È special feature tutorial glow discharge mass spectrometry… · 1998-08-08 · glow discharge...

JOURNAL OF MASS SPECTROMETRY, VOL. 30, 1061È1075 (1995)

SPECIAL FEATURE :TUTORIAL¤

Glow Discharge Mass Spectrometry : TraceElement Determinations in Solid Samples

F. L. King, J. Teng and R. E. SteinerDepartment of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045, USA

Glow discharge mass spectrometry (GDMS) is a mature, versatile technique for the direct determination of traceelements in a variety of materials. The technique is an extension of the earliest forms of mass spectrometry.Processes inherent to the glow discharge, namely cathodic sputtering coupled with Penning ionization, yield an ionpopulation from which semi-quantitative results can be directly obtained. QuantiÐcation in GDMS is achieved boththrough standard elemental mass spectrometric procedures and more innovative approaches. The analytical per-formance of GDMS compares favorably with competing elemental mass spectrometric methods and newer experi-ments use this ionization method for both molecular and elemental analysis. As with any analytical technique, thefuture of GDMS lies in improvements with respect to instrumental implementation and extension to new areas ofapplication. If the method is to remain competitive, commercial GDMS systems must incorporate advances inmass spectrometric technology to increase analytical performance while decreasing the size, complexity and cost ofthe technique. Continued e†orts to develop improved quantitation procedures are needed to provide greater accu-racy. The method should continue to mature as sustained e†orts demonstrate its utility in the solution of new andmore varied problems.

INTRODUCTION

Like most mass spectrometric methods, glow dischargemass spectrometry (GDMS) is a versatile tool with arange of applications spanning many disciplines.Whereas the analytical chemistry community viewsGDMS as a method for the direct determination oftrace elements in solid state materials,1 the materialsscience community often thinks of glow discharge massspectrometry as a diagnostic tool for the character-ization and control of reactive and non-reactive deposi-tion plasmas.2 In fact, members of the materials sciencecommunity were the Ðrst to recognize the potential forGDMS as an analytical method capable of providinginformation regarding the composition of solidsamples.3,4 This tutorial focuses on the realization ofGDMS as an analytical chemistry tool. The reader

¤ EditorÏs Note : This is the second of several “TutorialÏ articleswhich will appear this year in the Special Features section of thejournal. These tutorials will describe fundamental aspects and applica-tions of mass spectrometry with the general reader, not the authorÏspeer group, in mind. The aim will be to cover some speciÐc areas ofmass spectrometry in a manner in which a teacher might present thesubject in a graduate level course. Although these articles are nor-mally invited, comments and suggestions from readers are welcome.Please address these to the Special Features Coordinator ; GrahamCooks, Dept of Chemistry, Purdue University, W. Lafayette, IN,USA, 47907. Further, as a special o†er to readers, copies of the Ðguresof this “TutorialÏ article, as color slides, are available free of charge onrequest from the editor-in-chiefÏs office. Supplies are limited and slideswill be sent out in the order requests are received.

interested in plasma monitoring GDMS should refer toreviews such as those listed in Ref. 2.

The Ðrst report of GDMS as a true analytical chem-istry technique appeared in 1974,5 and in the sub-sequent years it has matured as an analytical technique.The steady increase in publications that occurred overthe past 25 years, shown in Fig. 1, reÑects the growinginterest in GDMS. Since 1975 the volume of pub-lications in this Ðeld has nearly doubled every 5 years.

Figure 1. GDMS publication vs. 5-year time periods, 1970–94.Publications in GDMS have doubled every 5 years from 1975 tothe present. The relatively small number of total publications isreflective of the specialized nature of GDMS.

CCC 1076È5174/95/081061È15 Received 11 April 1995( 1995 John Wiley & Sons, Ltd. Accepted 19 May 1995

1062 F. L. KING, J. TENG AND R. E. STEINER

Although the total number of publications seems small,this observation is more indicative of the techniqueÏsspecialized nature than it is reÑective of its impact.GDMS has carved out an important niche where theavailability of electronic and optical materials of thegreatest possible purity is important. It is GDMS thatoften provides certiÐcation of these materials at therequired purity levels. In recent times, research hasfocused on extending the applicability of GDMS toinclude environmental monitoring of soils6 and ambientatmospheres,7 the characterization of organic and bio-logical compounds,8 the identiÐcation of impurities inpetroleum9 and the examination of polymers.10

