Session 4
Atomic Mass Spectrometry
Comparison of different
techniques for trace analysis
Crucial steps in atomic spectroscopies
and -metries and other methods
Laser ablation etc.
Solid/liquid
sample
Nebulisation
Solution
Sample
preparation
M+
X-
Desolvation
M+
Atoms in gas
phase
Ionisation
MX(g)
Vaporisation
M(g) + X(g)
Sputtering, etc.
Molecules in
gas phase
Atomisation=
Dissociation
Excitation
Ions
ICP-MS and
other MS methods
(also: ICP-OES)
Adapted from www.spectroscopynow.com (Gary Hieftje)
Excited
Atoms
 AAS and AES,
X-ray methods
ICP-MS
Mass spectrometry method: detects ions
distinguished by their mass-to-charge ratio (m/z
value)
 Based on ions moving under influence of
electrical or magnetic field
Mass analysers generally require operation under
vacuum, to avoid ions colliding with other
particles


Recommended series of short articles: Robert Thomas: A
beginner’s guide to ICP-MS
ICP-MS instrumentation and principle
Plasma generates
positive ions
Under vacuum
Detector
(e.g.
electron
multiplier)
nebuliser
Interface
Spray chamber
Sorted by mass analyser,
e.g. quadrupole,
magnetic sector,
according to m/z ratio
sample
http://www.cee.vt.edu/ewr/environmental/teach/smprimer/icpms/icpms.htm
ICP-MS instrumentation
Sampling cone
Collision cell
Detector
(discrete
dynode)
ICP Torch
Cones and
Ion Optics
Mass Analyser:
Quadrupole
Skimmer cone
Modern instrument with collision/reaction cell
5
Recap: Ion formation in an
inductively-coupled plasma

Mostly, singly
charged positive
ions are generated
(>90% efficiency)
Interface and ion optics
Room temperature,
vacuum
6000 K, ambient pressure
ICP torch


Major challenge in instrumentation: large differences in
temperature and pressure. Interface (consisting of two cones)
allows connecting ion source to mass analyser (requires vacuum)
Lens focuses ions. Necessary for getting as many ions as possible
into analyser (maximising signal)
Mass analysers for ICP-MS

Quadrupole: High mass stability, fast



Time-of-Flight (rare)
HR (High-resolution): Uses magnetic sector
mass analyser



Lowest cost option
Highest sensitivity and resolution, but slow and
requires stable working environment
Expensive
Multi-collector (MD): Also with magnetic
sector, but with detector array


Good for accurate and precise isotope ratios
Isotope dilution measurements – e.g. for accurate
elemental ratios
Quadrupole mass analyser



Four parallel metal rods
with dc and ac voltage
(alternating with
radiofrequency)
Works as mass filter:
allows passage of
particular m/z ions only
Can scan over m/z
range  spectrum
Atomic mass spectra
Ray and Hieftje, J. Anal. At.
Spectrom., 2001, 16, 12061216
http://www.wcaslab.com/tech/
tbicpms.htm
Typical detection limits of ICP-MS instrument
http://las.perkinelmer.co.uk/content/TechnicalInfo/TCH_ICPMSThirtyMinuteGuide.pdf
Multi-collector mass analyser




Magnetic sector mass analyser separates ions
according to m/z
Simultaneous detection with
array of collectors (Faraday cups)
Best for detn. of isotope ratios
Applications in geochemistry
and biomedical research
Possible factors that can affect the
performance of ICP-MS



Variations in plasma ionization efficiency
Possible clogging or corrosion of cone apertures
Differing concentrations of other components in
matrix (e.g. acid, bulk elements) in samples could
result in matrix suppression



Ion current influenced by matrix composition
Temperature and humidity fluctuations in the
laboratory environment
Isobaric elemental and polyatomic interferences:
Used to be greatest limitation for applicability
Polyatomic interferences
in ICP-MS: Origins


Spectral interference:
caused by presence of
species with same mass
as analyte
Often derived from
compounds with Ar
Analyte
Interference
39K+
38Ar1H+
40Ca+
40Ar+
51V+
35Cl16O+
52Cr+
40Ar12C+
56Fe+
40Ar16O+
63Cu+
23Na40Ar+
75As+
40Ar35Cl+
80Se+
40Ar +
2
Overcoming polyatomic interferences:


Collision/reaction cells (CRC technology)
Various modes of action:






Collision-induced dissociation (less important)
Chemical reaction (major mechanism)
Requires reactive gas
Electron transfer (major mechanism)
KED: kinetic energy discrimination (monoatomic analyte
and interfering molecules are retarded differently)
Can either affect analyte or interference
Commonly used gases: He, H2, ammonia
http://breeze.thermo.com
/collisioncells/
(Webinar)
Polyatomics and high-resolution ICP-MS

also no problem with polyatomics, as there are
small, resolvable differences in mass:
31P
16O16O
14N16O1H
15N16O
30.95
31.00
Mass (u)
32S
31.95
32.00
Stable isotopes and their uses





