy of the results achieved.
With limitations due to instrumental performance and sample properties the greatest
sources of error result from a lack of accuracy in sample and cell treatment.
This chapter shows possible sources of error and helps the user to eliminate them in order
to achieve the best measurement results possible.
117
Sample Handling and Measurement
3.1. Sample Handling
Introduction
Assuming that the instrument works properly with given limitations due to the instrument's
performance and sample properties, the largest sources of error in spectrophotometry are
related to sample and cell handling.
The following experiment demonstrates the effect of sample and cell handling on the
reproducibility of results.
Equipment and Reagents
t quinoline yellow (yellow dye)
t distilled water
t 1.0-l volumetric flask
t disposable glass pipettes (minimum 2)
t 10-mm path length quartz cell
Experiment
Time: about 30­60 min
1 Prepare a solution of about 12 mg quinoline yellow in 1.0 l distilled water.
2 Measure a reference on distilled water.
3 Measure the spectrum of the quinoline yellow solution in the wavelength range from 190 to
900 nm.
If you have a slow scanning spectrophotometer, first determine the wavelength of the
absorbance maximum in the visible range and set the instrument to this wavelength instead
of measuring the whole spectrum.
4 Measure the first series of ten measurements without touching the cell.
5 Measure the second series of ten measurements, taking the cell out of the cell holder after
each measurement and replacing it again with the same orientation.
6 Measure the third series of ten measurements rotating the cell by 180 degrees after each
measurement.
118
Sample Handling and Measurement
Evaluation
1 Determine the wavelength of the absorbance maximum in the visible range and its
absorbance value.
2 Complete the following table by entering the absorbance values at the absorbance maximum
for the three series of measurements. Calculate mean , standard deviation and relative
standard deviation for each of the series of measurements and enter them in the
following table below.
Evaluation Table 3.1. Measured Absorbance Values and Calculation Results at the Absorbance Maximum
Measured Absorbance Values [AU] for Different Series of Measurements
Number of
Measurements
First Series
Cell Untouched Between
Measurements
Second Series
Cell Repositioned Between
Measurements
Third Series
Cell Rotated Between
Measurements
1
2
3
4
5
6
7
8
9
10
Mean
Standard deviation
% RSD
x Sx
%RSDx
119
Sample Handling and Measurement
The equations to calculate mean , standard deviation and relative standard deviation
are given below.
where: n = number of values
x = values
3 Do you observe any difference in the standard deviation from one series of measurements
to the next? If yes, explain why.
4 Based on these results, can you make any recommendation for cell handling to improve
accuracy and precision of measurements?
x S
%RSD
x
x
n
----------=
S
nx
2
x( )
2
n
n 1­( )
---------------------------------=
%RSD
S
x
---
 
