Session 4Session 4

Atomic Mass SpectrometryAtomic Mass Spectrometry

Comparison of different Comparison of different techniques for trace analysistechniques for trace analysis

Crucial steps in atomic Crucial steps in atomic spectroscopies and -metries and spectroscopies and -metries and

other methodsother methods

Adapted from www.spectroscopynow.com (Gary Hieftje)

Solid/liquid sample Solution

Molecules in gas phase

Sample preparation

Nebulisation

Atomisation=Dissociation

Vaporisation

Desolvation

Atoms in gas phase

IonsExcited Atoms

Laser ablation etc.

Sputtering, etc.

ICP-MS and other MS methods(also: ICP-OES) AAS and AES,

X-ray methods

IonisationExcitation

M+ X-

MX(g)

M(g) + X(g)

M+

ICP-MSICP-MS Mass spectrometry method: detects ions Mass spectrometry method: detects ions

distinguished by their mass-to-charge ratio distinguished by their mass-to-charge ratio (m/z value)(m/z value)

Based on Based on ions moving under influence of ions moving under influence of electrical or magnetic fieldelectrical or magnetic field

Mass analysers generally require operation Mass analysers generally require operation under vacuum, to avoid ions colliding with under vacuum, to avoid ions colliding with other particlesother particles

Recommended series of short articles: Robert Recommended series of short articles: Robert Thomas: A beginner’s guide to ICP-MS Thomas: A beginner’s guide to ICP-MS

ICP-MS instrumentation and ICP-MS instrumentation and principleprinciple

Plasma generatesPlasma generates

positive ionspositive ions

nebuliser

Spray chamber

Detector (e.g. electron multiplier)

Sorted by Sorted by mass analyser, mass analyser, e.g. quadrupole, e.g. quadrupole, magnetic sector, magnetic sector, according to m/z ratioaccording to m/z ratio

http://www.cee.vt.edu/ewr/environmental/teach/smprimer/icpms/icpms.htm

Under vacuum

sample

Interface

55

ICP Torch

Mass Analyser:Quadrupole

Collision cellDetector(discrete dynode)

Cones and Ion Optics

ICP-MS instrumentationICP-MS instrumentation

Modern instrument with collision/reaction cell

Sampling cone

Skimmer cone

Recap: Ion formation in an Recap: Ion formation in an inductively-coupled plasmainductively-coupled plasma

Mostly, singly Mostly, singly charged positive charged positive ions are ions are generated (>90% generated (>90% efficiency)efficiency)

Interface and ion opticsInterface and ion optics

Major challenge in instrumentation: large differences in temperature and pressure. Interface Major challenge in instrumentation: large differences in temperature and pressure. Interface (consisting of two cones) allows connecting ion source to mass analyser (requires vacuum)(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 Lens focuses ions. Necessary for getting as many ions as possible into analyser (maximising signal)signal)

6000 K, ambient pressure

Room temperature,vacuum

ICP torch

Mass analysers for ICP-MSMass analysers for ICP-MS Quadrupole: High mass stability, fastQuadrupole: High mass stability, fast

Lowest cost optionLowest cost option Time-of-Flight (rare)Time-of-Flight (rare) HR (High-resolution): Uses magnetic sector mass HR (High-resolution): Uses magnetic sector mass

analyseranalyser Highest sensitivity and resolution, but slow and requires Highest sensitivity and resolution, but slow and requires

stable working environmentstable working environment ExpensiveExpensive

Multi-collector (MD): Also with magnetic sector, but with Multi-collector (MD): Also with magnetic sector, but with detector detector arrayarray Good for accurate and precise isotope Good for accurate and precise isotope ratiosratios Isotope dilution measurements – e.g. for accurate elemental Isotope dilution measurements – e.g. for accurate elemental

ratiosratios

Quadrupole mass analyserQuadrupole mass analyser Four parallel metal Four parallel metal

rods with dc and ac rods with dc and ac voltage (alternating voltage (alternating with radiofrequency)with radiofrequency)

