Session 4Atomic Mass SpectrometryComparison of different techniques for trace analysis

Crucial steps in atomic spectroscopies and -metries and other methodsAdapted from www.spectroscopynow.com (Gary Hieftje)Solid/liquid sampleSolutionMolecules in gas phaseSample preparationNebulisationAtomisation=DissociationVaporisationDesolvationAtoms in gas phaseIonsExcited AtomsLaser ablation etc.Sputtering, etc.ICP-MS and other MS methods(also: ICP-OES) AAS and AES, X-ray methodsIonisationExcitationM+ X-MX(g)M(g) + X(g)M+

ICP-MSMass spectrometry method: detects ions distinguished by their mass-to-charge ratio (m/z value)Based on ions moving under influence of electrical or magnetic fieldMass analysers generally require operation under vacuum, to avoid ions colliding with other particles

Recommended series of short articles: Robert Thomas: A beginners guide to ICP-MS

ICP-MS instrumentation and principlePlasma generatespositive ionsnebuliserSpray chamberDetector (e.g. electron multiplier)Sorted by mass analyser, e.g. quadrupole, magnetic sector, according to m/z ratiohttp://www.cee.vt.edu/ewr/environmental/teach/smprimer/icpms/icpms.htmUnder vacuumsampleInterface

ICP TorchMass Analyser:QuadrupoleCollision cellDetector(discrete dynode)Cones and Ion OpticsICP-MS instrumentationModern instrument with collision/reaction cellSampling coneSkimmer cone

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

Interface and ion opticsMajor 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 pressureRoom temperature,vacuumICP torch

Mass analysers for ICP-MSQuadrupole: High mass stability, fastLowest cost optionTime-of-Flight (rare)HR (High-resolution): Uses magnetic sector mass analyserHighest sensitivity and resolution, but slow and requires stable working environmentExpensiveMulti-collector (MD): Also with magnetic sector, but with detector arrayGood for accurate and precise isotope ratiosIsotope dilution measurements e.g. for accurate elemental ratios

Quadrupole mass analyserFour parallel metal rods with dc and ac voltage (alternating with radiofrequency)Works as mass filter: allows passage of particular m/z ions onlyCan scan over m/z range spectrum

Atomic mass spectraRay and Hieftje, J. Anal. At. Spectrom., 2001, 16, 1206-1216 http://www.wcaslab.com/tech/tbicpms.htm

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

Multi-collector mass analyserMagnetic sector mass analyser separates ions according to m/zSimultaneous detection with array of collectors (Faraday cups)Best for detn. of isotope ratiosApplications 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 compositionTemperature and humidity fluctuations in the laboratory environment Isobaric elemental and polyatomic interferences: Used to be greatest limitation for applicability

Polyatomic interferences in ICP-MS: OriginsSpectral interference: caused by presence of species with same mass as analyteOften derived from compounds with Ar

AnalyteInterference 39K+38Ar1H+40Ca+40Ar+51V+35Cl16O+52Cr+40Ar12C+56Fe+40Ar16O+63Cu+23Na40Ar+75As+40Ar35Cl+80Se+40Ar2+

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 interferenceCommonly used gases: He, H2, ammonia

http://breeze.thermo.com/collisioncells/(Webinar)Requires reactive gas

Polyatomics and high-resolution ICP-MSalso no problem with polyatomics, as there are small, resolvable differences in mass:31P15N16O14N16O1H30.9531.00Mass (u)32S16O16O31.9532.00

Stable isotopes and their usesMost elements have more than one isotopeE.g. 32S and 34S, or 56Fe and 57FeCan use more than one mass for one element for measurements in ICP-MSIDSM: Isotope dilution mass spectrometry: Use particular isotope of desired analyte as internal standard in ICP-MSCan buy enriched compounds, e.g. 67ZnO, and use as tracers

Example for use of stable isotopesMetal-binding protein with 4 Zn(II)Are all four zinc ions exchangeable ?

Isolated with natural abundance Zn(II):Isotope%Incubated overnight at 37C with 40 mol equivalents of 67Zn(II) (93% isotopic purity)Measured isotopic ratios67Zn: 4.1%

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 M water. For blank subtraction, the 10 mM NH4Acetate buffer was diluted 1+49 with 18 M water.Results for sample: S: Zn ratio: 9:4 (as expected; the protein contains 9 sulfurs)

Total S2.45 mg/L ( 0.2 %)Total Zn2.21 mg/L ( 0.6 %)Ratios:66Zn / 64Zn0.657 0.0028 (n = 7)67Zn / 64Zn4.17 0.025 (n = 7)68Zn / 64Zn0.490 0.0037 (n = 7)70Zn / 64Zn0.01325 0.00007 (n = 7)

Comparison of experimental and calculated isotopic ratiosFor 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 exchangeAs measuredCalculated for 4 exchanging Zn(II)Calculated for 3 exchanging Zn(II)66/6467/6468/6470/64

ICP-MS and hyphenationICP-MS can be coupled with a variety of separation techniques:Liquid chromatography HPLC-ICP-MSCapillary electrophoresis CE-ICP-MSAdvantages of hyphenated techniques: better control over matrixAllows separation of different components: direct access to speciationLaser ablation LA-ICP-MSFor surface analysisFor 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 materialCaveat: Calibration ?

Laser ablationLaserTo ICPCarrier gas inmonitorcameraUV lightsampleUseful for surface analysis of solid samples

The ablation processPlume of molecules and ions from a surface hit by a laserhttp://kottan-labs.bgsu.edu/pictures/

Comparison: AAS, ICP-OES, and ICP-MSAAS: Single element, ppm/ppb rangeCheap, simple Small dynamic rangeGFAAS about 100 times more sensitive than FAAS, but also more challengingICP-OES: Multi-element, ppb rangeLimited spectral interferences, good stability, low matrix effectsICP-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 rangeshttp://pubs.acs.org/hotartcl/tcaw/99/oct/element.html

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

Technique Type of InterferenceMethod of CompensationFlame AAS IonizationChemical

PhysicalIonization buffers Releasing agent or nitrous oxide-acetylene flame Dilution, matrix matching, or method of additionsGraphite Furnace AASPhysical and chemicalMolecular absorptionSpectralSpectral Standard Temperature Platform Furnace (STPF), conditions, standard additions Zeeman or continuum source background correction Zeeman background correctionICP-OES Spectral

MatrixBackground correction or the use of alternate analytical lines Internal standardizationICP-MSSpectral

MatrixInter-element correction, use of alternate masses, higher resolution systems or reaction/collision cell technology Internal standardization

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

Other inorganic mass spectrometry methodsMainly 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 techniqueSputtered 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 spectrometryPrinciple: bombard surface with ions, secondary ions are sputtered from surfaceHigh sensitivity for all elementsAny 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 One of the most widespread surface analysis techniques for advanced material research Dynamic SIMS: Bulk composition or depth distribution of trace elements. Depth resolution ranging from one to 20-30 nm