  100=
120
Sample Handling and Measurement
Example Results & Discussion
Samples: quinoline yellow in distilled water (12mg/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 190­900 nm
absorbance range: 0.742­0.750 AU
1 The wavelength of the absorbance maximum in the visible range is at 414 nm. The
absorbance value for the maximum is approximately 0.75 AU. In our case the y-scaling of the
graphic display was set to an absorbance range from 0.742 to 0.750 AU before creating the
diagram below which shows the measured absorbance values at 414 nm.
Diagram: Measured Absorbance Values at 414 nm
Absorbance
[AU]
0 5 10 15 20 25
0.743
0.744
0.745
0.746
0.747
0.748
0.749
Absorbance in fixed cell
Absorbance after repositioning
Absorbance after rotation
Number of measurements
Measured Wavelength: = 414 nm
121
Sample Handling and Measurement
2 The following table shows the measured absorbance values and calculation results at the
absorbance maximum in the visible range ( = 414 nm).
3 The cell used to measure a sample is also part of a spectrophotometer's optical system.
Therefore, the position and geometry of the cell have an influence on the accuracy and
precision of absorbance measurements.
In the first series of measurements, we did not touch the cell and the precision of the
measurements should be best. The standard deviation is very low (% RSD should be equal
or less than 0.1 %). This demonstrates that UV-visible spectroscopy is a highly precise
measurement technique.
In the second series of measurements we removed the cell after each measurement. This
procedure demonstrates the influence of the position of the cell in the optical path,
determined by the cell holder. Using a well-designed cell holder, that locks the cell in
exactly the same position, the standard deviation of this second series of measurements
should be comparable to the one of the first series. A poorly designed cell holder, or an
unlocked cell will significantly worsen the standard deviation of the results.
In the third series of measurements we rotated the cell by 180 degrees after each
measurement. Ideally the two windows of a cell should be optically identical and absolutely
parallel. In practice, the absorbance of the windows can differ slightly and the two
windows can have nonflat or nonparallel surfaces. Therefore, the cell acts as an active
component which changes properties with the orientation. Rotating the cell between the
Results Table 3.1. Measurements
Measured Absorbance Values [AU] for Different Series of Measurements
Number of
Measurement
First Series
Cell Fixed Between
Measurements
Second Series
Cell Repositioned
Between Measurements
Third Series
Cell Rotated Between
Measurements
1 0.74614 0.74517 0.74618
2 0.7468 0.7458 0.74778
3 0.7459 0.7451 0.74583
4 0.74588 0.74492 0.74869
5 0.74618 0.74517 0.7456
6 0.74676 0.74486 0.74777
7 0.74611 0.74555 0.74579
8 0.74673 0.74586 0.74898
9 0.74645 0.74489 0.7465
10 0.74555 0.7452 0.74871
Mean: 0.74625 0.745252 0.747183
Standard
Deviation 0.000424 0.000364 0.001345
% RSD 0.056835 0.048853 0.180016
122
Sample Handling and Measurement
measurements demonstrates this effect and the standard deviation for the third series is
significantly worse compared to series one or two.
4 For best measurement practice, the cell should remain in the cell holder between the
measurements, or, if removed, the cell should always face the same direction in the cell
holder, for example, label to the light source. This ensures that the optical effects are
identical for both reference and sample measurements.
123
Sample Handling and Measurement
3.2. Cell Types
Introduction
The cell used to measure a sample is a part of the spectrophotometer's optical system.
Therefore, the position and geometry of the cell can have an influence on the accuracy and
precision of absorbance measurements.
The following experiment demonstrates the effect of cell quality on the accuracy and
reproducibility of the absorbance measurements.
Equipment and Reagents
t quinoline yellow (yellow dye)
t distilled water
t 1.0-l volumetric flask
t disposable glass pipettes (minimum 2)
t high quality matched quartz cell (cell a)
t quartz cell from a different set (badly treated cell, quartz of different quality or cell with path
length error, (cell b)
t disposable cell made of plastic (cell c)
Experiment
Time: about 30­45 min
1 Prepare a solution of about 12 mg quinoline yellow in 1.0 l distilled water.
2 Measure a reference on distilled water in cell a.
3 Measure the spectra of the quinoline yellow solution in the wavelength range from 190 to
500 nm using cells a, b and c.
124
Sample Handling and Measurement
Evaluation
1 Determine the wavelengths of the three absorbance maxima in the measured wavelength
range.
2 Complete the following table by entering the absorbance values at the three wavelengths of
the absorbance maxima.
3 Explain the differences between the three cells.
Evaluation Table 3.2. Absorbance Values at the Three Different Wavelengths of the Absorbance Maxima with the Different
Cells
Absorbance Values [AU] for the Different Cells
Wavelengths of Maximum
Absorbance
Cell a: Cell b: Cell c:
1 =
2 =
3 =
125
Sample Handling and Measurement
Example Results & Discussion
Sample: quinoline yellow in distilled water (12 mg/l)
Cells: high quality 10-mm path length quartz cell (cell a)
10-mm path length quartz cell with a path length error (cell b)
10-mm path length disposable plastic cell (cell c)
Instrument
Parameters: wavelength range: 190­500 nm
absorbance range: 0.0­1.0 AU
Spectra: Measured Spectra of the Sample in the Different Cells
1 The wavelengths of maximum absorbance can be found at 224, 289, and 414 nm.
2 The following table shows the absorbance values at the wavelengths of maximum
absorbance for the different cells.
Absorbance
[AU]
200 300 400 500
0.00
0.25
0.50
0.75
1 Quartz suprasil cell 1.00 cm
2 Quartz suprasil cell with path length error (1.01 cm)
3 Plastic cell
Wavelength [nm]
1
2
3
Results Table 3.2. Absorbance Values at the Three Different Wavelengths of the Absorbance Maxima
with the Different Cells
Absorbance Values [AU] for the Different Cells
Wavelengths of
Maximum Absorbance
Cell a: Cell b: Cell c:
1 = 224 nm 0.78387 0.79980 0.08590
2 = 289 nm 0.58291 0.59134 0.57814
3 = 414 nm 0.73820 0.74290 0.75149
126
Sample Handling and Measurement
3 Cell a is used as the reference cell to which the results of the other two cells are compared
to. For best results cell a is used for both reference and sample measurement.
Cell b is a cell of the same quality but with a slightly different path length. According to
Beer's law, the absorbance reading is proportional to the path length. Any difference/error
in the path length results in an error of the same magnitude of the absorbance reading.
Cell c is a disposable cell made of methylacryl. This material absorbs in the UV range and
acts like a cut-off filter. No measurements can be made in the UV range.
Note that for best results reference and sample measurements must be done using the same
cell.
127
Sample Handling and Measurement
3.3. Cleanliness
The cell, as part of the optical system of a spectrophotometer, needs the same care as all
other optical components.
The following experiment demonstrates the effect of cell cleanliness on the results.
Equipment and Reagents
t quinoline yellow
t distilled water
t chalk powder
t photographic lens tissue
t 1.0-l volumetric flask
t disposable glass pipettes (minimum 2)
t 10-mm path length quartz cell
Experiment
Time: about 30­45 min
1 Prepare a solution of about 12 mg quinoline yellow in 1.0 l distilled water.
2 Measure a reference on distilled water.
3 Measure the spectrum of the quinoline yellow solution in the wavelength range from 325 to
500 nm.
4 Remove the cell and touch the optical surfaces with your fingertips. Some fingerprints
should remain in the area where the light passes the cell. Measure the spectrum of the yellow
dye again using this dirty cell.
5 Remove the fingerprints by wiping the optical surface carefully with the photographic lens
tissue.
6 Contaminate the outer surface of the cell with some drops of sample solution and measure
the spectrum again.
7 Remove the liquid by wiping the optical surface carefully with the photographic lens tissue.
8 To examine the effect of dust or floating particles, add a small amount of chalk powder to
the sample in the cell. Shake well and measure the spectrum again.
128
Sample Handling and Measurement
Evaluation
1 Overlay all spectra in a graph.
2 Compare the results of the various measurements and explain the reasons for the
differences of the spectra.
129
Sample Handling and Measurement
Example Results & Discussion
Sample: quinoline yellow in distilled water (12 mg/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 325­500 nm
absorbance range: 0.0­1.0 AU
1 The following diagram shows the overlaid absorbance spectra of the different samples.
Diagram: Overlaid Spectra for the Differently Treated Samples
2 In all cases, the absorbance is higher than for the clean cell because of an additional
absorbing component (see experiment "Principle of Additivity" on page 77).
The fats in fingerprints are significant absorbers in the UV region and, if left on optical
surfaces, will cause erroneous results. Wipe off all fingerprints and contaminants carefully
before using a sample cell. Use only high quality lens tissues to avoid scratches on the
surface.
A wet outer surface of the cell can also contain absorbing components and in addition, act
as a `lens' and in this way influence the beam shape of the optics.
Finally, floating particles in the cell will deflect the light beam and lead to a background
absorbance. The effect of very fine particles is also known as light scattering.
Absorbance
[AU]
350 375 400 425 450 475 500
0.00
0.25
0.50
0.75
1.00
Wavelength [nm]
1 chalk in the sample
2 cell with fingerprints
3 cell wet outside
4 clean cell
1
2
3
4
130
Sample Handling and Measurement
3.4. Influence of Instrumental
Parameters
Introduction
The accuracy of results depends on the characteristics of the instrument used.
The following experiments cannot be performed with a diode array detector for two main
reasons:
* Some of the parameters that must be varied are not variable with this instrument, for
example, slit width and scan speed.
* Diode array detectors do not suffer from some of the effects that have to be
investigated, for example, variation of peak shape and peak position depending on the
scan speed.
The following experiment shows the influence of different scan speeds, different slit widths
and of varying damping factors on the accuracy of the results.
Equipment and Reagents
t benzene (a)
t conventional scanning spectrophotometer
with variable slit width and variable scan speed
t disposable glass pipette
t set of two 10-mm path length quartz cells
Experiment
Time: about 120­180 min
1 Take a drop of benzene with the glass pipette and place it on the bottom of the cell. Seal the
cell with a stopper. Evaporation in the cell will create enough vapor of benzene to carry out
the measurements.
2 Measure a reference on air (no cell in the cell holder).
3 Measure the spectra of benzene in the wavelength range from 220 to 280 nm, varying the slit
widths (0.1, 0.5, 1, 2 and 5 nm) with a constant scan speed (30 nm/min).
4 Repeat the speed measurements varying the scan speed (7.5, 30, 120 and 480 nm/min) with
a constant slit width (0.1 nm).
5 Examine the effect of the damping factor (damping factors 0 and 10). Choose a small slit
width (0.1 nm) and a low scan speed (15 nm/min) for your measurements.
(a)
131
Sample Handling and Measurement
Evaluation
1 Overlay the spectra with the identical scan speeds but different slit widths. Read the values
of the five significant benzene bands in the UV range and enter them in the table below.
2 Overlay the spectra with the identical slit widths but different scan speeds. Read the values
of the five significant benzene bands in the UV range and enter them in the table below.
Evaluation Table 3.3. Absorbance Values for the Different Slit Widths
Absorbance Values [AU] of the 5 Different Benzene Bands
Slit Width
[nm]
at band 1:
____nm
at band 2:
____nm
at band 3:
____nm
at band 4:
____nm
at band 5:
____nm
Evaluation Table 3.4. Absorbance Values for the Different Scan Speeds
Absorbance Values [AU] of the 5 Different Benzene Bands
Scan Speed
[nm/min]
at band 1:
___nm
at band 2:
___nm
at band 3:
___nm
at band 4:
___nm
at band 5:
___nm
132
Sample Handling and Measurement
3 Compare the absorbance values of the five significant benzene bands for the different
damping factors. Enter the values in the table below.
Evaluation Table 3.5. Absorbance Values for the Different Damping Factors
Wavelength of Benzene Band
[nm]
Absorbance Value [AU]
for a 0.1-nm Slit Width
15 nm/min Scan Speed
Damping Factor 0
Absorbance Value [AU]
for a 0.1-nm Slit Width
15 nm/min Scan Speed
Damping Factor 10
Band 1 ______nm
Band 1 ______nm
Band 3 ______nm
Band 4 ______nm
Band 5 ______nm
133
Sample Handling and Measurement
Example Results & Discussion
Sample: benzene vapor
Cell: set of two 10-mm path length quartz cells
Instrument
Parameters: wavelength range: 220­280 nm
absorbance range: 0.0­1.5 AU
scan speed: 7.5 nm/min to 480 nm/min
slit width: 0.1 nm to 5 nm
damping factors: 0 or 10
Benzene vapor has several sharp absorbance bands in the ultraviolet wavelength range.
The five significant bands in the wavelength range from 220 to 280 nm were used for this
experiment, i.e 236.8, 241.8, 274.4, 253.0 and 259.3 nm.
1 The following diagram shows the absorbance spectra for a constant scan speed of 30
nm/min and with varying slit widths.
Diagram: Absorbance Spectra with Varying Slit Widths
The diagram above indicates that the resolution increases with the reduction of the slit
width. Resolution is closely related to the instrumental spectral bandwidth (SBW). The
SBW is determined by the bandwidth of wavelengths passing the slits for a certain
wavelength setting. The smaller the slit width, the higher the resolution. However,
decreasing the slit width also decreases the energy throughput which results in a lower
signal-to noise-ratio (S/N-ratio).
Doubling the resolution requires an increase of the measurement time by a factor of 16 to
achieve the same S/N ratio.
Absorbance
[AU]
220 230 240 250 260 270 280
0.0
0.2
0.4
0.6
0.8
Scan speed 30 nm/min
1 slit width 0.1
2 slit width 0.5
3 slit width 1.0
4 slit width 2.0
5 slit width 5.0
Wavelength [nm]
1
2
3
4 5
134
Sample Handling and Measurement
The following table shows the absorbance values detected with the constant scan speed of
30 nm/min and with varying slit width.
2 The following diagram shows the absorbance spectra for a constant slit width of 0.1 nm with
varying scan speeds.
Diagram: Absorbance Spectra for Varying Scan Speeds
The diagram shows the loss of information when increasing the scan speed--the
absorbance values measured for the highest scan speeds are significantly lower than the
values measured with the slowest scan speed. The true absorbance of the bands cannot be
measured using high scan speed, because the peaks are cut off. This also creates distorted
peak shapes and incorrect peak positions.
Results Table 3.3. Absorbance Values for Varying Slit Widths
Absorbance Values [AU] of the 5 Different Benzene Bands
Slit Width
[nm]
at
236.8 nm
at
241.8 nm
at
247.4 nm
at
253.0 nm
at
259.3 nm
0.1 0.12085 0.29099 0.60012 0.89518 0.61497
0.5 0.07647 0.16698 0.26621 0.31353 0.23361
1 0.04923 0.09838 0.14884 0.16969 0.11923
2 0.03646 0.06785 0.09939 0.11305 0.0783
5 0.03926 0.06233 0.08291 0.08786 0.06154
Absorbance
[AU]
220 230 240 250 260 270 280
0.0
0.2
0.4
0.6
0.8
Slit width 0.1 nm
1 scan speed 7.5 nm/min
2 scan speed 30 nm/min
3 scan speed 120 nm/min
4 scan speed 480 nm/min1
2
3
4
Wavelength [nm]
135
Sample Handling and Measurement
The following table shows the absorbance values detected with the constant slit width of
0.1 nm and with varying scan speed.
3 The following diagram shows the absorbance spectra for varying damping factors.
Diagram: Absorbance Spectra for Varying Damping Factors
The diagram above indicates that an increase of the damping factor reduces the noise, but
