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.
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3
ICP-MS Mass analysers
according to the method of ion separation
• Scanning (Filter)
• Linear Quadrupole
• Sector
• Pulsed (Batch)
• Ion Trap
• Time-of-Flight
( Separation in Space)
( Separation in Time)
Quadrupole mass filter is most common (90%) and economical, but there are also magnetic sector,
time-of-flight, and collision/reaction cell systems. 4
• Low: 300-400 (quadrupole)
• Medium: 2000-4000 (TOF)
• High: 8 000 – 10 000 (SF)
Resolving power
of ICP mass spectrometer
5
Magnetic sector analyzer
Sector field SF
Ions are accelerated towards the magnetic field. Each charged particle will experience a sideways force
that is proportional to the strength of the magnetic field, the velocity of the particle and its charge. Each
appropriate mass can be selected in turn by setting the magnetic field strength to a value that will direct the
selected ions through a narrow window in front of the detector. 6
Magnetic sector analyzer
Sector field SF
7
qvB
r
mv
F 
2
v
rB
q
m
2
22

maF 
qEF 
qVmvEK  2
2
1
..
qvBF 
m – mass
a – acceleration
B – Magnetic Field
q – charge
E - electric field
F – Force
K.E. – kinetic energy
V – electric potential
v – velocity of accelerated ion
Magnetic sector analyzer
Ion physics
8
Magnetic sector analyzer
Sector field SF
Positive ions with a certain value m/z accelerated by a negative
potential V enter a magnetic field with magnetic induction B,
which curves the movement of ions on a trajectory with radius r.
Entering the magnetic field, the ions have a kinetic energy EK
corresponding to z.V obtained in an accelerating electric field.
(In the magnetic field acting on the ion B.z.v centripetal force,
which must be balanced with the centrifugal force m.v2 /r
From which we derive the basic equation for a magnetic analyzer:
v
rB
q
m
2
22
 radius of the ion path depends on m/z, B, v
Ek = z.V = 1/2 m.v2
B.z.v = m.v2/r
z 9
Double focusing magnetic mass analyzer
Sector field SF
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Double focusing magnetic mass analyzer
Sector field SF
The mass-to-charge ratio can be calculated from the induced radius of
curvature, or bend R sub E through the electrostatic analyzer, and the bend
R sub M through the magnetic sector analyzer, as we show here:
Double focusing magnetic sector instruments
combine a magnetic sector with an electrostatic
analyzer to compensate for the spread in ion
kinetic energy caused by the ion source.
11
Consists of some combination of a large electromagnetic ('B' sector), and some kind of
electrostatic focussing device ('E' sector) - different manufactures use differing geometries.
Double focusing magnetic mass analyzer
Sector field SF
Reverse Nier-Johnson geometry
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Double focusing magnetic mass analyzer
Reverse geometry
13
Double focusing magnetic mass analyzer
Reverse geometry
14
Time of flight ICP-MS
scheme
TOF measures the time of ion flight required to cross a certain distance.
Rapid analysis.
15
Time of flight ICP-MS
scheme
Schematics of an axial time-of-flight ICPMS (after Ray and Hieftje 2001).
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Quadrupole analyzer (Q)
linear filter
Quadruple analyzer filters out ions of a specific m/z
depending on the FR (AC) voltage and DC voltage applied
to quadrupole rods.
Only one mass (m/z) is allowed to reach the detector at
given time.
Quadrupole consists of 4 parallel rods with different
charges:
• One pair supplied with a positive DC voltage and RF
voltage
• Second pair supplied with a negative DC voltage and RF
(AC) voltage180° out of phase with the other pair
(U+Vcos(wt))
-(U+Vcos(wt)) 17
Quadrupole analyzer (Q)
linear filter
• The applied voltages affect the trajectory of ions traveling down the flight path
• For given dc and ac voltages, only ions of a certain mass-to-charge ratio pass
through the quadrupole filter and all other ions are thrown out of their original
path
Heavy ions: average (DC) potential
Lighter ions: Motion corrected by AC field
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Quadrupole analyzer (Q)
linear filter
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Quadrupole analyzer (Q)
linear filter
The values of U range from 500 to 2000 volts and V in the
above equation ranges from 0 to 3000 volts.
