Trace element analysis of
geological materials by ICP-MS I
DSP analytical geochemistry
Markéta Holá, MU Brno
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
C9067
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Outline
1. Mass spectrometry. General introduction and history.
2. Ion sources for mass spectrometry. Inductively coupled plasma.
3. Interface. Ion optics. Mass discrimination. Vacuum system.
4. Spectral interferences. Resolution, ion resolution calculations.
5. Mass analyzers. Elimination of spectral interferences.
6. Non-spectral interference.
7. Detectors, expression of results.
8. Introduction of samples into plasma.
9. Laser ablation for ICP-MS.
10.Excursion in the laboratory.
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3
Detectors
for ICP-MS
• Detection system — an important area of the mass spectrometer that
counts the number of ions emerging from the mass analyzer.
• The detector converts the ions into electrical pulses, which are then
counted by its integrated measurement circuitry.
• The magnitude of the electrical pulses corresponds to the number of
analyte ions present in the sample. Trace element quantitation in an
unknown sample is then carried out by comparing the ion signal with
known calibration or reference standards.
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• For some applications where ultratrace detection limits are not required, the
ion beam from the mass analyzer is directed into a simple metal electrode
(channeltron), or Faraday cup.
• The most common type is discrete dynode electron multiplier.
Detectors
for ICP-MS
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Detectors
Faraday cup
The incident ion strikes the dynode surface which emits electrons and induces a current
which is amplified and recorded. The dynode electrode is made of a secondary emitting
material like CsSb, GaP or BeO.
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Detectors
Channel electron multiplier
• Curved ('horn' shaped) continuous
dynode where amplifications occur
through repeated collisions with the
dynode surface.
• Open glass cone coated with a
semiconductor type material — that
generates electrons from ions impinging
on its surface.
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Detectors
Discrete dynode electron multiplier
• series of discrete dynodes maintained at
increasing potentials resulting in a series
of amplifications.
• Ions pass the conversion dynode and
strike the initial amplification dynode
surface producing an emission of
secondary electrons which are then
attracted either to the second dynode, or
into the continuous dynode where more
secondary electrons are generated in a
repeating process ultimately resulting in a
cascade of electrons.
• Linear range up to 9 orders
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Detectors
Electron multiplier
Working in dual mode:
• pulse – small ion signals
• analog – higher ion fluxes
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- discrete dynode detection system that
enables the quantification of both trace
and major elements.
- mass independent detector response,
enabling fully automatic cross calibration
between the counting and analog modes.
- manufacturer recommendation for
isotope ratios - with the minor isotope in
counting mode and the abundant isotope
in analog mode the highest precision and
accuracy can be obtained.
- We prefer measuring all isotopes in
counting mode which means appropriate
sample dilution!!!
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Detectors
Analog + counting mode
HR ICP MS Element2
11
For the digital counting mode, the concentrations should
generally not be greater than 100 µg/l at a resolution of R=300
for the upper mass range. The upper limit of the count rate for
the digital counting mode is about 5*106 cps; at higher count
rates, measurements must be made in the analog mode. The
linearity of the calibration function, and the acceptable
working ranges need to be checked for each particular
application.
In the analog mode, the concentration limits are in the mg/l
range.
Dynamic range
Detectors
Analog + counting mode
HR ICP MS Element2
Detectors
Analog + counting mode
After cross calibration, the
two curves are normalized,
which means the detector is
suitable for concentration
levels between 0.1 ppt and
100 ppm — typically 8 - 9
orders of magnitude for
most elements.
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Detectors
Electron multiplier
Scanning protocol of a multielement scan of
three different masses.
Quadrupole is scanned to mass A. The
electronics are allowed to settle (settling
time) and left to dwell for a fixed period of
time at one or multiple points on the peak
(dwell time); intensity measurements are
then taken (based on the dwell time). The
quadrupole is then scanned to masses B
and C and the measurement protocol
repeated. The complete multielement
measurement cycle (sweep) is repeated as
many times as needed to make up the
total integration per peak.
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Detectors
Electron multiplier
Impact of integration time on the overal analysis time for Pb isotope ratios.
More ions that are counted = the better the precision will be.
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15
The dead time of a detector is defined as the minimum time interval that two consecutive counts
must be separated in order to be recorded as two different events. The effect of having a dead
time in a detector used to monitor counting rates is that the measured counting rates will be lower
than the real ones. However, the real counting rate can be determined from the measured one if the
dead time of the detector is known.
Dead time changes with life of a detector and for accurate isotopic ratio measurement must be
determined. It is performed on the measured intensities (counts per second) using the equation:
Icor and Iexp are the corrected and experimental intensities, and τ is the dead-time (in seconds).
It is prudent to check this internal correction by measuring the isotope ratios of a standard solution at
different concentration levelsand updating the detector deadtime value in the sofware.