again cuts off the peaks.
Results Table 3.4. Absorbance Values for Varying Scan Speeds
Absorbance Values [AU] of the 5 Different Benzene Bands
Scan Speed
[nm/min]
at
236.8 nm
at
241.8 nm
at
247.4 nm
at
253.0 nm
at
259.3 nm
7.5 0.12513 0.33284 0.67429 0.9468 0.69723
30 0.11484 0.26891 0.58976 0.7549 0.45915
120 0.10664 0.24791 0.44817 0.55828 0.37854
480 0.06203 0.11479 0.1765 0.1848 0.17182
Absorbance
[AU]
220 230 240 250 260 270 280
0.0
0.2
0.4
0.6 Slit width 0.1 nm
Scan speed 15 nm/min
1 damping 0
2 damping 10
Wavelength [nm]
1
2
136
Sample Handling and Measurement
The following table shows the measured absorbance values for varying damping factors.
Results Table 3.5. Absorbance Values for the Different Damping Factors
Wavelength of Benzene Band
[nm]
Absorbance Value [AU]
for a 0.1 nm Slit Width
15 nm/min Scan Speed
Damping Factor 0
Absorbance Value [AU]
for a 0.1 nm Slit Width
15 nm/min Scan Speed
Damping Factor 10
236.5 0.08286 0.03756
241.8 0.21309 0.08328
247.2 0.43881 0.13763
253.0 0.65523 0.15583
259.1 0.44824 0.11343
137
Sample Handling and Measurement
3.5. Properties of Solvents
Introduction
An ideal solvent dissolves a sample completely, it is easy to handle and completely
transparent at the wavelengths of interest. Besides water, all common solvents absorb
more or less in the UV range of the spectrum and no sample measurements can be
performed there.
The following experiment demonstrates the different cut-off wavelengths in the UV range
of some common solvents.
Equipment and Reagents
t ethanol (CH3CH2OH)
t methanol (CH3OH)
t 2-propanol (a)
t acetone (b)
t acetonitrile (CH3CN)
t hexane (CH3(CH2)4CH3)
t tetrahydrofurane (c)
t N,N'-dimethylformamide (d)
t distilled water
t disposable glass pipettes (minimum 9)
t 10-mm path length quartz cell
Experiment
Time: about 60­120 min
1 Measure a reference on air (no cell in the cell holder).
2 Measure transmittance spectrum of distilled water in the range from 190 to 500 nm.
3 Measure transmittance spectra of the solvents listed above.
CH
OH
CH3 CH3 (a) C
O
CH3 CH3 (b)
O
(c)
C
O
N(CH3)2
H
(d)
138
Sample Handling and Measurement
Evaluation
1 Overlay the spectra of the different solvents.
2 Determine the cut-off wavelengths (transmittance lower than 50 %) of the different solvents.
Enter the data in the table below.
3 What are the lowest wavelengths in the UV range that can be used with the different
solvents?
4 Discuss the advantages and the disadvantages of the different solvents.
5 Does the type of solvent affect the position and intensity of the absorbance bands of
molecules?
Evaluation Table 3.6. Cut-Off Wavelengths of the Different Solvents
Solvent Cut-Off Wavelength [nm]
Water
Ethanol
Methanol
2-Propanol
Acetone
Acetonitrile
Hexane
Tetrahydrofuran
N,N'-Dimethylformamide
139
Sample Handling and Measurement
Example Results & Discussion
Samples: ethanol, methanol, 2-propanol, acetone, acetonitrile, hexane,
tetrahydrofurane, N,N'-dimethylformamide, distilled water
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 190­500 nm
transmittance range: 0­100 %
1 The following diagram shows the overlaid transmittance spectra of the different solutions.
Diagram: Overlaid Transmittance Spectra of the Different Solutions
2 The following table shows the cut-off wavelengths of the different solvents.
Transmittance [%]
200 250 300 350 400 450 500
0
10
20
30
40
50
60
70
80
1 Water
2 Acetonitrile
3 N-hexane
4 Methanol
5 Ethanol
6 2-propanol
7 Tetrahydrofuran
8 Benzene
9 Dimethyleformamide
10 Acetone
Wavelength [nm]
90
1 2 3 4 5 6 7 8 109
Results Table 3.6. Cut-off Wavelengths of the Different Solvents
Solvent Cut-Off Wavelength [nm]
Water < 190
Ethanol 208
Methanol 217
2-Propanol 209
Acetone 332
Acetonitrile 191
N-Hexane 200
Tetrahydrofuran 245
N,N'-Dimethylformamide 272
140
Sample Handling and Measurement
3 Normally solvents can be used at wavelengths higher than their cut-off wavelength. Ideally,
a solvent should absorb as little light as possible in the measured range.
4 Distilled water is transparent in the whole UV-visible wavelength range. It dissolves many
polar compounds and is easy to handle. Water is not suitable for many nonpolar organic
compounds. For these compounds usually organic solvents with different polarity are used.
All of the organic solvents used in this experiment have a cut-off wavelength in the UV range.
Below this wavelength the absorbance is too strong for sample measurements. In addition
organic solvents are more difficult to handle. They can be flammable, toxic and involve
health hazards.
5 The solvents can modify the electronic environment of the absorbing chromophore. This
can cause a shift of the absorbance band. For more information, refer to "The Effect of
Solvents on UV-visible Spectra" on page 21.
141
Sample Handling and Measurement
3.6. Background Absorbance
Introduction
Ideally, the measured absorbance only depends on the target compound. In practice,
however, additional absorbencies which interfere with the measurement often occur for
chemical or physical reasons. The presence of any other compound that absorbs in the
same region as the target compound will result in an error in the absorbance measurement.
The following experiment demonstrates various methods of background correction and
their influence on the results.
Equipment and Reagents
t acetone (a)
t tetrahydrofurane (b)
t 50-ml volumetric flask
t 0.5-ml pipette or syringe
t disposable glass pipettes (minimum 2)
t 10-mm path length quartz cell
Experiment
Time: about 30­45 min
1 Prepare a solution of 0.5 ml acetone in 50 ml tetrahydrofurane.
2 Measure a reference on air.
3 Measure the spectrum of the acetone solution in the wavelength range from 225 to 375 nm.
4 Measure the spectrum of the pure tetrahydrofurane in the same wavelength range.
C
O
CH3 CH3 (a) O
(b)
142
Sample Handling and Measurement
Evaluation
1 Draw an overlay of the spectra of the acetone solution and of the pure tetrahydrofurane.
2 Determine the wavelength of the absorbance maximum of the acetone solution.
3 Use the table on the next page for the following evaluations:
Determine the absorbance value at max of the acetone solution without any background
correction.
a) Correct the absorbance value at max of the acetone solution for the background
absorbance at a single reference wavelength ref .
The reference wavelength ref should be selected according to the following two
criteria:
* ref should be close to the wavelength of interest (max ).
* The absorbance at ref should only depend on the background.
b) To correct a sloped baseline, the so-called three-point drop-line or Morton-Stubbs
correction can be used. This correction uses two reference wavelengths  ref 1 and
 ref 2, one on each side of the wavelength max . The absorbance due to background is
estimated by linear interpolation.
* Select two reference wavelength one at a shorter wavelength, for example, 240 nm
and one at a longer wavelength, for example, 325 nm as max applying the previous
rules.
* Estimate the background absorbance A interpolated at the wavelength max by a linear
interpolation between the absorbance values at  ref 1 and  ref 2 and subtract it from
the absorbance value at max of the sample solution (spectrum 1).
If the spectrum of the compound which causes the background absorbance is
available, it can be used for the correction.
143
Sample Handling and Measurement
c) Subtract the absorbance value at max of the pure tetrahydrofurane from the absorbance
value at max of acetone.
4 Which kind of baseline correction is the most accurate one?
Evaluation Table 3.7. Correcting Results with Different Correction Methods for Background Absorbance
Applied Background Correction Resulting Absorbance Values [AU]
No correction Absorbance at max :
Constant background single reference
wavelength) Absorbance at max:
ref = ________nm Absorbance at ref: Corrected
absorbance at max: =
Three-point drop-line or Morton-Stubbs
correction Absorbance at max :
ref 1 = ________nm
Interpolated background Absorbance at
max: ref
2 = ________nm Corrected absorbance at max: =
Subtraction of the underlying background
spectrum Absorbance at max:
Background absorbance at max : Corrected
absorbance at max: =
144
Sample Handling and Measurement
Example Results & Discussion
Sample: acetone in tetrahydrofurane (10 ml/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 225­375 nm
absorbance range: 0.0­1.5 AU
1 The following diagram shows the overlaid spectra for the different methods of background
correction.
Diagram: Overlaid Spectra for the Different Methods of Background Correction
Absorbance
[AU]
250 275 300 325 350 375
0.00
0.25
0.50
0.75
1.00
1.25
1 no baseline correction
2 baseline of pure solvent
3 subtraction of solvent baseline
4 subtraction of absorbance
at a single reference wavelength
5 Morton-Stubbs correction
(three point correction)
Wavelength [nm]
1
2
4
3
5
145
Sample Handling and Measurement
2 The wavelength of maximum absorbance is 269 nm.
3 The following table shows the results of the background correction calculations.
Background absorbance is an additional absorbance which is not caused by the compound
of interest, but by the matrix of the sample, an interfering impurity, or scattering. Different
techniques can be used to minimize the influence of the background absorbance on the
analytical result depending on its nature.
* Correcting a constant background absorbance.
A constant background absorbance over a large wavelength range can be eliminated
using a single reference wavelength.
* Correcting a background absorbance with a constant slope
A background absorbance with a constant (linear) slope over a large wavelength
range can be eliminated by a three-point drop line.
* Correcting a background absorbance by subtracting a spectrum.
Best results can be achieved by this correction, if the background absorbance can be
measured separately.
4 In this experiment, the subtraction of the background absorbance spectrum gives the most
accurate result. A non-constant background cannot be eliminated by a linear correction.
Note: Internal reference can also be used for a dual wavelength measurement to correct
the additional absorbance of a second interfering compound. In this case, the
absorbance of the second compound at the reference and the analytical wavelength
have to be identical.
Results Table 3.7. Correcting Results with Different Correction Methods for Background Absorbance
Applied Background Correction Resulting Absorbance Values [AU]
No correction Absorbance at max : 1.388
Constant background (single reference
wavelength) Absorbance at max: 1.388
ref = 335 nm Absorbance at ref: -0.039
Corrected absorbance at max: = 1.349
Three-point drop-line or Morton-Stubbs
correction Absorbance at max : 1.388
ref 1 = 240 nm Interpolated background absorbance at max: -0.783
ref 2 = 325 nm Corrected absorbance at max: = 0.605
Subtraction of the underlying background
spectrum Absorbance at max: 1.388
Background absorbance at max : - 0.335
Corrected absorbance at max: = 1.053
146
Sample Handling and Measurement
3.7. Sample Decomposition
Introduction
In some cases samples are very sensitive to oxygen, especially in the presence of impurities
acting as catalysts. Ascorbic acid is oxidized fast in the presence of even low
concentrations of Cu 2+
­ ions. These concentrations of Cu 2+
­ ions can already be found in
tap water.
The following experiment shows the ongoing oxidation of ascorbic acid in the presence of
oxygen with Cu2+
­-ions present in tap water as catalysts.
Equipment and Reagents
t ascorbic acid (a)
t tap water
t 100-ml volumetric flask
t disposable glass pipettes (minimum 2)
t thermostattable cell holder with stirring capabilities
t stop watch or time drive program included in the spectrophotometer software
t 10-mm path length quartz cell
Experiment
Time: about 90­120 min
1 Prepare a solution of about 8 mg ascorbic acid in 100 ml tap water.
2 Seal the flask immediately to avoid contact with air (oxygen).
3 Measure a reference spectrum on tap water.
4 Measure a spectrum of the ascorbic acid solution in the wavelength range from
190 to 400 nm using a cell with a stopper. If you use a conventional scanning
spectrophotometer, determine the wavelength of maximum absorbance of the solution and
continue your decomposition measurements at this single wavelength.
5 Open the cell and measure the absorbance at the wavelength of the maximum absorbance
every two minutes for 80 minutes. The cell has to be kept open during this time to allow
contact with air. Stir the solution in the cell to accelerate the oxidation process. Set the cell
temperature to 30 °C.
(a)
O O
OHHO
CHCH2OH
OH
147
Sample Handling and Measurement
Evaluation.
1 Draw a graph showing the change of the absorbance value at max versus time.
Diagram: Absorbance as a Function of Concentration
2 Guess the order of the reaction.
Absorbance [AU]
5010 20 30 40 60 70 80 90 100
Time [min]
0
0
0.75
1.5
148
Sample Handling and Measurement
Example Results & Discussion
Samples: ascorbic acid in tap water (82 mg/l)
Cell: 10-mm pass length quartz cell
Instrument
Parameters: wavelength range: 220­320 nm
absorbance range: 0.0­2.0 AU
acquisition time: every 2 minutes for 75 minutes
Sample temperature: 30 °C
stirrer speed: 300 RPM
The wavelength of maximum absorbance is 266 nm.
Diagram: Overlaid Spectra Indicating Sample Decomposition with Advancing Time
Absorbance
[AU]
220 230 240 250 260 270 280 290 300 310 320
0.00
0.25
0.50
0.75
1.00
1.25
Wavelength [nm]
t = 0 min
t = 80 min
149
Sample Handling and Measurement
1 The following diagram shows the change of the absorbance value at max versus time.
Diagram: Change of the Absorbance Value at max Versus Time
2 The absorbance shows a exponential decay over time. From this point of view the reaction
can be described by first order kinetics.
Absorbance
[AU]
0 10 20 30 40 50 60 70 80
0.00
0.25
0.50
0.75
1.00
1.25
Absorbance at = 266 nm
Time [min]
150
Sample Handling and Measurement
Part 4
151
8 Applications
152
Applications
4.1. Single Component Analysis
Introduction
The most common quantitative method in UV-visible spectroscopy is single component
analysis. Here we analyze an analgetics tablet containing acetyl salicylic acid as the only
active ingredient.
Reagents and Equipment
t acetyl salicylic acid
t an analgetics tablet containing acetyl salicylic acid as the only active ingredient
(for example, aspirinTM)
t distilled water
t 100-ml volumetric flask
t 500-ml volumetric flask
t disposable filter (size of pores < 1 m)
t disposable glass pipettes (minimum 3)
t 10-mm path length quartz cell
t magnetic stirrer
Experiment
Time: about 60­90 min
1 Prepare a standard solution:
* acetyl salicylic acid (about 13 mg in 100 ml distilled water).
Make sure that the acetyl salicyclic acid is completely dissolved. Use a magnetic
stirrer if necessary.
2 Determine the tablet weight.
3 Break the tablet into small pieces and prepare a sample solution:
* tablet material (about 50 mg in 500 ml distilled water)
4 Stir this solution for at least 30 min.
5 Use distilled water for the reference measurement in the wavelength range from
200 to 400 nm.
6 Measure the acetyl salicylic acid standard absorbance spectrum in a wavelength range from
200 to 400 nm.
7 Use the filtered sample solution to measure the sample spectrum in the range from
200 to 400 nm.
153
Applications
Evaluation
1 Determine the wavelength of the absorbance maximum of your standard solution.
2 Get the absorbance value Astd
max of your standard solution at the absorbance maximum.
3 Calculate the molar concentration cstd
of your acetyl salicylic acid standard solution. The
molecular weight of acetyl salicylic acid MW is 180.15 g / mol.
4 Determine the sample concentration using Beer's law:
where: = wavelength [nm]
= absorbance value [AU] at
= molar extinction coefficient [l/(mol cm)] at
= concentration [mol/l]
= path length [cm]
According to Beer's law we determine the molar extinction coefficient  using the
standard absorbance value Astd
max first.
max = Astd
max /(cstd
d)
Get the absorbance value Asmp
max of your sample solution at the absorbance maximum.
max = nm
Astdmax = AU
cstd = mol/l
max = l/(molcm)
Asmp
max = AU
A  c d=

