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
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.
Mass Spectrometry
inorganic
Mass number=(# protons)+(# neutrons)
1 AMU = 1 mu = 1 Da = 1.66053904020 x 10-27 kg
AMU atomic mass unit - It is a unit of mass used to express atomic or molecule
masses. When the mass is expressed in AMU, it roughly reflects the sum of the
number of protons and neutrons in the atomic nucleus (electrons have so much less
mass that they are assumed to have a negligible effect).
Mass number vs. Atomic mass unit
Mass number - also called atomic mass number or nucleon number, is the total
number of protons and neutrons (together known as nucleons) in an atomic nucleus.
56Fe amu 55.9349363
Mass number
mu = m(12C)/12
Mass Spectrometry
natural isotopes
Isotopes are atoms of the same element,
which have different masses – by having
varying numbers of neutrons in their nuclei.
Isotopes of elements that occur in nature have
a constant abundance relative to another –
RELATIVE NATURAL ABUNDANCE
• Spectral
mass overlap of the interfering particle and the measured
isotope (same m/z - indistinguishable from each other)
• Non-spectral
influencing the signal intensity of the analyte by the presence of
various substances in the sample matrix
Interferences
ICP-MS
Spectral interferences
isobaric, polyatomic, multiply charged ions
Mass interferences on a given mass-to-charge-ratio (m/z) are possible due to
the presence of isobars (e.g. 204Hg, 204Pb),
polyatomic/molecular species (e.g., 40Ar16O vs 56Fe, 40Ar:40Ar vs. 80Se) formed
by various recombinations of sample, matrix and Ar ions in cooler parts of
the plasma
multiply charged ions (e.g., 138Ba2+ vs. 69Ga+), also formed in the plasma.
Spectral interferences
isobaric, polyatomic, double charged species
Spectral interferences
isobaric
• Are cause by isotopes of different elements forming atomic ions with the same nominal
mass-to-charge ratio (m/z)
• 58Fe on 58Ni, 64Ni on 64Zn, 48Ca on 48Ti
• They are bet avoided by choosing alternative, noninterfered analyte isotopes, if available
• Given acknowledge of the natural abundances of the isotopes of all elements, isobaric
interferences are easily corrected by measuring the intensity of another isotope of the
interfering element and substracting the appropriate correction factor from the intensity
of the interfered isotope.
Isobaric interferences
geochronology
U-(Th)-Pb system
238U
235U
232Th
208Pb
207Pb
206Pb
204Pb
202Hg
https://resources.perkinelmer.com/
• are formed in the plasma by a combination of different ions
• the degree of interference can be influenced by the conditions in
the plasma - ionization conditions (power input to the plasma,
position of the plasma torch…) - tuning of the device
• Ions originate from:
working gas (argon, laser ablation He)
sample matrix
solvent
example 40Ar16O vs. 56Fe
solutoin: use of alternative isotope 57Fe
analyser with high resolution (10 000)
40Ar16O 55,957 vs. 56Fe 55,935
Spectral interferences
polyatomic
Quadrupole
analyser
Sector
analyser
REE in geological samples
Spectral interferences
polyatomic
Spectral interferences
multiply charged ions
• Are due to relatively rare doubly-charged matrix or sample ions with twice the mass of the
analyte and hence the same m/z. exaple 90Zr++ on 45Sc+
• The formation of doubly-charged species can be minimized by optimizing instrument
operating conditions.
• For most elements is second ionisation potential higher than first ionization potential of Ar
Spectral interferences
doubly charged ions
The formation of a
doubly charged ion
is significant in the
case of Sr, Ba, (Pb).
Resolving power is the ability of a mass spectrometer to distinguish between ions of
different mass mass-to-charge ratios. Therefore, greater resolving power
corresponds directly to the increased ability to differentiate ions.
Resolving power
of mass spectrometer
• Width of one peak
RP = m / Δm
• Overlay of two peaks
RP = m1 / (m2 - m1)
Resolving power
of mass spectrometer
Resolving power
of mass spectrometer
Ion beam image
Collector Slit
Source Slit
Peak Profile
Low RP High RP
R = 400 R = 10 000
Resolution vs resolving power
of mass spectrometer
Sometimes used for low resolution analyzers (quadrupoles, ion traps)
„Resolution“ instead of „Resolving Power“
The resolution is expressed e.g. as a unit resolution (typical for
quadrupoles).
RP must be related to a certain m/z value or m/z range,
manufacturers often define a resolution valid for the whole mass
range of the analyzer, (e.g. 2 000 – 4 000).
• Low: 300-400 (quadrupole)
• Medium: 2000-4000 (TOF)
• High: 8 000 – 10 000 (SF)
Resolving power
of ICP mass spectrometer
Resolving power
of ICP mass spectrometer
Resolving power
of ICP mass spectrometer
Calculate the resolution power necessary to distinguish ions with amu:
28Si+ 27.9769284 vs. 14N2
+ 28.006148
40Ca+ 39.9625907 vs. 40Ar+ 39.962383
56Fe+ 55.9349393 vs. 40Ar 16O+ 59.957298
Resolving power
calculations