Atomic Mass Spectrometry: Techniques & Applications

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- MX(g) Vaporisation 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 www.spectroscopynow.com (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 http://www.cee.vt.edu/ewr/environmental/teach/smprimer/icpms/icpms.htm

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 • 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) 6000 K, ambient pressure ICP torch

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, 1206-1216 http://www.wcaslab.com/tech/tbicpms.htm

11. Typical detection limits of ICP-MS instrument http://las.perkinelmer.co.uk/content/TechnicalInfo/TCH_ICPMSThirtyMinuteGuide.pdf

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 geochemistryand 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

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 http://breeze.thermo.com/collisioncells/ (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. 60 50 40 30 20 10 0 64 66 67 68 70 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): % 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:  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) • 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 As measured Calculated for 3 exchanging Zn(II) 68/64 70/64 66/64 67/64

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 ionsfrom a surface hit by a laser http://kottan-labs.bgsu.edu/pictures/

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 http://pubs.acs.org/hotartcl/tcaw/99/oct/element.html

26. Synopsis:Interferences in atomic spectroscopy http://pubs.acs.org/hotartcl/tcaw/99/oct/table1.html

27. A technique decision matrix Exercise: How is this decision matrix correlated with strengths and limitations of the various techniques ? http://las.perkinelmer.com/content/relatedmaterials/brochures/bro_atomicspectroscopytechniqueguide.pdf

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 20-30 nm