A 
 
c
d
154
Applications
Based on the molar extinction coefficient max and the measured sample absorbance
Asmp
max we can calculate the sample concentration.
5 Determine the total amount of acetyl salicylic acid contained in your tablet.
Using the sample concentration result and the total volume of our sample
solution we get the number of molecules.
With the molecular weight of acetyl salicylic acid MW we can calculate the weight .
Based on the fraction of the tablet we used to prepare our sample solution and the
total tablet weight we can calculate the total amount of acetyl salicylic acid / tablet
.
6 Compare the determined value with the specified content on the packing note of your
tablets.
7 What can be improved in the single component analysis procedure?
csmp
= mol/l
c
smp
Amax
smp
maxd( )/=
nsmp
= mol
wsmp
= mg
mass = mg
c
smp
V
smp
n
smp
c
smp
V
smp
=
w
smp
w
smp
n
smp
MW=
m
smp
m
tab
m
ass
m
ass
w
smp
m
tab
m
smp
/=
Evaluation Table 4.1. Comparison of Labeled Content and Determined Content
Labeled Content Determined Content
mass = mg mass = mg
m
ass
155
Applications
Advanced Experiment
Time: about 60­90 min
1 Prepare five standard solutions:
* acetyl salicylic acid about 6 mg in 100 ml distilled water
* acetyl salicylic acid about 8 mg in 100 ml distilled water
* acetyl salicylic acid about 9 mg in 100 ml distilled water
* acetyl salicylic acid about 11 mg in 100 ml distilled water
* acetyl salicylic acid about 13 mg in 100 ml distilled water
2 Use distilled water for the reference measurement.
3 Measure all acetyl salicylic acid standards at 274 nm in absorbance.
Advanced Evaluation
1 Calculate the concentrations of your five standard solutions in mol/l based on your actual
acetyl salicylic acid weights.
2 Enter your absorbance measurement data in the table below.
c1 = mol/l
c2 = mol/l
c3 = mol/l
c4 = mol/l
c5 = mol/l
Evaluation Table 4.2. Measured Absorbance Values at 274 nm
Name Absorbance[AU] at 274 nm
Standard (c1)
Standard (c2)
Standard (c3)
Standard (c4)
Standard (c5)
156
Applications
3 Calculate a linear regression y = ax. The x,y pairs are the measured absorbance data (y) and
the corresponding concentration values (x). Enter the calculation results in the table below.
4 What does the slope value tell you?
5 How is the slope value related to the molar extinction coefficient  ?
6 What can the correlation coefficient be used for?
7 Calculate the total amount of acetyl salicyclic acid of your tablet using the statistically
improved molar extinction coefficient and evaluation data from the basic experiment.
Evaluation Table 4.3. Correlation Coefficient and Slope of Measured Curve
Name Value
Slope a
Correlation coefficient R
mass
= mg
157
Applications
Example Results and Discussion
Standard: acetyl salicylic acid (131 mg /l)
Sample: tablet material (116 mg/l)
Tablet: tablet weight (604.85 mg)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 250­310 nm
absorbance range: 0.0­0.8 AU
Diagram: Measured Absorbance Spectra of Acetyl Salicyclic Acid Standard and of
Sample Tablet
1 The wavelength of the absorbance maximum of the standard solution is:
2 The absorbance value of the standard solution is:
Absorbance
[AU]
250 260 270 280 290 300 310
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Wavelength [nm]
1
2
1 Standard
2 Sample tablet
max = 274 nm
Astd
max = 0.5988 AU
158
Applications
3 Based on the concentration of 131 mg/l the molar concentration can be calculated as:
4 The molar extinction coefficient of 274 nm can be calculated as:
The measured absorbance of the sample solution at 274 nm is:
Using the molar extinction coefficient at 274 nm, the molar concentration of the sample
solution is:
5 Using the volume of the sample solution (100 ml) the number of acetyl salicyclic acid
molecules can be found:
Using the molecular weight of acetyl salicyclic acid the amount of the substance can be
determined as:
The total amount of acetyl salicylic acid per tablet can be calculated as:
cstd
= 7.272 ˇ10-4
mol/l
max = 8.2343 ˇ102 l/(mol cm)
Asmp
max = 0.4368 AU
csmp = 5.305 ˇ 10-4 mol/l
nsmp
= 2.653 ˇ 10-4
mol
wsmp
= 47.79 mg
mass
= 498.3 mg
159
Applications
6 The table below shows a comparison of the labeled content with the analysis result:
7 The analysis result depends on the quality of the determination of the molar extinction
coefficient. Usually a series of standards is prepared to minimize the probability of errors.
These errors are due to the limited precision that can be achieved in standard preparation.
Weighing errors and volume errors affect the standard concentration accuracy. If these
errors are not systematic, a series of standards minimizes those errors and improves the
quality of the molar extinction coefficient.
Advanced Example Results and Discussion
Standards: acetyl salicylic acid (65.5 mg /l)
acetyl salicylic acid (81.75 mg /l)
acetyl salicylic acid (90.69 mg /l)
acetyl salicylic acid (109.17 mg /l)
acetyl salicylic acid (131.05 mg /l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength: 274 nm
absorbance range: 0.0­0.8 AU
1 Standard concentrations:
Results Table 4.1. Comparison of Labeled and Determined Content
Labeled Content Determined Content
mass
= 500 mg mass
= 498.3 mg
c1 = 3.636 10-4
mol/l
c2 = 4.538 10-4
mol/l
c3 = 5.034 10-4
mol/l
c4 = 6.060 10-4
mol/l
c5 = 7.272 10-4 mol/l
160
Applications
2 The following table shows the measured absorbance values.
3 The following table shows the linear regression calculation results.
4 The slope is the linear dependence of the measurement data as a function of the
concentration values of the standards. The graphical representation of this relationship is
called calibration curve and shown below.
Results Table 4.2. Measured Absorbance Values at 274 nm
Name Absorbance [AU] at 274 nm
Standard (c1) 0.2955
Standard (c2) 0.3723
Standard (c3) 0.4189
Standard (c4) 0.5075
Standard (c5) 0.5988
Results Table 4.3. Correlation Coefficient and Slope of Measured Curve
Name Value
Slope a 8.27 10-2
Correlation coefficient R 0.9984
Absorbance
[AU]
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
Concentration [10-4
mol/l]
161
Applications
5 Due to the regression formula used in the calculation, the slope is the reciprocal of the molar
extinction coefficient . This is true because we used a 10-mm path length cell. The
statistically improved molar extinction coefficient  is 8.271102
l/(mol cm). The unit of the
molar extinction is [l/(mol  cm)] because we used a 10-mm path length quartz cell.
6 The correlation coefficient is a measure of the quality of a calibration and tells you how good
a straight line fits all of your calibration points.
7 Using the statistically improved molar extinction coefficient the total amount of acetyl
salicyclic acid can be calculated as:
mass
= 495.7 mg
162
Applications
4.2. Multicomponent Analysis
Introduction
Mixtures can be analyzed using UV-visible spectroscopic data. The method is based on the
assumption that Beer's law is obeyed by all components of the mixture and all possible
components forming the mixture are known. This method is called multicomponent
analysis (MCA).
As an example methyl orange solutions at different values of pH are evaluated. Due to the
spectra shown in the experiment "Influence of pH -- Buffered Methyl Orange Solutions" on
page 64 where the isosbestic points are in the pH dependent spectra, the assumption is
made that an equilibrium of two different molecular forms exists. Therefore the two
solutions with the highest pH (pH = 5.2) and lowest pH (pH = 2.2) are used as standard
solutions of two hypothetical pure forms A and B.
The solutions with pH 3.0, 3.8, and 4.6 are considered to be mixtures of these two
hypothetical components A and B.
Reagents and Equipment
t methyl orange
t disodium hydrogen ortho-phosphate (a)
t citric acid (b)
t distilled water
t five 25-ml volumetric flasks
t 100-ml volumetric flask
t 200-ml volumetric flask
t 500-ml volumetric flask
t five 50-ml beakers
t 0.5-ml pipette
t 1-ml pipette
t 5-ml pipette
t 10-ml pipette
t 25-ml pipette
t disposable glass pipettes (minimum 5)
t 10-mm path length quartz cell
t magnetic stirrer
t calculator with matrix operation or spreadsheet program with matrix operation capabilities
P
Na
O O
O
H
Na
O
(a) COOHH2C
COOHHOC
COOHH2C (b)
163
Applications
Experiment
Time: about 4 h
1 Prepare a stock solution of 0.05 g methyl orange in 100 ml distilled water.
2 Prepare the following solutions:
a) a 0.2 M disodium hydrogen orthophosphate solution:
Dissolve 5.68 g Na2HPO4 in 200 ml distilled water. If necessary, use a magnetic stirrer for
complete dissolution.
b) a 0.1 M citric acid solution:
Dissolve 9.61 g citric acid in 500 ml distilled water.
c) The five McIlvaine's buffer solutions are prepared by mixing aliquots of the citric acid
solution and the disodium hydrogen orthophosphate solution. Mix the buffer solutions
shown in the table below.
3 Divide each mixture into two equal portions of 25 ml.
4 Add 0.5 ml of methyl orange stock solution to one of the portions of each of the buffer
solutions. These are the sample solutions to be measured. The other portions of buffer have
to be used for the reference measurements.
5 The following two steps have to be repeated for each corresponding pair of the buffer and
the sample (same pH values):
* Measure a reference on the pure buffer.
* Measure the spectrum of the corresponding sample solution in the wavelength range
from 250 to 650 nm.
Approximate pH Volume [ml] of Na2HPO4 Volume [ml] of Citric Acid
2.2 1.0 49.0
3.0 10.0 40.0
3.8 18.0 32.0
4.6 23.0 27.0
5.2 27.0 23.0
164
Applications
Evaluation
Experiment "Influence of pH -- Buffered Methyl Orange Solutions" on page 64 showed that
the assumed two forms of methyl orange do have different spectra. This measurable
spectral difference is one of the requirements for UV-visible multicomponent analysis.
In addition we assume that Beer's law is obeyed by the protonated form A (component A)
and the deprotonated form B (component B).
Then the absorbance at a given wavelength is the sum of the absorbance of component A
and the absorbance of component B. In contrast to the single component analysis of the
previous experiment, we cannot calculate the two concentrations by solving a single
equation. We need at least as many equations as components.
The measured absorbance of the sample at the wavelength is:
where: m = number of components
n = number of wavelengths
p = number of samples
d = path length [cm]
ij= extinction coefficient of the ith component of the jth
wavelength [mol/l]
cik= concentration of the component of the ith component of
the kth standard [mol/l]
Such a set of equations can be written in matrix notation:
where:
A = the matrix of sample data
 = the calibration coefficients matrix
C = the sample concentration modules matrix
To simplify the equation above we have already assumed concentration modules. A
concentration module is a concentration value multiplied by its path length.
The above equation can be solved for the calibration matrix:
In the calibration of an MCA analysis these coefficients are determined by measuring the
standards and calculating the coefficients.
A =   C
 = A  CT(CCT)-1
Ajk k
th
j
th
Ajk d cik ij
i 1=
m