Works as mass filter: Works as mass filter: allows passage of allows passage of particular m/z ions particular m/z ions onlyonly

Can scan over m/z Can scan over m/z range range spectrum spectrum

Atomic mass spectraAtomic mass spectra

Ray and Hieftje, J. Anal. At. Spectrom., 2001, 16, 1206-1216

http://www.wcaslab.com/tech/tbicpms.htm

Typical detection limits of ICP-MS Typical detection limits of ICP-MS instrumentinstrument

http://las.perkinelmer.co.uk/content/TechnicalInfo/TCH_ICPMSThirtyMinuteGuide.pdf

Multi-collector mass analyserMulti-collector mass analyser Magnetic sector mass analyser separates ions according to m/zMagnetic sector mass analyser separates ions according to m/z Simultaneous detection withSimultaneous detection with

array of collectors (Faraday cups) array of collectors (Faraday cups) Best for detn. of isotope ratiosBest for detn. of isotope ratios Applications in geochemistryApplications in geochemistry

and biomedical research and biomedical research

Possible factors that can affect Possible factors that can affect the performance of ICP-MSthe performance of ICP-MS

Variations in plasma ionization efficiency Variations in plasma ionization efficiency Possible clogging or corrosion of cone Possible clogging or corrosion of cone

apertures apertures Differing concentrations of other components Differing concentrations of other components

in matrix (e.g. acid, bulk elements) in in matrix (e.g. acid, bulk elements) in samples could result in samples could result in matrix suppressionmatrix suppression Ion current influenced by matrix compositionIon current influenced by matrix composition

Temperature and humidity fluctuations in the Temperature and humidity fluctuations in the laboratory environment laboratory environment

Isobaric elemental and polyatomic Isobaric elemental and polyatomic interferences: Used to be greatest limitation interferences: Used to be greatest limitation for applicability for applicability

Polyatomic interferences Polyatomic interferences in ICP-MS: Originsin ICP-MS: Origins

Spectral interference:Spectral interference:caused by presence caused by presence of species with same of species with same mass as analytemass as analyte

Often derived from Often derived from compounds with Arcompounds with Ar

Analyte Interference 39K+ 38Ar1H+

40Ca+ 40Ar+

51V+ 35Cl16O+

52Cr+ 40Ar12C+

56Fe+ 40Ar16O+

63Cu+ 23Na40Ar+

75As+ 40Ar35Cl+

80Se+ 40Ar2+

Overcoming polyatomic Overcoming polyatomic interferences:interferences:

Collision/reaction cells (CRC technology)Collision/reaction cells (CRC technology) Various modes of action:Various modes of action:

Collision-induced dissociation (less important)Collision-induced dissociation (less important) Chemical reaction (major mechanism)Chemical reaction (major mechanism) Electron transfer (major mechanism)Electron transfer (major mechanism) KED: kinetic energy discrimination (monoatomic analyte and interfering KED: kinetic energy discrimination (monoatomic analyte and interfering

molecules are retarded differently)molecules are retarded differently) Can either affect analyte or interferenceCan either affect analyte or interference Commonly used gases: He, HCommonly used gases: He, H22, ammonia , ammonia

http://breeze.thermo.com/collisioncells/(Webinar)

Requires reactive gas

Polyatomics and high-resolution ICP-Polyatomics and high-resolution ICP-MSMS

also no problem with polyatomics, as also no problem with polyatomics, as there are small, resolvable differences in there are small, resolvable differences in mass:mass:

31P

15N16O

14N16O1H

30.95 31.00Mass (u)