Most elements have more than one isotope
E.g. 32S and 34S, or 56Fe and 57Fe
Can use more than one mass for one element
for measurements in ICP-MS
IDSM: Isotope dilution mass spectrometry:
Use particular isotope of desired analyte as
internal standard in ICP-MS
Can buy enriched compounds, e.g. 67ZnO, and
use as “tracers”
Example for use of stable isotopes
• Metal-binding protein with 4 Zn(II)
• Are all four zinc ions exchangeable ?
• Isolated with natural abundance Zn(II):
60
%
50
40
67Zn:
30
4.1%
20
10
0
64
66
67
68
70
Isotope
• Incubated overnight at 37°C with
40 mol equivalents of 67Zn(II)
(93% isotopic purity)
• Measured isotopic ratios
Measurement and output
(Thermofinnigan Element2)
Total Zn and total S were determined using standard addition. For Zn quantification, the sum of
the Zn isotopes 64, 66, 67, 68 and 70 was used. S was measured on the 32S isotope. Zn isotopic
distribution (64, 66, 67, 68, 70) was determined. All elements and isotopes were measured in
Medium Resolution (R = 4000). No internal standard has been used. No mass bias correction
(using certified materials) was used for the isotopic distribution measurement.
Sample preparation:
Sample was diluted 1+49 with 18 MΩ water. For blank subtraction, the 10 mM NH4Acetate buffer
was diluted 1+49 with 18 MΩ water.
Results for sample:
Total S
2.45 mg/L (± 0.2 %)
Total Zn
2.21 mg/L (± 0.6 %)
Ratios:
66Zn
/ 64Zn
0.657 ± 0.0028 (n = 7)
67Zn
/ 64Zn
4.17 ± 0.025 (n = 7)
68Zn
/ 64Zn
0.490 ± 0.0037 (n = 7)
70Zn
/ 64Zn
0.01325 ± 0.00007 (n = 7)
 S: Zn ratio: 9:4 (as
expected; the protein contains
9 sulfurs)
Comparison of experimental and
calculated isotopic ratios
16
Calculated for 4 exchanging Zn(II)
14
12
10
As measured
8
6
Calculated for 3
exchanging Zn(II)
4
2
0
1
66/64
2
67/64
3
68/64
4
70/64
 For each isotopic ratio, results agree best with the scenario for 3
exchanging zinc: Clear demonstration that only 3 out of 4 Zn exchange:
 The protein has one zinc that is inert towards exchange
ICP-MS and hyphenation




ICP-MS can be coupled with a variety of
separation techniques:
Liquid chromatography  HPLC-ICP-MS
Capillary electrophoresis  CE-ICP-MS
Advantages of hyphenated techniques:



Laser ablation  LA-ICP-MS




better control over matrix
Allows separation of different components: direct access
to speciation
For surface analysis
For materials that are difficult to digest (e.g. alloys)
Is being developed in scanning fashion with mm spatial
resolution: Imaging the metal composition of a material
Caveat: Calibration ?
Laser ablation
Useful for surface analysis of solid samples
monitor camera
Laser
UV light
Carrier gas in
To ICP
sample
The ablation process
Plume of molecules and ions
from a surface hit by a laser
http://kottan-labs.bgsu.edu/pictures/
Comparison: AAS, ICP-OES, and ICP-MS

AAS: Single element, ppm/ppb range




ICP-OES: Multi-element, ppb range


Cheap, simple
Small dynamic range
GFAAS about 100 times more sensitive than FAAS,
but also more challenging
Limited spectral interferences, good stability, low
matrix effects
ICP-MS: Multi-element, possible to reach ppt
(or even ppq)

Most complex, most expensive, lowest detection
limits, isotope analysis possible
Comparison: Detection limits and
working ranges
http://pubs.acs.org/hotartcl/tcaw/99/oct/element.html
Synopsis:
Interferences in atomic spectroscopy
Technique Type of
Interference
Ionization
Flame
Chemical
AAS
Physical
Graphite
Furnace
AAS
Physical and
chemical
Molecular
absorption
Spectral
ICP-OES
Spectral
Matrix
ICP-MS
Spectral
Matrix
Method of Compensation
Ionization buffers
Releasing agent or nitrous oxide-acetylene
flame
Dilution, matrix matching, or method of
additions
Spectral Standard Temperature Platform
Furnace (STPF), conditions, standard additions
Zeeman or continuum source background
correction
Zeeman background correction
Background correction or the use of alternate
analytical lines
Internal standardization
Inter-element correction, use of alternate
masses, higher resolution systems or
reaction/collision cell technology
Internal standardization
http://pubs.acs.org/hotartcl/tcaw/99/oct/table1.html
A technique decision matrix
Exercise: How is this
decision matrix
correlated with
strengths and
limitations of the
various techniques ?
http://las.perkinelmer.
com/content/relatedm
aterials/brochures/bro
_atomicspectroscopyte
chniqueguide.pdf
Other inorganic mass spectrometry
methods








Mainly for surface analysis (depth profiling, imaging) in different
materials (e.g. conducting, semiconducting, and nonconducting solid
samples; technical, environmental, biological, and geological
samples)
Spark source mass spectrometry (SSMS)
Glow discharge mass spectrometry (GDMS)
Laser ionization mass spectrometry (LIMS)
Thermal ionization mass spectrometry (TIMS)
Secondary ion mass spectrometry (SIMS): most sensitive elemental
and isotopic surface analysis technique
Sputtered neutral mass spectrometry (SNMS)
Detection limits for the direct analysis of solid samples by inorganic
solid mass spectrometry: down to ppb levels
SIMS: secondary ion mass spectrometry









One of the most widespread surface analysis techniques for
advanced material research
Principle: bombard surface with ions,
“secondary” ions are sputtered from surface
High sensitivity for all elements
Any type of material that can stay
under vacuum (insulators, semiconductors,
metals)
Potential for high-resolution imaging
(down to 40 nm)
Very low background: high dynamic range
(more than 5 decades)
Quantitative work complicated by variations
in secondary ion yields in dependence on
chemical environment and the sputtering
conditions (ion, energy, angle)
Rapid deterioration of bombarded surface
Static SIMS: Molecular and elemental
characterisation of top monolayer
• Dynamic SIMS: Bulk composition or depth distribution of trace
elements. Depth resolution ranging from one to 20-30 nm