 
 
 
 
= i 1 ... m, ,= j 1 ... n, ,= k 1 ... p, ,=
165
Applications
1 Create the concentration matrix C assuming a concentration of 1 for component A at a pH
of 2.2 and component B at a pH of 5.2.
2 Multiply the concentration matrix by its transposed form:
3 Invert the concentration matrix product :
Evaluation Table 4.4. Concentration Matrix C
Component A Component B
Standard (pH 2.2)
Standard (pH 5.2)
C  CT
Evaluation Table 4.5. Matrix Product
i = 1 i = m
i = 1
i = m
(C  CT
)-1
Evaluation Table 4.6. Inverted Matrix
i = 1 i = m
i = 1
i = m
C C
T

166
Applications
4 Multiply the transposed concentration matrix by the inverted the concentration matrix
product :
5 Set up your absorbance calibration data matrix A:
CT
(C  CT
)-1
Evaluation Table 4.7. Matrix Product
i = 1 i = m
i = 1
i = m
C C
T
( )
1Evaluation
Table 4.8. Calibration Data Matrix A
Component A (pH 2.2) Component B(pH 5.2)
430 nm
440 nm
450 nm
460 nm
470 nm
480 nm
490 nm
500 nm
510 nm
520 nm
167
Applications
6 Multiply your calibration data matrix A by the matrix product mentioned before.
Based on the above calibration, concentration results can be calculated for measured
sample data.
To be able to calculate the MCA results we go back to the basic equation:
Now we have to solve this equation for the unknown concentration matrix C:
The matrix A is now the sample data matrix.
In preparation for the calculation of the concentration results the following steps have to
be performed in advance:
 = A  CT
(C  CT
)-1
Evaluation Table 4.9. Calibration Coefficient Matrix
Component A Component B
430 nm
440 nm
450 nm
460 nm
470 nm
480 nm
490 nm
500 nm
510 nm
520 nm
 =   C
C = (T  )-1  T 
168
Applications
7 Transpose the calibration coefficient matrix :
8 Multiply the transposed calibration coefficient matrix by the calibration coefficient
matrix :
9 Invert the calibration coefficient matrix product :

Evaluation Table 4.10. Transposed Calibration Coefficient Matrix
430 nm 440 nm 450 nm 460 nm 470 nm
Component A
Component B
480 nm 490 nm 500 nm 510 nm 520 nm
Component A
Component B

 
Evaluation Table 4.11. Matrix Product
1 2
1
2
(
 )-1

T


T

Evaluation Table 4.12. Inverted Matrix
1 2
1
2
169
Applications
10 Multiply the inverted the calibration coefficient matrix product by the transposed
calibration coefficient matrix 
:
Now we are able to analyze our sample data. We perform the final steps:
11 Set up your sample absorbance data matrices A:
(
 )-1
 
Evaluation Table 4.13. Result Matrix
430 nm 440 nm 450 nm 460 nm 470 nm
Component A
Component B
480 nm 490 nm 500 nm 510 nm 520 nm
Component A
Component B
Evaluation Table 4.14.
Absorbance [AU] of Sample (pH 3.0)
430 nm
440 nm
450 nm
460 nm
470 nm
480 nm
490 nm
500 nm
510 nm
520 nm
170
Applications
12 Multiply the result matrix (Evaluation Table 4.13) by your sample measurement data
matrices A (Evaluation Tables 4.14­4.15):
Evaluation Table 4.15. Measured Absorbance Values at a pH of 3.8
Absorbance [AU] of Sample (pH 3.8)
430 nm
440 nm
450 nm
460 nm
470 nm
480 nm
490 nm
500 nm
510 nm
520 nm
Evaluation Table 4.16. Measured Absorbance Values at a pH of 4.6
Absorbance [AU] of Sample (pH 4.6)
430 nm
440 nm
450 nm
460 nm
470 nm
480 nm
490 nm
500 nm
510 nm
520 nm
C = (
 )-1
 
 
171
Applications
13 How many data points per standard/sample measurement are required in the above
example?
14 What is the advantage of more than the minimum required wavelength wavelengths?
15 How many standards have to be used as a minimum in the example for calibration?
16 What is the advantage of using more than the minimum number of standards?
17 What is the major difference to single component analysis?
Evaluation Table 4.17. Calculated Concentrations at a pH of 3.0
Sample (pH = 3.0)
Component A
Component B
Evaluation Table 4.18. Calculated Concentrations at a pH of 3.8
Sample (pH = 3.8)
Component A
Component B
Evaluation Table 4.19. Calculated Concentrations at a pH of 4.6
Sample (pH = 4.6)
Component A
Component B
172
Applications
Example Results and Discussion
Samples: stock solution: methyl orange in distilled water (0.5 g/l)
buffered solution at pH values of 2.2, 3.0, 3.8, 4.6, 5.2 preparation
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 250­650 nm
absorbance range: 0.0­1.5 AU
Diagram: Pure Components A and B and Mixture of A and B (Buffered Solution of
Methyl Orange, pH 3.8)
Calculations
1 Concentration matrix C:
Absorbance
[AU]
250 300 350 400 450 500 550 600 650
0.00
0.25
0.50
0.75
1.00
1.25 1 Component A
2 Component B
3 Mixture of A and B
Wavelength [nm]
1
2
3
Results Table 4.4. Concentration Matrix C
Component A Component B
1 0 Standard (pH 2.2)
0 1 Standard (pH 5.2)
173
Applications
2 Concentration matrix product:
3 Inverted concentration matrix product:
4 Matrix product:
5 Calibration data matrix A:
C  CT
Results Table 4.5. Matrix Product
i = 1 i = m
0 1 i = 1
1 0 i = m
(C  CT
)-1
Results Table 4.6. Inverted Matrix
i = 1 i = m
1 0 i = 1
0 1 i = m
CT
(C  CT
)-1
Results Table 4.7. Matrix Product
i = 1 i = m
0 1 i = 1
1 0 i = m
Results Table 4.8. Calibration Data Matrix A
Compound A Compound B
0.186481 0.608564 430 nm
0.283442 0.663574 440 nm
0.412372 0.713896 450 nm
0.575434 0.748984 460 nm
0.766198 0.752545 470 nm
0.954226 0.714345 480 nm
1.126214 0.635997 490 nm
1.269060 0.526056 500 nm
1.302705 0.400055 510 nm
1.241117 0.280303 520 nm
174
Applications
6 Calibration coefficient matrix:
7 Transposed calibration coefficient matrix:
8 Calibration coefficient matrix product:
 = A  CT
(C  CT
)-1
Results Table 4.9. Calibration Coefficient Matrix
Component A Component B
0.186481 0.608564 430 nm
0.283442 0.663574 440 nm
0.412372 0.713896 450 nm
0.575434 0.748984 460 nm
0.766198 0.752545 470 nm
0.954226 0.714345 480 nm
1.126214 0.635997 490 nm
1.269060 0.526056 500 nm
1.302705 0.400055 510 nm
1.241117 0.280303 520 nm
T
Results Table 4.10. Transposed Calibration Coefficient Matrix
430 nm 440 nm 450 nm 460 nm 470 nm
0.186481 0.283442 0.412372 0.575434 0.766198 Component A
0.608564 0.663574 0.713896 0.748984 0.752545 Component B
480 nm 490 nm 500 nm 510 nm 520 nm
0.954226 1.126214 1.26906 1.302705 1.241117 Component A
0.714345 0.635997 0.526056 0.400055 0.280303 Component B
T
 