VANHAECKE, Frank a Patrick DEGRYSE. Isotopic Analysis: Fundamentals and Applications Using ICP-MS. Weinheim: WILEY-VCH Verlag GmbH & Co., 2012. ISBN 978-3-527-32896-3.
Detectors
Dead time
Detectors
multicollector
MC-ICP-MS: These instruments have similar
sensitivities and can achieve precision for isotope ratio
measurements in the range of 0.001–0.002%.
Isotope ratio – 87Sr/86Sr obtained by
ablation SRM NIST 1486 by LA-ICP-(Q)MS.
Certified ratio: 0,70931
Quadrupole filter is not suitable for isotope
ratio measurements?
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17
- fast electric scanning mode (EScan) - magnetic field is kept constant and scanning is performed electrically
by varying the accelerating voltage and the ESA high voltage
- the low mass resolution mode with flat top peaks
- peak jump (scan time ~ 0.1 s), points per peak – 1-3 for LA (TRA), 3-10 for SN
- detection mode - counting
- dead time changes with life of a detector and for accurate isotopic ratio measurement must be determined
Detectors
SF-ICP-MS
Signal evaluation
Peak reading
mass
count
on peak (laser ablation)
Gaussian shape
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Results expression
arithmetic mean, standard deviation
Q. Yang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 399 (2014) 225–235
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Results expression
arithmetic mean, standard deviation
T. Himmler et al. / Chemical Geology 277 (2010) 126–136 20
Results expression
arithmetic mean, standard deviation
D.V.M. Sousa, et al., Talanta 219 (2020) 121239521
Results expression
errors
• Random errors (indeterminate) – influence
precision, causes data to be scattered more or
less symmetrically around a mean value. Standard
deviation, confidence interval (uncertainty)
• Systematic errors (determinate) – influence
accuracy, causes the mean of a data set to differ
from the accepted value (wrong calibration,
interference …) – (Student's t-test)
• Gross errors – influence precision and accuracy,
occur only occasionally, are often large, are often
the product of human errors; lead to outliers (test
to detect outliers)
Results can be precise without being accurate and accurate without being precise.
It is impossible to perform a chemical analysis
that is totally free of errors or uncertainties.
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Results expression
Gaussian Distribution and Reference Range
Mean±1 SD contain 68.2% of all values.
Mean±2 SD contain 95.5% of all values.
Mean±3 SD contain 99.7% of all values
Gaussian distribution (also known as normal distribution) is
a bell-shaped curve, and it is assumed that during any
measurement values will follow a normal distribution with
an equal number of measurements above and below the
mean value.
In a normal distribution the mean is zero and the standard
deviation is 1.
Normal distribution is characterized by two parameters:
µ – mean value
SD (σ) – standard deviation
the mean value µ or SD cannot be determined => an estimate of
these values is determined
Estimation of µ: median, arithmetic or geometric mean
Estimation of SD: range, standard deviation 23
• arithmetic mean – almost eliminates the effect of random errors, use only in the
case of normal distribution


n
x
ix
n
X
1
__
1
xi – measured value; n – number of measurements
• estimation of the standard deviation for n < 7 using R
Rks n  kn – tabulated values; R = xn – x1
 1
1
2_










n
Xx
s
n
i
i xi – measured value; X – arithmetic mean;
n – number of measurements
Results expression
arithmetic mean, standard deviation
• estimation of the standard deviation for n > 7
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• estimation of the standard deviation for n < 7 using R
Rks n  kn – tabulated values; R = xn – x1
Results expression
arithmetic mean, standard deviation
n kn
2 0.886
3 0.591
4 0.486
5 0.430
6 0.395
7 0.370
8 0.351
9 0.337
10 0.325
Example:
The following Cd contents were determined in the soil sample by
ICP MS method:
14.2, 15.7, 14.9, 13.7, 15.1 ng/g
Express analytical result with SD.
Mean = 14.72, R = 15.7 – 13.7 = 2, SD = 2*0.43 = 0.86
Cd: 14.72 ± 0.86 ng/g 25
Results expression
Limit of detection, limit of quantification
• Limit of detection (LOD) is defined as the smallest possible concentration that can be
distinguished with a predetermined probability from random background fluctuations.
• We do not directly measure concentration, but the signal intensity. The relationship between
signal and concentration is determined by calibration.
• Assuming that background fluctuations have a Gaussian distribution, the noise is expressed as
the standard deviation of the distribution σ.
• Limit of quantification (LOQ) is the lowest concentration determined with acceptable accuracy,
considering the statistical fluctuations between the background and an alytical signal.
LOD =
3𝜎
𝑆
LOQ =
10𝜎
𝑆
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Results expression
Limit of detection, limit of quantification
LOD LOQ
10𝜎
SLOD
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Results expression
arithmetic mean, SD, LOD, LOQ
D.V.M. Sousa, et al., Talanta 219 (2020) 121239528