32S

16O16O

31.95 32.00

Stable isotopes and their usesStable isotopes and their uses

Most elements have more than one Most elements have more than one isotopeisotope

E.g. E.g. 3232S and S and 3434S, or S, or 5656Fe and Fe and 5757FeFe Can use more than one mass for one Can use more than one mass for one

element for measurements in ICP-MSelement for measurements in ICP-MS IDSM: Isotope dilution mass IDSM: Isotope dilution mass

spectrometry: Use particular isotope of spectrometry: Use particular isotope of desired analyte as internal standard in desired analyte as internal standard in ICP-MSICP-MS

Can buy enriched compounds, e.g. Can buy enriched compounds, e.g. 6767ZnO, ZnO, and use as “tracers”and use as “tracers”

Example for use of stable Example for use of stable isotopesisotopes

• Metal-binding protein with 4 Zn(II)• Are all four zinc ions exchangeable ?

• Isolated with natural abundance Zn(II):

Isotope

%

• Incubated overnight at 37°C with 40 mol equivalents of 67Zn(II) (93% isotopic purity)

• Measured isotopic ratios

0

10

20

30

40

50

60

64 66 67 68 70

67Zn: 4.1%

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: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)

Measurement and outputMeasurement and output(Thermofinnigan Element2)(Thermofinnigan Element2)

S: Zn ratio: 9:4 (as expected; the protein contains 9 sulfurs)

Comparison of experimental and Comparison of experimental and calculated isotopic ratioscalculated isotopic ratios

For each isotopic ratio, results agree best with the scenario for 3 For each isotopic ratio, results agree best with the scenario for 3 exchanging zinc: Clear demonstration that only 3 out of 4 Zn exchange:exchanging zinc: Clear demonstration that only 3 out of 4 Zn exchange:

The protein has one zinc that is inert towards exchangeThe protein has one zinc that is inert towards exchange

0

2

4

6

8

10

12

14

16

1 2 3 4

As measured

Calculated for 4 exchanging Zn(II)

Calculated for 3 exchanging Zn(II)

66/64 67/64 68/64 70/64

ICP-MS and hyphenationICP-MS and hyphenation

ICP-MS can be coupled with a variety of ICP-MS can be coupled with a variety of separation techniques:separation techniques:

Liquid chromatography Liquid chromatography HPLC-ICP-MS HPLC-ICP-MS Capillary electrophoresis Capillary electrophoresis CE-ICP-MS CE-ICP-MS Advantages of hyphenated techniques: Advantages of hyphenated techniques:

better control over matrixbetter control over matrix Allows separation of different components: direct Allows separation of different components: direct

access to access to speciationspeciation Laser ablation Laser ablation LA-ICP-MS LA-ICP-MS

For surface analysisFor surface analysis For materials that are difficult to digest (e.g. alloys)For materials that are difficult to digest (e.g. alloys) Is being developed in scanning fashion with Is being developed in scanning fashion with m m

spatial resolution: Imaging the metal composition of spatial resolution: Imaging the metal composition of a materiala material

Caveat: Calibration ?Caveat: Calibration ?

Laser ablationLaser ablation

Laser

To ICPCarrier gas in

monitor camera

UV light

sample

Useful for surface analysis of solid samples

The ablation processThe ablation process

Plume of molecules and ionsfrom a surface hit by a laserhttp://kottan-labs.bgsu.edu/pictures/

Comparison: AAS, ICP-OES, and ICP-Comparison: AAS, ICP-OES, and ICP-MSMS

AAS: Single element, ppm/ppb rangeAAS: Single element, ppm/ppb range Cheap, simple Cheap, simple Small dynamic rangeSmall dynamic range GFAAS about 100 times more sensitive than FAAS, GFAAS about 100 times more sensitive than FAAS,

but also more challengingbut also more challenging ICP-OES: Multi-element, ppb rangeICP-OES: Multi-element, ppb range

Limited spectral interferences, good stability, low Limited spectral interferences, good stability, low matrix effectsmatrix effects