Results Table 4.11. Matrix Product
1 2
8.230179 4.538105 1
4.538105 3.877759 2
175
Applications
9 Inverted calibration coefficient matrix product:
10 Matrix product:
11 Sample absorbance data matrices A:
(T
 )-1
Results Table 4.12. Inverted Matrix
1 2
0.342551 -0.40088 1
-0.40088 0.727031 2
(T
 )-1
 T
Results Table 4.13. Result Matrix
430 nm 440 nm 450 nm 460 nm 470 nm
-0.18008 -0.16892 -0.14493 -0.10314 -0.03922 Component A
0.367688 0.368812 0.353712 0.313853 0.239968 Component B
480 nm 490 nm 500 nm 510 nm 520 nm
0.040501 0.130824 0.22383 0.285867 0.312776 Component A
0.136818 0.010909 -0.12629 -0.23138 -0.29375 Component B
Results Table 4.14. Measured Absorbance Values at a pH of 3.0
Absorbance [AU] of Sample (pH 3.0)
0.28029 430 nm
0.36811 440 nm
0.47692 450 nm
0.61471 460 nm
0.76364 470 nm
0.90020 480 nm
1.01720 490 nm
1.10440 500 nm
1.10220 510 nm
1.02930 520 nm
176
Applications
Results Table 4.15. Measured Absorbance Values at a pH of 3.8
Absorbance [AU] of Sample (pH 3.8)
0.42128 430 nm
0.49539 440 nm
0.58063 450 nm
0.67284 460 nm
0.75884 470 nm
0.81955 480 nm
0.85307 490 nm
0.85608 500 nm
0.80123 510 nm
0.70799 520 nm
Results Table 4.16. Measured Absorbance Values at a pH of 4.6
Absorbance [AU] of Sample (pH 4.6)
0.56573 430 nm
0.62513 440 nm
0.68320 450 nm
0.73143 460 nm
0.75347 470 nm
0.73828 480 nm
0.68539 490 nm
0.60180 500 nm
0.49263 510 nm
0.37885 520 nm
177
Applications
12 Concentration result matrices:
13 For a two component systems at least two data points are required. In our example above
we used 10 data points. The system is redundant. Therefore the least squares algorithm was
applied.
14 The advantage of a redundant system is that the precision of the result is improved. Under
the assumption that the noise of the measurement data is normally distributed, redundancy
in data points minimizes the standard deviation of the result.
15 As with the data points at least two standards must be used in calibration. In this example
we used two "pure component" standards. Also two mixtures of the components A and B
could have been used for calibration.
16 If more than the minimum number of standards is used in calibration, the precision of the
calibration coefficient matrix can be improved. Statistical errors due to standard
preparation like volumetric errors and weighting errors are minimized.
17 Instead of a single linear equation a set of linear equations has to be evaluated. According to
this set of equations multiple data points and multiple standards have to be used. In addition
the components of the mixture must have different extinction coefficients at the data points
used in calibration and analysis.
C = (T
 )-1
   