Stability diagram illustrating a single value for U and V where
only particle of mass m are allowed to reach the detector.
peak width
20
Quadrupole ICP-MS
scheme
https://www.youtube.com/watch?v=6_mavZ_WKoU
Watch Dave  https://www.youtube.com/watch?v=vuLrmgmJ54E
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Ion trap ICP-MS
scheme
• Ion trap mass spectrometers operate on a principle
similar to a quadrupole mass filter. However, it
does not operate as a filter; the ions are stored for
subsequent experiments and analysis.
• Electric fields are applied to electrodes arranged as
a ring electrode in the middle with cap electrodes
on each end. Conceptually, an ion trap can be
considered as a conventional quadrupole folded
on itself to form a closed loop.
• Within a selected range of m/z ratios determined
by the applied voltages, the device traps ions in
the space bounded by the electrodes. A mass
spectrum is produced by scanning the applied RF
voltages to eject ions sequentially of increasing
m/z ratio through an end cap opening for
detection.
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Ar-containing ions
• Ar is introduced for plasma generation (~20 l/min) => Ar+ and
Ar2+ always present in normal plasma
• Ar + elements from solvent, ambient air and/or matrix
Doubly charged ions
•Ar++, M++
Oxide and hydroxide ions
•MO+(m/z + 16) and MOH+(m/z + 17)
•MO+/M+ determined by M-O bond strength
•Usually MO+/M+ > MOH+/M+
•Optimization of instrumental settings MO+/M < 5 %
•Still problematic if m/z(M1O+)=m/z(M2
+) and c(M1)>c(M2)
Solvent and matrix based polyatomic ions
•H2O+, H3O+, SO+, NO+, MNa+
Spectral interferences
isobaric, polyatomic, multiply charged ions
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Spectral interferences
isobaric, polyatomic, multiply charged ions
ICP-MS background spectrum of high purity water
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Elimination of spectral interferences
isobaric, polyatomic, multiply charged ions
Selection of strategy depends on type of interference
• Analyzer with high resolution
• Sample processing
• Blank correction
• Mathematical correction
• Optimization of instrument settings
• Cool plasma
• Collision/reaction cell
• Tripple quadrupole
25
Sample processing
for elimination of spectral interferences
Appropriate selection of chemical reagents
• Dissolution preferably using HNO3 , H2O2 - Compared to H2O & surrounding air: no additional
elements
• Avoid use of HCl (if possible) - 35Cl16O+/ 51V+, 40Ar35Cl+/ 75As+
• Avoid use of H2SO4 (if possible) - 32S16O+/ 48Ti+, 32S16O2
+/ 64Zn+
Use of matrix/trace separation techniques
• Liquid/liquid extraction, ion exchange chromatography, ...
• Pre concentration of trace elements
• Powerful but: Labor intensive + time consuming + each new matrix new strategy + Higher risk
of contamination & analyte losses
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analyzovaný izotop interferující ionty
24Mg (79,0 %) 12C2
28Si (92,2 %) 12C16O, 14N2
31P (100 %) 15N16O, 14N16OH
32S (95,0 %) 16O2, 14N18O, 31PH
39K (93,3 %) 38ArH
45Sc (100 %) 31P14N
47Ti (7,5 %) 31P16O
51V (99,8 %) 36Ar15N
52Cr (83,8 %) 40Ar12C, 36Ar16O, 38Ar14N
55Mn (100 %) 40Ar15N, 38Ar17O
56Fe (91,8 %) 40Ar16O, 14N42Ca, 38Ar18O
60Ni (26,2 %) 44Ca16O, 42Ca18O
69Ga (60,1 %) 38Ar31P,
72Ge (27,7 %) 36Ar2
78Se (23,8 %) 38Ar40Ar
80Se (49,6 %) 40Ar2, 36Ar44Ca
85Rb (72,2 %) 42Ca43Ca
Sample processing
for elimination of spectral interferences analyte interferent
Overview of significant polyatomic
interferences from hydroxyapatite
matrix decomposed in a mixture of
nitric acid-hydrogen peroxide.