Through this tutorial we seek to present the readerwith an overview of analytical GDMS. In keeping withthe educational focus of this feature, the overviewbegins with a brief description of the historical relation-ship between plasma spectroscopy and mass spectrom-etry. The roots of GDMS are found in the early historyof mass spectrometry. An introduction to the glow dis-charge and its inherent atomization and ionization pro-cesses provides the background necessary to appreciatethe analytical capabilities of this ion source. Require-ments for a generic GDMS system frame the variousinstrumental approaches to the implementation ofGDMS. Methods of quantiÐcation and degrees towhich it is achieved are summarized with a workedhypothetical GDMS analysis. The relation of GDMS toother elemental mass spectrometric methods is exam-ined to deÐne better its realm of application. Althoughthis tutorial concludes with some representative appli-cations of GDMS, the interested reader should refer torecent technical reviews and the current literature fordetailed information regarding applications.

HISTORICAL PERSPECTIVE

“It will be seen that of the 80 non-reactive elementsnow known to exist, 53 have been analyzed and theirconstitutions, isotopic or otherwise, determined infour and a half yearsÏ work. Failing the discovery ofnew and more general methods of producing massrays, it is not to be expected that progress with theremaining ones will be so rapid, but it seems reason-able to hope that in a comparatively moderate timeevery stable atomic species existent in any consider-able quantity on earth will have been identiÐed andweighed.Ï

F. W. Aston, in Isotopes, 192414This passage is merely a remainder that the mass

spectrometric determination of the elements and theirisotopes is not a recent development. The modern shiftof trace element analysis from optical spectrometry tomass spectrometry is a replay of history. Mass “spectros-copyÏ originated as a tool for investigations into the for-mation and characterization of the positive, or anode,rays produced in “ordinaryÏ gas tubeÈinvestigations forwhich optical spectroscopic techniques would notsuffice. Because the origins of modern GDMS date tothe adolescence of mass spectrometry, a short recap ofthis history is in order.

In 1886,11 there appeared a report describing lumi-nous streamers arising from rays that had passedthrough perforations in a cathode and travelling in theopposite direction to the cathode rays. Because the raysgiving rise to these streamers were found to possesspositive charge, Thomson termed them positive rays.12Today we know, in large part because of work thatbegan in ThomsonÏs laboratory, that these positive rayswere loosely focused ion beams. Using the parabolamethod, Thomson embarked upon investigations intothe nature of positive rays.12 Dempster followed upthese investigations employing an instrument equippedwith focusing slits to resolve the components of positiverays more clearly.13 This instrument of DempsterÏs wasthe Ðrst that could be called a mass spectrograph.14ThomsonÏs assistant, Aston, soon reported on the devel-opment of a mass spectrograph that achieved improvedresolution through velocity and direction focusing.15This instrument and its descendants were employed inAstonÏs investigations of positive rays that led to thecharacterization of isotopes.14,16 Aston, employing elec-trical discharge ionization, and Dempster, employingthermal ionization, pioneered techniques that evolvedinto modern methods for trace element determinationssuch as plasma source and thermal ionization massspectrometry.17

The early work by Thomson revealed an importantfact regarding the spectra of positive (or mass) raysÈnot only were these spectra characteristic of the dis-charge support gas, but also they were considerably lesscomplex than the corresponding optical spectra.Because mass spectrometry exhibited both high sensi-tivity and spectral simplicity, its combination with amethod of generating a large, representative populationof sample ions made it a powerful tool for traceanalyses. Although ion generation from gaseoussamples was relatively straightforward using electronbeams, the generation of representative sample ionsfrom solid materials presented more of a challenge. Thischallenge was met by the glow discharge because it con-veniently yielded ionized sample atoms not only fromthe discharge support gas but also from contiguoussolid state materials.

Until the advent of GDMS, trace element analyses ofsolid-state materials were achieved by means of spark-source mass spectrometry (SSMS).18 SSMS employs anenergetic spark for the thermal atomization and ioniza-tion of sample material. Mass spectrometric monitoringof the resulting ions yields detection limits in the partsper billion (ppb) range. However, the spark provides arelatively unstable ion beam characterized by a widedistribution of ion kinetic energies. Although signalintegration and double-focusing mass spectrometricsystems were employed to alleviate these undesirableattributes to SSMS, new methods were sought thatwould improve analytical performance and minimizeinstrumental complexity.