Results Table 4.17. Calculated Concentrations at a pH of 3.0
Sample (pH = 3.0)
0.778232 Component A
0.222047 Component B
Results Table 4.18. Calculated Concentrations at a pH of 3.8
Sample (pH = 3.8)
0.44404 Component A
0.55621 Component B
Results Table 4.19. Calculated Concentrations at a pH of 4.6
Sample (pH = 4.6)
0.102103 Component A
0.897806 Component B
178
Applications
4.3. Derivative Spectroscopy
Introduction
The number of aromatic amino acids in proteins can be determined by a method using 2nd
order derivative spectroscopy.
To apply this method developed by Levine and Federici, only the concentration and the
molecular weight of the protein must be known.
In this experiment we analyze Angiotensin III.
Reagents and Equipment
t N-acetyl-tryptophan-ethylester (MW = 274 g/mol)
t N-acetyl-tyrosine-ethylester (MW = 251 g/mol)
t N-acetyl-phenylalanin-ethylester (MW = 235.2 g/mol)
t guanidine hydrochloride
t angiotensin III (MW = 931.1 g/mol)
t 0.02 M phosphate buffer at a pH of 6.5
t distilled water
t 100-ml volumetric flask
t four 10-ml volumetric flasks
t disposable glass Pasteur pipettes (minimum 5)
t 10-mm path length quartz cell with a stopper
Experiment
Time: about 120­150 min
1 Prepare 100 ml stock solution of 0.02 M phosphate buffer at a pH of 6.5 with 6 mol guanidine
hydrochloride dissolved in the buffer.
2 Prepare 10 ml of an N-acetyl-tyrosine-ethylester (Tyr) standard solution with a
concentration of about 1  10-3
mol/l in the stock solution.
3 Prepare 10 ml of an N-acetyl-tryptophan-ethylester (Trp) standard solution with a
concentration of about 0.3  10-3
mol/l in the stock solution.
4 Prepare 10 ml of an N-acetyl-phenylalanin-ethylester (Phe) standard solution with a
concentration of about 10  10-3
mol/l in the stock solution.
5 Prepare 10 ml of an Angiotensin III sample solution with a concentration of about 110-4
mol/l in the stock solution.
179
Applications
6 Use the stock solution for the reference measurement.
7 Measure all standards (Tyr, Trp, Phe) using the 2nd
order derivative of absorbance in a
wavelength range from 220 to 400 nm.
8 Measure the sample (Angiotensin III) using the 2nd
order derivative of absorbance in a
wavelength range from 220 to 400 nm.
180
Applications
Evaluation
1 Use the 2nd
order derivative data of your standards for the analysis of the three aromatic
amino acids: Tyr, Trp, and Phe. Use a wavelength range from 240 to 300 nm for the
calibration. The algorithm for the calibration is the same as explained in the experiment
"Multicomponent Analysis" on page 162
Analyze the Angiotensin III sample 2nd
order derivative data using the above calibration:
2 Calculate the aromatic amino acid units in the Angiotensin III protein. The amino acid units
per protein molecule are calculated:
where: =aromatic acid units [units]
=result amino acid concentration [mol/l]
=protein concentration [mol/l]
Enter your calculation results in the table below.
3 Why can the unit content of aromatic amino acids be calculated using the above method?
4 Why is derivative spectroscopy used for the MCA?
Evaluation Table 4.20. 2nd Order Derivative Data
Tyr [10-3
mol/l] Trp [10-3
mol/l] Phe [10-3
mol/l]
Angiotensin III
XProt cAcd cProt/=
XProt
cAcd
cProt
Evaluation Table 4.21. Amino Acid Units
Tyr [units] Trp [units] Phe [units]
Angiotensin III
181
Applications
Example Results and Discussion
Standards: N-acetyl-tyrosine-ethylester (0.93  10-3 mol/l)
N-acetyl-tryptophan-ethylester (0.27  10-3 mol/l)
N-acetyl-phenylalanin-ethylester (10  10-3 mol/l)
Sample: Angiotensin III (9.5  10-5 mol/l)
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength range: 220­400 nm
absorbance range: 0.1­0.6 AU
Diagram: Measured Spectra of the Standards
Absorbance
[AU]
Wavelength [nm]
220 240 260 280 300 320 340 360
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04 Phe
Tyr
Trp
182
Applications
Diagram: Measured Spectrum of the Sample
1 The analysis results using 2nd order derivative data are shown in the table below.
2 The units of amino acids per protein molecule can be integers only. The calculated units in
the Angiotensin III protein are given in the table below. The amino acid sequence of
Angiotensin III is: Arg-Val-Tyr-Ile-His-Pro-Phe.
3 The absorbance spectrum of the denatured protein can be approximated by the sum of the
absorbance spectra of the constituent amino acids. Due to the buffer used for denaturation,
only the spectra of the aromatic amino acids can be seen. They absorb in a range from 250
to 300 nm. These aromatic systems in the protein seemed to be not very much affected by
the amino acid sequence. Therefore the aromatic amino acids can be treated as individual
units in the calculation.
4 Derivative spectroscopy is applied to eliminate the scattering background due to light
scattering of higher molecular weight proteins.
References
R. L. Levine, A. M. Federici, "Biochemistry 1982, 21", page 2600ff
Absorbance
[AU]
Wavelength [nm]
220 240 260 280 300 320 340 360
-0.002
0
0.002
0.004
0.006
0.008
Results Table 4.20. 2nd Order Derivative Data
Tyr [10-3
mol/l] Trp [10-3
mol/l] Phe [10-3
mol/l]
Angiotensin III 0.081 0.002 0.092
Results Table 4.21. Amino Acid Units
Tyr [units] Trp [units] Phe [units]
Angiotensin III 1 0 1
183
Applications
4.4. Kinetic Analysis - Studying
Melting Temperatures of DNA
Introduction
The denaturation and renaturation of deoxyribonucleic acid (DNA) can be detected by
UV-visible spectroscopy. We measure the denaturation profile and determine the melting
temperature. In addition we calculate the percentage of the guanine/cytosine base pairs
(% GC).
Reagents and Equipment
t highly polymerized calf thymus DNA (MW  8.6 MDa)
t citrate buffer at a pH of 7.0
* 1.5  10-2 M sodium chloride
* 1.5  10-3 M sodium citrate
t 0.5-ml pipette with adjustable volume
t 3-ml pipette with adjustable volume
t 10-mm path length quartz cell with a stopper
Experiment
Time: about 120 - 240 min
1 Fill the quartz cell with 2.7 ml of the citrate buffer.
2 Set the temperature of your cell holder to 40 °C.
3 Place the cell in the cell holder and allow to equilibrate to the temperature set.
4 Prepare a DNA stock solution of about 0.35 mg/ml DNA in the citrate buffer.
5 Measure the reference on the stock solution in the cell.
6 Add 0.3 ml of the stock DNA solution to your cell, gently stir the solution.
7 Start your measurements with the current temperature of 40 °C. Then increase the
temperature in 5 degree steps up to 90 °C. For each temperature do the following:
* Allow to equilibrate to the temperature set.
* Measure the actual temperature of the DNA solution in the cell.
* Measure the absorbance value at 260 nm.
184
Applications
Evaluation
1 Enter your measured values in the table below.
Evaluation Table 4.22. Absorbance Measurements at 260 nm for Different Temperatures
Set Temperature [°C] Sample Temperature [°C] Absorbance at 260 nm [AU]
40
45
50
55
60
65
70
75
80
85
90
185
Applications
2 Enter your measured absorbance values at 260 nm versus the sample temperature in the
following diagram.
Diagram: Absorbance at 260 nm as a Function of Temperature
3 Determine the melting temperature Tm in the above diagram using the inflection point of the
melting interval.
Absorbance [AU]
2
1.5
0.5
1
40 50 70 80 90
Temperature [0 C]
2.5
60
Tm = °C
186
Applications
4 Calculate the percentage of guanine/cytosine base pairs (% GC) using Tm and the salt
molarity M of the solvent using the equation below:
where: =guanine/cytosine content [%]
= melting temperature [°C]
= salt molarity of the solution [mol/l]
Salt molarity:
Logarithm log [M]:
Result % GC:
M = mol/l
Log(M) =
% GC = %
% GC 2.44 Tm 81.5 16.6 M( )log­­( )=
%GC
Tm
M
187
Applications
Example Results and Discussion
Sample Stock:
DNA in citrate buffer pH = 7 (0.195 mg/ml)
Sample: 1:10 dilution of sample stock solution
Cell: 10-mm path length quartz cell
Instrument
Parameters: wavelength: 260 nm
absorbance range: 0.0 - 1.0 AU
1 The following table shows the measured values.
Results Table 4.22. Absorbance Values at 260 nm for Different Temperatures
Set Temperature [°C] Sample Temperature [°C] Absorbance at 260 nm [AU]
40 38.9 0.477
45 43.6 0.477
50 49.0 0.478
55 54.0 0.491
60 58.9 0.555
65 63.8 0.608
70 68.6 0.632
75 73.7 0.643
80 78.3 0.647
85 83.4 0.650
90 88.0 0.652
188
Applications
2 The following diagram shows the measured absorbance versus temperature.
Diagram: Absorbance at 260 nm as a Function of Temperature
3 The melting temperature can be determined as:
4 The percentage of guanine/cytosine base pairs (% GC) can be calculated as:
0.3
0.4
0.5
0.6
0.7
40 50 60 70 80 90
Temperature [0 C]
Absorbance
[AU]
Tm = 60 °C
M = 1.6510-2
mol/l
Log(M) =-1.7825
%GC = 19.7 %
189
Applications
4.5. Isosbestic Points
Introduction
UV-visible spectroscopy is an ideal tool to study the kinetics of a chemical reaction. The
measurement is fast and the acquired data can be used to understand the molecular
mechanism as well as for quantitative calculation of reaction constants.
As an example here we use the hydrolysis of p-nitrophenolphenyl acetate. In principle it is a
second order reaction, but with water in large excess it can be expected to show pseudo
first-order kinetics. The reaction rate constant is sensitive to pH and temperature.
Reagents and Equipment
t p-nitrophenyl acetate
t dry acetonitrile
t 0.1 molar phosphate buffer at pH 8.5
t 3-ml pipette
t 50-l syringe or pipette with adjustable volume
t 10-mm path length quartz cell with stopper
t thermostattable cell holder
Experiment
Time: about 50­60 min
1 Prepare a stock solution of about 0.5 mg p-nitrophenyl acetate in about 2 ml dry acetonitrile.
2 Fill the quartz cell with 3 ml of the phosphate buffer.
3 Set the temperature of your cell holder to 60°C.
4 Place the cell in the cell holder and allow to equilibrate to the temperature set.
5 Measure the reference on your prefilled cell.
6 Prepare your system to acquire absorbance spectra every 2 minutes for 30 minutes using a
wavelength range from 214 to 550 nm.
O C CH3
O
NO2
+ H2O
, pH
OH
NO2
+ HO C CH3
O
190
Applications
7 Add 30 l of the p-nitrophenyl acetate stock solution to the cell, quickly stir or shake the
solution and immediately start the data acquisition. Do not shake or stir too strong to avoid
bubble formation.
Evaluation
1 Determine the isosbestic points of the measured absorbance spectra:
Evaluation Table 4.23. Wavelengths of Isosbestic Points
Name Wavelength [nm]
Isosbestic Point 1
Isosbestic Point 2
Isosbestic Point 3
Isosbestic Point 4
Isosbestic Point 5
191
Applications
2 Enter your measured absorbance values at 400 nm versus reaction time in the diagram
below.
Diagram: Absorbance at 400 nm as a Function of Time
3 What type of time dependence does the above diagram show?
Absorbance [AU]
2
1.5
0.5
1
400 800 1200 1600 2000
Time [s]
0
2.5
192
Applications
4 Calculate the rate constant k. Assuming a pseudo first-order reaction the following
equations can be used:
where: =absorbance at time t
= initial absorbance at time 0
= absorbance at infinite time
= first order rate constant
= reaction time
For the evaluation the above equation can be expressed as:
Calculate a linear regression with offset for the above equation. Use the calculation formula
given in the appendix for . Here the x,y pairs are the reaction time and the
logarithms of the absorbance differences .
Determine your A: value at 400 nm using the data value at 1800 seconds:
Simplification:
(t) =  + (0 - )e-kt
ln(t- ) = kt + ln (0 - )
400,  = 400, 1800
A400,  = [AU]
A t( )
A0
A
k
t
y ax b+= x( )
At A­( ) y( )ln
193
Applications
Enter the values for linear regression calculation in the table below.
Calculate the linear regression and enter the results in the table below.
Enter the pseudo first order rate constant:
Evaluation Table 4.24. Linear Regression Calculations
Reaction time t [s] A400, [AU] A400,-A400, [AU] ln(|A400,-A400,|)
0
120
240
360
480
600
720
840
960
1080
1200
1320
1440
1560
1680
1800
Evaluation Table 4.25. Calculation Results
Name 400 nm, T = 60 °C
Slope a
Intercept b
Correlation coefficient R
k = s-1
194
Applications
5 How does the rate constant depend on the wavelength used for rate constant evaluation?
6 What is the result if one of the isosbestic points' wavelengths is used for evaluation?
7 At which wavelength can the most precise rates constant results be expected?
Advanced Experiment
Time: 90­120 min
1 Repeat the above experiment
* using a set temperature of 50 °C
* using a set temperature of 55 °C
2 Measure spectra every 2 minutes for half an hour.
Advanced Evaluation
1 Isosbestic points of the measured absorbance spectra:
Evaluation Table 4.26. Wavelength Measurements of Isosbestic Points at 50 °C
Name Wavelength [nm]
Isosbestic Point 1
Isosbestic Point 2
Isosbestic Point 3
Isosbestic Point 4
Isosbestic Point 5
Evaluation Table 4.27. Wavelength Measurements of Isosbestic Points at 55 °C
Name Wavelength [nm]
Isosbestic Point 1
Isosbestic Point 2
Isosbestic Point 3
Isosbestic Point 4
Isosbestic Point 5
195
Applications
2 Enter your measured absorbance values at 400 nm at 50 °C and 55 °C versus reaction time
in the following diagrams.
Diagram: Absorbance at 400 nm and 50 °C as a Function of Time
Absorbance [AU]
2
1.5
0.5
1
1000200 400 600 800 1200 1400 1600 1800 2000
Time [s]
0
2.5
196
Applications
Diagram: Absorbance at 400 nm and 55 °C as a Function of Time
3 Calculate the rate constants k according to the basic kinetics experiment equations at 50°C
and 55°C. Due to the much slower speed, the absorbance value A1800 is no longer a good