Sample matrix: Ca5(OH)(PO4)3
Plasma: Ar
Reagents: HNO3 , H2O2
Air: N2, O2, CO2 …
27
Blank correction
for elimination of spectral interferences
Procedure blank
• Same sample pre-treatment as samples (no sample intake)
• Correction for contamination, spectral overlap – molecular ions containing Ar and/or elements
from reagents and solvent used
Matrix-matched blank (imitation of matrix)
• Addition of high purity chemical compounds to blank
• Extension of application range
• Impossible for complex matrices
• Increased risk of contamination
Only succesful when degree of overlap is limited 28
Mathematical correction
for elimination of spectral interferences
Based on known isotopic abundances
29
„Cold“ plasma
for elimination of spectral interferences
• Reduce plasma Ar species, such as ArO+ and 56Fe, 40Ar and K or Ca.
• Cold (cool) plasma = reduced forward (generator) power (500 – 800 W) and
increaced nebulizer gas flow rate (1.5 – 1.8 l/min)
• Problem of secondary discharges - metal shield inserted between torch and
load coil
• Reduced background noise level and enhanced ion transmission for elements
in the lower mass region
• Elements of higher mass generally show higher detection limits compared to
normal plasma mode.
30
„Cold“ plasma
for elimination of spectral interferences
Limits of detection in pg/g and sensitivitties in
counts/pg/g in water and 5% HNO3, resp.,
measured with cold plasma and normal mode
Improvements of analytical response in cold
plasma
Wollenweber et al., Fres. J Anal Chem, 364, 433-437 (1999)
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„Cold“ plasma
for elimination of spectral interferences
The Guard Electrode (GE) feature of this Torch decreases the ion energy spread to increase ion
transmission for improved sensitivity. The GE feature is a requirement for Cold Plasma measurements.
32
HOT PLASMA
„Cold“ plasma
for elimination of spectral interferences
33
„COLD“ PLASMA
„Cold“ plasma
for elimination of spectral interferences
34
Collision/reaction cell
polyatomic interferences
35
Collision/reaction cell
polyatomic interferences
The collision cell itself is a type of ion lens located between the interface cones of the instrument and the
quadrupole mass spectrometer. When operated in collision cell mode, a flow of gas is passed into the
cell. Ions coming through the interface cones from the plasma interact with the gas and a range of
processes then occur depending on which gas is passed into the cell.
• chemical reaction
• charge transfer
• collisional retardation (kinetic energy
discrimination KED)
using H2 (in practice, mixed with He for
safety reasons) and He as example gases.
36
Collision/reaction cell
polyatomic interferences
• Molecular interference (ArX+) has
larger cross section than the analyte
(M+)
• More frequent interactions with He
• A significant reducion in kinetic
energy relative to the analyte (M+).
Energy filtering can be used to
ensure only the analyte enters the
quadrupole analyzer.
37
Collision/reaction cell
polyatomic interferences
38
Triple quadrupole mass spectrometer
QQQ MS, TQMS, polyatomic interferences
Tandem MS configuration, ICP-QQQ instruments - with two fully functioning mass filters Q1 and Q2, and
collision/reaction cell (CRC).
Q1 - rejects all non-target masses/elements, ensuring more consistent reaction processes in the CRC
CRC - analyte ions are separated from overlapping interfering ions
Q2 - the resulting product ions that emerge from the CRC are filtered
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Triple quadrupole mass spectrometer
QQQ MS, TQMS, polyatomic interferences
ICP-MS/MS measurement modes:
On-mass Mass-shift
40
Triple quadrupole mass spectrometer
QQQ MS, TQMS, polyatomic interferences
Removal of Interference on Sulfur and Phosphorus
Mass-shift mode 41
Triple quadrupole mass spectrometer
QQQ MS, TQMS, polyatomic interferences
Removal of REE++ Interference on Arsenic and Selenium
42
Triple quadrupole mass spectrometer
QQQ MS, TQMS, polyatomic interferences
Removal of REE++ Interference on Arsenic and Selenium
43
Elimination of spectral interferences
example
40Ar16O+ ……… 56Fe
mass: 55.9349393 ….. 55.957298
• High resolution mass analyser –
separation of peaks (2500 resolution
required)
• Cold plasma – reduction of Ar+ formation
(and so ArO+)
• Collision/reaction cell:
ArO+ + NH3 → ArO + NH3
+
https://doi.org/10.1144/geochem2019-049 44