ICP-MS: Multi-element, possible to reach ppt ICP-MS: Multi-element, possible to reach ppt (or even ppq)(or even ppq) Most complex, Most complex, most expensivemost expensive, lowest detection , lowest detection

limits, isotope analysis possiblelimits, isotope analysis possible

Comparison: Detection limits Comparison: Detection limits and working rangesand working ranges

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

Synopsis:Synopsis:Interferences in atomic Interferences in atomic

spectroscopyspectroscopyTechnique Type of Interference

Method of Compensation

Flame AAS

IonizationChemical

Physical

Ionization buffersReleasing agent or nitrous oxide-acetylene flame Dilution, matrix matching, or method of additions 

Graphite Furnace AAS

Physical and chemicalMolecular absorptionSpectral

Spectral Standard Temperature Platform Furnace (STPF), conditions, standard additionsZeeman or continuum source background correctionZeeman background correction

ICP-OES Spectral

Matrix

Background correction or the use of alternate analytical linesInternal standardization

ICP-MS Spectral

Matrix

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

http://las.perkinelmer.com/content/relatedmaterials/brochures/bro_atomicspectroscopytechniqueguide.pdf

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

A technique decision matrix

Other inorganic mass spectrometry Other inorganic mass spectrometry methodsmethods

Mainly for surface analysis (depth profiling, imaging) in Mainly for surface analysis (depth profiling, imaging) in different materials (e.g. conducting, semiconducting, and different materials (e.g. conducting, semiconducting, and nonconducting solid samples; technical, environmental, nonconducting solid samples; technical, environmental, biological, and geological samples) biological, and geological samples)

Spark source mass spectrometry (SSMS)Spark source mass spectrometry (SSMS) Glow discharge mass spectrometry (GDMS) Glow discharge mass spectrometry (GDMS) Laser ionization mass spectrometry (LIMS)Laser ionization mass spectrometry (LIMS) Thermal ionization mass spectrometry (TIMS)Thermal ionization mass spectrometry (TIMS) Secondary ion mass spectrometry (SIMS): most sensitive Secondary ion mass spectrometry (SIMS): most sensitive

elemental and isotopic surface analysis techniqueelemental and isotopic surface analysis technique Sputtered neutral mass spectrometry (SNMS) Sputtered neutral mass spectrometry (SNMS)

Detection limits for the direct analysis of solid samples by Detection limits for the direct analysis of solid samples by inorganic solid mass spectrometry: down to ppb levelsinorganic solid mass spectrometry: down to ppb levels

SIMS: secondary ion mass SIMS: secondary ion mass spectrometryspectrometry

Principle: bombard surface with ions, Principle: bombard surface with ions, “secondary” ions are sputtered from “secondary” ions are sputtered from surfacesurface

High sensitivity for all elementsHigh sensitivity for all elements Any type of material that can stay Any type of material that can stay

under vacuum (insulators, semiconductors, under vacuum (insulators, semiconductors, metals) metals)

Potential for high-resolution imagingPotential for high-resolution imaging(down to 40 nm)(down to 40 nm)

Very low background: high dynamic range Very low background: high dynamic range (more than 5 decades)(more than 5 decades)

Quantitative work complicated by variations Quantitative work complicated by variations in secondary ion yields in dependence on in secondary ion yields in dependence on chemical environment and the sputtering chemical environment and the sputtering conditions (ion, energy, angle)conditions (ion, energy, angle)

Rapid deterioration of bombarded surface Rapid deterioration of bombarded surface Static SIMS:Static SIMS: Molecular and elemental Molecular and elemental

characterisation of top monolayer characterisation of top monolayer

One of the most widespread surface analysis techniques for One of the most widespread surface analysis techniques for advanced material research advanced material research

• Dynamic SIMS:Dynamic SIMS: Bulk composition or depth distribution Bulk composition or depth distribution of trace of trace elements. Depth resolution ranging from one to 20-30 elements. Depth resolution ranging from one to 20-30 nm nm