approximation for the A value A. Try to extrapolate the value out of the time trace diagram.
If not possible, use the value at 60°C as an approximation.
Absorbance [AU]
2
1.5
0.5
1
1000200 400 600 800 1200 1400 1600 1800 2000
Time [s]
0
2.5
A400,(50 °C) = AU A400,(55 °C) = AU
197
Applications
Enter the values for linear regression calculation in the tables below.
Evaluation Table 4.28. Linear Regression Calculations at 50 °C
Reaction time t [s] A400, [AU] A400, - A400, [AU] ln(lA400, - A400,l)
0
120
240
360
480
600
720
840
960
1080
1200
1320
1440
1560
1680
1800
198
Applications
4 Calculate the linear regressions and enter the results in the table below:
Evaluation Table 4.29. Linear Regression Calculations at 55 °C
Reaction time t [s] A400, [AU] A400, - A400, [AU] ln(lA400, - A400,l)
0
120
240
360
480
600
720
840
960
1080
1200
1320
1440
1560
1680
1800
199
Applications
Calculate the linear regression and enter the results in the table below.
Enter the pseudo first order rate constants:
5 How are the isosbestic wavelength affected by temperature changes?
6 How are the rate constants affected by temperature changes?
Evaluation Table 4.30. Calculation Results
Name 400 nm, T = 50 °C 400 nm, T = 55 °C
Slope a
Intercept b
Correlation coefficient R
k(50 °C) = s-1 k(55 °C) = s-1
200
Applications
Example Results and Discussion
Sample: p-nitrophenyl acetate (244 mg/l)
(dry acetonitrile)
Assay: 30 l sample injected in 3 ml
0.1 molar phosphate buffer at pH 8.5
Cell: 10-mm path length quartz cell with a stopper
Instrument
Parameters: run time: 30 min
cycle time: 2 min
wavelength range: 214­550 nm
absorbance range: 0.0­2.0 AU
Temperature: temperature set: 60 °C
Diagram: Reaction Spectrum at 60 °C
1 The table below shows at which wavelengths the isosbestic points can be found.
Absorbance
[AU]
Wavelength [nm]
300 400
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
500
Results Table 4.23. Wavelengths of Isosbestic Points
Name Wavelength [nm]
Isosbestic Point 1 221
Isosbestic Point 2 247
Isosbestic Point 3 320
201
Applications
2 The following diagram shows the absorbance at 400 nm as a function of time:
Diagram: Absorbance at 400 nm as a Function of TimeReaction Spectrum at 60 °C
3 The shape of the curve indicates an exponential relationship. This shape of time traces is an
indication for a simple, mono-molecular reaction mechanism.
4 Estimated absorbance value at infinite time:
Absorbance
[AU]
Absorbance (400 nm), T = 60 0 C
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
0 500 1000 1500 2000
Time [s]
A400, = 1.752 AU
202
Applications
The table below shows the values for linear regression calculation.
The table below shows linear regression results.
The pseudo first-order rate constant can be calculated as:
5 The rate constants are independent of the wavelength chosen for evaluation. Dependent on
the precision of the data, the algorithm used for data evaluation and the total absorbance
difference, the precision of the rate constant determination varies. In our example the
absorbance value at infinite time (due to the algorithm used) and the total absorbance
difference have most impact on the precision of the rate constant results.
Results Table 4.24. Linear Regression Calculations
Reaction time t [s] A400, [AU] A400, - A400, [AU] ln(lA400, - A400,l)
0 0.037 -1.7154 0.53962
120 0.539 -1.2134 0.19339
240 0.889 -0.8628 -0.14763
360 1.139 -0.6131 -0.48918
480 1.316 -0.4363 -0.8294
600 1.442 -0.3104 -1.16976
720 1.531 -0.2210 -1.50964
840 1.593 -0.1592 -1.83778
960 1.640 -0.1125 -2.18462
1080 1.671 -0.0814 -2.50789
1200 1.693 -0.0588 -2.83293
1320 1.710 -0.0422 -3.16486
1440 1.723 -0.0304 -3.49233
1560 1.731 -0.0212 -3.85517
1680 1.738 -0.0142 -4.25381
1800 1.741 -0.0113 -4.48384
Results Table 4.25. Calculation Results
Name 400 nm, T = 60°C
Slope a -0.99992
intercept b 0.529323
Correlation coefficient R -0.00282
k = 2.8 ˇ 10-3
s-1
203
Applications
6 The isosbestic point is characterized by its invariance with reaction time. Therefore no rate
constant can be calculated due to the lack of a measurable absorbance difference.
7 The most precise results can be expected at a wavelength where the absorbance change
with reaction time is high. In our example it is in the absorbance maximum of the growing
absorbance band at about 400 nm.
Advanced Example Results and Discussion
Sample: p-nitrophenyl acetate (244 mg/l)
(dry acetonitrile)
Assays: 30 l sample injected in 3 ml
Cell: 10-mm path length quartz cell with stopper
Instrument
settings: run time: 30 min
cycle time: 2 min
wavelength range: 214­550 nm
absorbance range: 0.0­2.0 AU
Temperature: set temperature: 50 °C
set temperature: 55 °C
Diagram: Reaction Spectrum at 50 °C
Absorbance
[mAU]
Wavelength [nm]
300 400
0
0.2
0.4
0.6
0.8
1
1.2
1.4
500
204
Applications
Diagram: Reaction Spectrum at 55 °C
1 The following tables show the wavelengths of the isosbestic points.
Absorbance
[AU]
Wavelength [nm]
300 400
0
0.2
0.4
0.6
0.8
1
1.2
1.4
500
Results Table 4.26. Wavelengths of Isosbestic Points at 50 °C
Name Wavelength [nm]
Isosbestic Point 1 221
Isosbestic Point 2 247
Isosbestic Point 3 320
Results Table 4.27. Wavelengths of Isosbestic Points at 55 °C
Name Wavelength [nm]
Isosbestic Point 1 221
Isosbestic Point 2 247
Isosbestic Point 3 320
205
Applications
2 The following diagram shows the absorbance at 400 nm and 50 °C as a function of time.
Diagram: Absorbance at 400 nm and 50 °C as a Function of Time
The following diagram shows absorbance at 400 nm and 55 °C as a function of time.
Diagram: Absorbance at 400 nm and 55 °C as a Function of Time
Absorbance
[AU] Absorbance (400 nm), T = 50 0 C
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
0 500 1000 1500 2000
Time [s]
Absorbance
[AU] Absorbance (400 nm), T = 55 0 C
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
0 500 1000 1500 2000
Time [s]
206
Applications
3 The estimated absorbance values at infinite time are:
The linear regression calculation data for the measurements at 50 °C listed in the table
below.
A400,(50 °C) = 1.75 AU A400,(55 °C) = 1.56 AU
Results Table 4.28. Linear Regression Calculations at 50 °C
Reaction time t [s] A400,[AU] A400, - A400, [AU] ln(lA400, - A400,l)
0 0.015 -1.7350 0.55101
120 0.223 -1.5270 0.42331
240 0.407 -1.3430 0.29491
360 0.570 -1.1800 0.16551
480 0.714 -1.0360 0.03537
600 0.841 -0.9090 -0.09541
720 0.954 -0.7960 -0.22816
840 1.052 -0.6980 -0.35954
960 1.139 -0.6110 -0.49266
1080 1.216 -0.5340 -0.62736
1200 1.283 -0.4670 -0.76143
1320 1.343 -0.4070 -0.89894
1440 1.396 -0.3540 -1.03846
1560 1.443 -0.3070 -1.18091
1680 1.485 -0.2650 -1.32803
1800 1.520 -0.2300 -1.46968
207
Applications
The linear regression calculation data for the measurements at 55 °C are listed in the table
below.
The linear regressions results are listed in the table below.
The pseudo first order-rate constants are:
4 The wavelengths of the isosbestic points are not affected by changes of the set temperature
as long as the molecular mechanism does not change.
5 The rate constants are increasing with temperature. This increase in reaction speed with
temperature is due to the increasing amount of molecules at higher energy levels. More
molecules have sufficient energy to undergo the chemical reaction.
Results Table 4.29. Linear Regression Calculations at 55 °C
Reaction time t [s] A400, [AU] A400, - A400, [AU] ln(lA400, - A400,l)
0 0.020 -1.5400 0.43178
120 0.306 -1.2540 0.22634
240 0.538 -1.0220 0.02176
360 0.727 -0.8330 -0.18272
480 0.881 -0.6790 -0.38713
600 1.005 -0.5550 -0.58879
720 1.105 -0.4550 -0.78746
840 1.188 -0.3720 -0.98886
960 1.251 -0.3090 -1.17441
1080 1.305 -0.2550 -1.36649
1200 1.348 -0.2120 -1.55117
1320 1.383 -0.1770 -1.73161
1440 1.412 -0.1480 -1.91054
1560 1.436 -0.1240 -2.08747
1680 1.454 -0.1060 -2.24432
1800 1.469 -0.0910 -2.3969
Results Table 4.30. Calculation Results
Name 400 nm, T = 50 °C 400 nm, T = 55 °C
Slope a -0.00112 -0.00159
Intercept b 0.569364 0.382845
Correlation coefficient R --0.99985 -0.9995
k[50 °C] = 1.1ˇ10-3
s-1
k[55 °C] = 1.6 ˇ 10-3
s-1
208
Applications
4.6. Biochemical Spectroscopy
Enzyme Activity
Introduction
The enzyme activity is a measure of the quality of an enzyme. During purification steps the
activity is monitored.
The activity of glutamate-oxaloacetate transaminase (GOT) can be measured by coupling
the production of Oxaloacetate by GOT to the oxidation of NADH using Malate
dehydrogenase (MDH):
Reagents and Equipment
t 0.1 molar phosphate buffer at pH 7.4
t 10 ml of 0.19 molar L-aspartate in phosphate buffer
t 1 ml of 0.6 molar 2-oxoglutarate in distilled water
t 1 ml of 0.012 molar NADH
t 1 ml of malate dehydrogenase (MDH) in 3.2 molar ammonium sulphate solution (about 1200
U/mg)
t glutamate-oxaloacetate transaminase (GOT)
t 0.1 M phosphate buffer at pH 7.4
t 3-ml pipette
t 50-l and 100-l syringe or pipette with adjustable volume
t 10-mm path length quartz cell with a stopper
t thermostattable cell holder
(1) 2-Oxoglutarate + L-Aspartate L-Glutamate + Oxaloacetate
GOT
(2) Oxaloacetate + NADH + H
+ L-Malate + NAD
MDH
209
Applications
Experiment
Time: about 30­50 min
1 Prepare the assay in the cell:
* add 3 ml of the L-aspartate in phosphate buffer solution
* add 100 l of the 2-oxoglutarate solution
* add 50 l of the NADH solution
* add 10 l of the MDH solution
2 Set the temperature of your cell holder to 37 °C.
3 Place the cell in the cell holder and allow to equilibrate to the temperature set.
4 Dilute your GOT enzyme suspension (about 10 mg/ml) 1:2000 with ice-cold phosphate buffer
5 Measure the reference on your cell.
6 Prepare your system to acquire absorbance values in 5 second intervals for 100 seconds (at
340 nm).
7 Add 20 l of your diluted GOT suspension to the cell, quickly stir or shake the solution and
immediately start the data acquisition. Do not shake or stir too strongly to avoid bubble
formation.
210
Applications
Evaluation
1 Calculate the rate constant k. Assuming a zero-order reaction the following equation can be
used:
with:
= absorbance at time t
= initial absorbance at time 0
Calculate a linear regression with offset for the above equation. Use the calculation formula
given in the appendix for . Here the x, y pairs are the reaction time (x) and the
absorbance values At at time (y).
Enter the absorbance values for linear regression calculation in the table below.
A t( ) k t A0+=
A t( )
A0
y ax b+=
Evaluation Table 4.31. Absorbance at Different Reaction Times
Reaction Time t [s] Absorbance [AU] at 340 nm
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
211
Applications
Calculate the linear regression and enter the results in the table below:
Enter the zero-order rate constant:
2 Use the absolute value of the zero-order rate constant and use a conversion factor of
7200 U  s/mg to convert to the specific activity.
3 Why are two enzyme reactions used to monitor the activity of GOT?
4 Why are not all data acquired used for the rate constant calculation?
Evaluation Table 4.32. Calculation Results
Name 340 nm, T = 37 °C
Slope a
Intercept b
Correlation coefficient R
k = s-1
Specific activity = U/mg
212
Applications
Example Results and Discussion
Sample: GOT enzyme suspension (10mg/ml)
(diluted 1:2000 with phosphate buffer )
Assay: 20 l sample injected in
* 3 ml L-aspartate
* 100 l 2-oxoglutarate
* 50 l NADH solution
* 10 l MDH solution
Cell: 10-mm path length quartz cell with a stopper
Instrument
Parameters: integration time: 0.1 s
run time: 100 s
cycle time: 5 s
wavelength:340 nm
absorbance range: 0.0­1.0 AU
Temperature: set temperature: 37 °C
1 The measured absorbance values are listed in the table below.
Results Table 4.31. Absorbance at Different Reaction Times
Reaction Time t [s] Absorbance [AU] at 340 nm
0 0.991
5 0.926
10 0.863
15 0.798
20 0.738
25 0.684
30 0.623
35 0.562
40 0.501
45 0.439
50 0.378
55 0.318
60 0.258
65 0.197
70 0.135
213
Applications
The linear regression results are listed in the table below.
The zero-order rate constant is:
2 The calculated enzyme activity is:
3 The second enzyme reaction (MDH) is used to monitor the first reaction. It must be much
faster, such that its reaction time is negligible compared to the GOT. In this indicator step
(2), NAD+
is formed, which has its absorbance maximum at 340 nm. This absorbance does
not interfere with any of the other components of the assay and its extinction coefficient is
quite high. This allows to detect even small amounts of the product (oxaloacetate) formed.
4 The conditions for the linear relationship of absorbance change with time are only valid, if
the substrate is available in large amounts. Therefore, the rate is usually determined as an
initial rate only. In our example the data acquired in the time interval from 0 to 70 s is by far
sufficient for the rate calculation.
Results Table 4.32. Calculation Results
Name 340 nm, T = 37 °C
Slope a -0.01215
Intercept b 0.985808
Correlation coefficient R -0.99995
k = -1.2ˇ10-2
s-1
Specific activity = 86.4 U/mg
214
Applications
Appendix
Linear
equation:
Least square
method:
y ax b+=
yi axi b+( )­( )
2
i 1=
n
 minimum
a
n xiyi
i 1=
n
 xi yi
i 1=
n

i 1=
n
n
xi
2
i 1=
n
 xi
i 1=
n

 
 
 
  2
­
--------------------------------------------------------=
b
xi
2
yi
i 1=
n

i 1=
n
 xi xiyi
i 1=
n

i 1=
n
n
xi
2
i 1=
n
 xi
i 1=
n

 
 
 
  2
­
---------------------------------------------------------------------=
R
xiyi
i 1=
n

1
n
--- xi yi
i 1=
n

i 1=
n
­
xi
2
i 1=
n

1
n
--- xi
i 1=
n

 
 
 
  2
­
 
 
 
 
yi
2
i 1=
n

1
n
--- yi
i 1=
n

 
 
 
  2
­
 
 
 
 
--------------------------------------------------------------------------------------------------------------=