Walczyk, Thomas

RERO DOC Digital Library

When J.J. Thomson discovered the isotopic nature ofneon in 1912 using a self-constructed mass spectrograph[1] it was difficult to foresee what impact stable isotopeanalysis would have in the natural sciences over the fol-lowing decades. By using isotopically enriched compounds,quantitative analysis can be performed at the highest ac-curacy by following isotope dilution principles. The sameprinciples can be employed to make use of stable isotopesas tracers in complex systems including the human body.Information on the origin and history of a sample can beobtained by studying natural changes in the isotopic com-position of an element induced either by radionuclide de-cay or by isotope-selective transport processes betweenphysical or chemical compartments. In either case, im-provements in precision, sensitivity and accuracy in iso-topic analysis have in the past opened new doors in basicand applied research.Among the plethora of mass spectrometric techniquesdeveloped in the 20th century, thermal ionization massspectrometry (TIMS) has established itself as the refer-ence technique for high-precision isotope ratio measure-ments of the heavier elements. As an example, most IUPACrecommendations for the standard atomic weights arebased on isotope abundance measurements by TIMS [2].The potential of inductively coupled plasma mass spec-trometry (ICP-MS) for isotopic analysis was recognizedsoon after it was launched as a new tool for elementalanalysis in the 1980s [3]. However, it took years to makeuse of this potential by overcoming major technical limi-tations immanent to the plasma ion source. Besides theion species of interest, other atomic and molecular ionsare formed in the plasma which can potentially interferewith the ion signals of interest. By using a double-fo-cussing magnetic sector field, mass spectrometric resolu-tion can be increased to resolve molecular isobaric inter-ferences in the mass spectrum [4]. Other approaches de-veloped so far involve the suppression of interfering mo-lecular ions either by collision with a residual gas in a col-lision cell [5], gas-phase reactions in a dynamic reactioncell [6] or a careful control of plasma conditions. By us-ing, in addition, a multicollector device (MC-ICP-MS) forion detection, another limitation of the plasma source forisotopic analysis could be overcome [7]. Flickers in signalintensity are significant in ICP-MS and limit the precisionin the isotope ratio measurement when jumping betweenpeaks. In MC-ICP-MS, these flickers simply cancel out asions are detected simultaneously.Recent instrumental developments made MC-ICP-MScompetitive to TIMS as the reference technique in thefield. MC-ICP-MS allows isotope ratio measurements forvarious elements at precisions below 50 ppm, which waspreviously only possible by using TIMS. At the sametime, isotopic analysis is less time consuming. For TIMSanalysis, the sample has to be separated from the matrix,presented in a solid state to the mass spectrometer andheated carefully to obtain a stable ion beam. On the otherhand, by using a plasma ion source, the sample can bepresented directly in solution or as an aerosol to the in-strument, which can make sample digestion superfluous(e.g. when using a laser beam for direct sampling). In ad-dition, ionization efficiencies are usually higher when us-ing ICP-MS which makes it even more sensitive. Consid-ering the obvious advantages of MC-ICP-MS over TIMSit seems as if TIMS has become superfluous. In recentyears more than 80 MC-ICP-MS systems have been soldworldwide. Surprisingly, about 60 MC-TIMS instrumentshave also been sold in the same period and at a similarprice. Is this just a belief in traditions or is scepticism grow-ing as to whether MC-ICP-MS can keep its promises?The precision and accuracy of an isotope ratio mea-surement by any mass spectrometric technique is limitedby mass-dependent isotopic fractionation of the sampleduring analysis. Isotope fractionation effects can be intro-duced during ion generation, mass separation or ion de-Thomas WalczykTIMS versus multicollector-ICP-MS: coexistence or struggle for survival?Anal Bioanal Chem (2004) 378 : 229–231DOI 10.1007/s00216-003-2053-4Received: 12 June 2003 / Revised: 16 July 2003 / Accepted: 17 July 2003 / Published online: 6 August 2003TRENDST. Walczyk (✉)Laboratory of Human Nutrition, Institute of Food Science and Nutrition, Swiss Federal Institute of Technology (ETH) Zurich, Seestrasse 72, 8003 Rüschlikon, Switzerlande-mail: thomas.walczyk@ilw.agrl.ethz.ch© Springer-Verlag 2003tection and often show a drift with time over the course ofthe measurement. In fact, not a single mass spectrometrictechnique exists which is free of isotope fractionation ef-fects and, therefore, delivers the true isotope ratio of thesample. Generated data can be corrected by determiningthe instrumental bias using gravimetrically prepared mix-tures of highly enriched isotopes (calibrated measurements).For elements having three or more stable isotopes, such amixture of known isotopic composition can be added tothe sample to correct for isotopic fractionation effects(double spike