Secondary ion mass spectrometry (SIMS)19 appearedto be the most likely successor to SSMS, but, it too wasnot without limitations. The principal limitations, largevariations in elemental sensitivity and severe matrixe†ects, arise through the direct production of analyteions from the solid-state sample.20 In SIMS, the bom-bardment of the sample surface by a primary beam of

GLOW DISCHARGE MASS SPECTROMETRY 1063

ions directly sputters neutral and ionized analyte atoms.Mass analysis of the secondary ions provides analyticaldata for the determination of elemental concentrationsin the sample. The chemical nature of the matrix con-trols not only the rate at which sample atoms are rel-eased by the sputtering process, but also the extent towhich these atoms are released as ions. Clearly, separa-tion of atomization and ionization into discrete stepswould be expected to improve analytical capabilities.

Many groups have investigated the use of auxiliaryionization of the neutral atom population generated byion beam sputtering.20 This auxiliary ionization couldincrease the analytical signal, because the sputteredneutral population greatly exceeded the sputtered ionpopulation, and reduce matrix e†ects because of theseparation of ionization and atomization. The resultingtechnique was termed sputtered neutrals mass spec-trometry, SNMS.21 Interestingly, one of the principalauxiliary ionization methods employed in SNMS wasthe use of an r.f.-powered glow discharge plasma.22 Theuse of the glow discharge allowed SNMS operationwith or without the primary ion beam needed in SIMS.SNMS was found to exhibit reduced matrix e†ects andsmaller variations in elemental sensitivity across thePeriodic Table. Analytical GDMS is a close relative ofboth SIMS and SNMS. In all three methods the solidsample is atomized by sputtering. Similarly to SNMS,GDMS reduces variations in elemental sensitivity bydecoupling the atomization and ionization processes.To some extent GDMS can be considered as a varia-tion of SNMS in which the plasma generates theprimary bombarding ions.

THE GLOW DISCHARGE

The development of a potential di†erence between twoelectrodes immersed in a low-pressure inert gas environ-ment results in an electrical discharge.23 Under certainconditions of pressure, current and voltage, a brilliantdischarge is established. Close observation of this dis-charge reveals the existence of various regions. A faintcathode glow region is directly adjacent to the nega-tively biased cathode. Moving away from the cathode,there exists a non-luminous cathode dark space. Thedark space extends one mean free path from thecathode glow region and hence is a region essentiallyfree from collisional excitation or recombination involv-ing electrons. In this dark region, where 80% of the dis-charge potential is dissipated, the cathode fall, electronsand ions undergo maximum acceleration and reachtheir maximum kinetic energy. At distances greater thanone mean free path from the cathode, collisional excita-tion and ionization processes abound. Radiative relax-ation of electronically excited species in this regionresults in the characteristic negative glow of the dis-charge. This glow is familiar as the light emitted from aconventional neon lamp. Glow discharges modiÐed toenhance the emission from the negative glow such asthe hollow-cathode glow discharge24 and Grimm glowdischarge25 lamps are familiar in atomic absorption andatomic emission spectrometry. A spectroscopic study ofanalytical glow discharges reveals that their optical

spectra are rich in transitions arising from excited statesof analyte ions. This observation alone suggests thepotential utility of the glow discharge as an ion sourcefor elemental mass spectrometry.

ANALYTE ATOMIZATION AND IONIZATION

In elemental mass spectrometry, it is necessary to estab-lish an ion population that is representative of thesample under study. Mass spectrometric sampling ofthis population provides the analytical signal employedin quantiÐcation. When a solution is to be sampled thepath from sample to ions is aspiration, atomization(nebulization) and ionization. Today, the inductivelycoupled argon plasma, the ICP,26 readily converts asolution sample in to gas-phase analyte ion. Solid sam-pling methods di†er in that atomization must occurdirectly from the solid state. Thermal, photon andparticle-induced desorption methods achieve such directatomization of solid-state samples.17 Ionization occursconcurrently with the atomization process in many ele-mental mass spectrometric methods such as thermalionization mass spectrometry (TIMS), laser ion-desorption mass spectrometry (LIMS) and SIMS.Although the simplicity of one-step atomization/ionization is appealing, the close coupling of these fun-damental processes results in the severe matrix e†ectsencountered for these methods. