1. Session 4 Atomic Mass Spectrometry
Comparison of different techniques for trace analysis

2. Crucial steps in atomic spectroscopies and -metries and other methods
Laser ablation etc.
Nebulisation
Solid/liquid sample
Molecules in gas phase
Solution
Desolvation
Sample
preparation
M+ X-
Vaporisation
MX(g)
M(g) + X(g)
Atomisation=
Dissociation
Atoms in gas phase
Sputtering, etc.
Excitation M+
Ionisation
Ions
Excited Atoms
ICP-MS and other MS methods
(also: ICP-OES)
 AAS and AES, X-ray methods
Adapted from (Gary Hieftje)

3. 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

4. ICP-MS instrumentation and principle
Detector (e.g. electron multiplier)
Plasma generates
positive ions
Under vacuum
nebuliser
Interface
Sorted by mass analyser, e.g. quadrupole, magnetic sector, according to m/z ratio
Spray chamber
sample

5. ICP-MS instrumentation
Sampling cone
Collision cell
Detector
(discrete dynode)
ICP Torch
Mass Analyser:
Quadrupole
Cones and Ion Optics
Skimmer cone
Modern instrument with collision/reaction cell

6. Recap: Ion formation in an inductively-coupled plasma
Mostly, singly charged positive ions are generated (>90% efficiency)

7. 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)

8. Mass analysers for ICP-MS
Quadrupole: High mass stability, fast
Lowest cost option
Time-of-Flight (rare)
HR (High-resolution): Uses magnetic sector mass analyser
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

9. 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

10. Atomic mass spectra
Ray and Hieftje, J. Anal. At. Spectrom., 2001, 16,

11. Typical detection limits of ICP-MS instrument

12. 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

13. 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

14. 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+
40Ar2+

15. Overcoming polyatomic interferences:
Collision/reaction cells (CRC technology)
Various modes of action:
Collision-induced dissociation (less important)
Chemical reaction (major mechanism)
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
Requires reactive gas
(Webinar)

16. Polyatomics and high-resolution ICP-MS
also no problem with polyatomics, as there are small, resolvable differences in mass:
31P
16O16O
14N16O1H
15N16O
32S
30.95
31.00
31.95
32.00
Mass (u)

17. 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”

18. 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): 10 20 30 40 50 60 64 66 67 68 70 %
67Zn: 4.1%
Isotope
Incubated overnight at 37°C with 40 mol equivalents of 67Zn(II) (93% isotopic purity)
Measured isotopic ratios

19. 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 ± (n = 7)
67Zn / 64Zn
4.17 ± (n = 7)
68Zn / 64Zn
0.490 ± (n = 7)
70Zn / 64Zn
± (n = 7)
 S: Zn ratio: 9:4 (as expected; the protein contains 9 sulfurs)

20. Comparison of experimental and calculated isotopic ratios
Calculated for 4 exchanging Zn(II)
As measured
Calculated for 3 exchanging Zn(II)
66/64
67/64
68/64
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

21. 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:
better control over matrix
Allows separation of different components: direct access to speciation
Laser ablation  LA-ICP-MS
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 ?

22. Laser ablation Useful for surface analysis of solid samples monitor
camera
UV light
Laser
To ICP
Carrier gas in
sample

23. The ablation process
Plume of molecules and ions from a surface hit by a laser

24. Comparison: AAS, ICP-OES, and ICP-MS
AAS: Single element, ppm/ppb range
Cheap, simple
Small dynamic range
GFAAS about 100 times more sensitive than FAAS, but also more challenging
ICP-OES: Multi-element, ppb range
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

25. Comparison: Detection limits and working ranges

26. Synopsis: Interferences in atomic spectroscopy
Technique
Type of Interference
Method of Compensation
Flame AAS
Ionization
Chemical
Physical
Ionization buffers Releasing agent or nitrous oxide-acetylene flame
Dilution, matrix matching, or method of additions 
Graphite Furnace AAS
Physical and chemical
Molecular absorption
Spectral
Spectral Standard Temperature Platform Furnace (STPF), conditions, standard additions Zeeman or continuum source background correction Zeeman background correction
ICP-OES
Matrix
Background correction or the use of alternate analytical lines Internal standardization
ICP-MS
Inter-element correction, use of alternate masses, higher resolution systems or reaction/collision cell technology
Internal standardization

27. A technique decision matrix
Exercise: How is this decision matrix correlated with strengths and limitations of the various techniques ?

28. 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

29. 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 nm