technique) or measured data can be normal-ized to a known isotope ratio in the sample by applyingtheoretically or empirically derived correction laws. Ingeneral, TIMS is less susceptible to isotopic fractionationthan ICP-MS. Fractionation effects in TIMS are usually inthe order of parts per thousands, relatively stable and sys-tematic, while isotopic fractionation in ICP-MS, commonlyreferred to as mass bias, can be easily in the percent rangeand depend on instrument settings and time [8].But how can isotope ratios be measured by MC-ICP-MSat a precision and, ideally, accuracy of 50 ppm or better ifthe mass bias is higher by 4 orders of magnitude and evenfluctuating? Different techniques are in use including a) internal normalization of measured isotope ratios [9]and double-spiking [10] as employed in TIMS, b) stan-dard sample bracketing techniques using an isotope refer-ence material [11] and c) external normalization to aknown isotope ratio of an element standard that has beenadded to the sample before analysis [12]. Although inprinciple all of these techniques are suitable for fractiona-tion correction, they have their limitations. For accuratedata correction, isotopic fractionation processes and frac-tionation laws have to be understood, at least empirically,and have to be independent from the sample matrix, ana-lyte concentration and instrument settings. This is not thecase in most situations. To perform isotope ratio measure-ments by MC-ICP-MS which are not only tremendouslyprecise but also accurate, the element has to be separatedas tediously from the sample matrix as in TIMS. This isnecessary not only for a better control of isotopic frac-tionation during the measurement but also to avoid arte-facts due to isobaric interferences which cannot be resolvedinstrumentally. It has to be noted that instrumental ap-proaches in ICP-MS made so far reduce the risk of biasdue to isobaric interferences but they still remain a majorobstacle in generating meaningful isotopic data.Even if all efforts have been made to minimize bias,use of isotopic reference materials is mandatory. How-ever, it is a severe misconception to assume that a MC-ICP-MS technique is validated only by showing thatgenerated data agree well with certified or commonly agreedreference values and that referring to a published valida-tion study is sufficient for claiming data accuracy. Whatwas found to work for one application/instrument/samplemight not be given in another situation, with externalmass bias correction being a good example. Furthermore,sources of systematic bias in ICP-MS are multiple andbias effects can cancel out when analysing a referencematerial. At least two reference materials of different iso-topic composition are needed to minimize the risk of anartefact. To date, no sets of isotopic reference materialsfor the ICP-MS community exist, with the exception ofuranium (IRM-072, IRM-073) [13]. If a certified isotopicreference material is available, which is not given for anumber of elements, this material commonly does notmatch the matrix of the samples to be analysed. This pointsagain to the need of matrix separation to perform accurateisotope ratio measurements by MC-ICP-MS.Quality control appears to become a major issue whenlooking at the potential risks of generating artefacts whenusing ICP-MS for isotopic analysis and the steadily grow-ing number of scientific conclusions that are drawn fromgenerated data. This refers in particular to applicationswhere findings may affect legal or political decisions, forexample when isotope ratio measurements are performedin food authentication, forensic analyses or nuclear safe-guarding. In biomedical research, wrong conclusions mayaffect the individual when findings are translated into pub-lic health programs, for example when stable isotope tech-niques are employed for the design and/or evaluation of agovernment-led food fortification program. For any ofthese applications, it is not the value of an isotope ratio it-self but the uncertainty of the measurement that decideswhat action should be taken. This requires more than asimple statement regarding the precision of the measure-ment. Possible sources of systematic bias have to be eval-uated carefully and considered by expanding the uncer-tainty interval of the measurement.To date, no commonly agreed guidelines exist on howto validate an ICP-MS technique for isotopic analysis andwhat measures of quality control should be taken when thetechnique is used routinely for sample analysis. As an ex-ample, three isotope plots are a useful tool to check for theabsence of isobaric interferences but their use is not oblig-atory to prove data accuracy. For elements having at leastthree stable isotopes, two isotope ratios can be measuredwith a common reference isotope. When analysing an iso-topic reference material of known isotopic composition,measured isotope ratios can be expressed on a δ-scale (i.e.as their relative deviation from the reference value in partsper thousand). When isotope ratios are plotted for the ref-erence material against each other, as illustrated in Fig. 1for iron, they should lie on a curve that is defined by theunderlying isotope fractionation law which should dependon the isotope masses. Offsets from the curve indicate thepresence of isobaric interferences. The same holds for thesample provided that the isotopic abundances are not af-fected by radioactive decay. Even if the element in thesample shows an altered isotopic composition due to nat-ural fractionation processes, measured data have to lie onthe fractionation curve. There is some debate as to whichof the proposed laws describe fractionation processes thebest [14]. However, differences between the fractionationlaws are usually too small in practice to be resolved withinthe achievable precision of the isotope ratio measurement.Three isotope plots are just one example of the possi-ble approaches to confirm the accuracy of measured data.For independent evaluation of published data by the sci-230entific community, data sets from quality control mea-surements and experiments have to be accessible to thereader. This is usually not the case. A lack of consent withinthe community, limited publication space in scientific jour-nals and a lack of stringent guidelines for data publicationby journal editors often leave the reader with due scepti-cism if data are trustworthy and scientific conclusions canbe justified. Unless firm quality control protocols are es-tablished for MC-ICP-MS measurements, TIMS will, there-fore, remain as the reference technique for isotope ratiomeasurements of the heavier elements. However, even ifthese standards are hopefully established in the comingyears, TIMS will still have its place in inorganic massspectrometry. The gap between TIMS and MC-ICP-MSnarrows in practical terms when taking into account thedifferent precautions necessary to minimize the risk of gen-erating artefacts using MC-ICP-MS. Chemical separationof the element from the matrix and the need for variousquality control tests make measurements by MC-ICP-MSnearly as time consuming as by TIMS. Thus, other factorsbecome decisive such as robustness, running costs andelement-specific advantages. As an example, the highionization potential of hafnium makes MC-ICP-MS moresuitable for this element [12]. In contrast, osmium isotoperatio measurements are technically less demanding to per-form by TIMS at higher sensitivity and precision bybleeding oxygen into the ion source [15, 16]. Withoutquestion, MC-ICP-MS has opened new doors in science,in particular for studying small natural isotope fractiona-tion effects for the heavier elements such as iron, whichare difficult to measure using TIMS [17]. However, it is inthe hands of the MC-ICP-MS community to make surethat it is used in the future as a key to a palace rather thana maze.References1. Thomson JJ (1912) Phil Mag 24:6682. Coplen TB (2001) J Phys Chem Ref Data 30:7013. Houk RS, Fassel VA, Flesch GD, Svec HJ, Gray AL, TaylorCE (1980) Anal Chem 52:22834. Giessmann U, Greb U (1994) Fresenius J Anal Chem 350:186–1935. Turner P, Merren T, Speakman J, Haines C (1997) In: HollandG, Tanner SD (eds) Special publication No 202. Royal Societyof Chemistry , Cambridge, pp 28–346. Baranov VI, Tanner SD (1999) J Anal Atom Spectrom 14:11337. Walder AJ, Platzner I, Freedman PA (1993) J Anal At Spec-trom 8:198. Heumann KG, Gallus SM, Rädlinger G, Vogl J (1998) J AnalAt Spectrom 13:10019. Vance D, Thirlwall M (2001) Chem Geol 185:22710. Siebert C, Nägler TF, Kramers JD (2001) Geochem GeophysGeosyst 2:2000GC00012411. Belshaw NS, Zhu XK, Guo Y O’Nions RK (2000) Int J MassSpectrom 197:200012. Rehkämper M, Halliday AN (1998) Int J Mass Spectrom 181:12313. Taylor PDP, deBièvre P, Walder AJ, Entwistle A (1995) J AnalAt Spectrom 10:39514. Maréchal CN, Télouk P, Albarède F (1999) Chem Geol 156:25115. Walczyk T, Hebeda EH, Heumann KG (1991) Fresenius J AnalChem 341:53716. Reisberg L, Meisel T (2002) Geostandard Newsletter 26:24917. Walczyk T, Blanckenburg F (2002) Science 295:2065231Fig. 1a–c Use of three isotope plots to check for isobaric inter-ferences in MC-ICP-MS. Each point represents the mean of aniron isotope ratio measurement of a standard (E) or a sample(❍) of natural isotopic composition. Isotope ratios (56Fe/54Fe and57Fe/54Fe, respectively) are plotted on a δ-scale (δ56Fe and δ57Fe,respectively), i.e. as relative deviations in parts per thousandfrom the known isotope ratio of an isotope reference material ofnatural isotopic composition (IRM-014). The diagonal line rep-resents the theoretical fractionation curve as defined by the iso-tope masses and an exponential fractionation law. a Absence ofisobaric interferences. Data points from standard and sample ploton the theoretical curve. b At least one isotopic signal in themass spectrum of the standard and the sample is interfered.c Matrix differences between standard and sample result in anoffset of the sample data points from the theoretical fractionationcurve due to isobaric interferences

TIMS versus multicollector-ICP-MS: coexistence or struggle for survival?

http://doc.rero.ch/record/319488/files/216_2003_Article_2053.pdf

TIMS versus multicollector-ICP-MS: coexistence or struggle for survival?

Abstract

Similar works

Full text

Available Versions

RERO DOC Digital Library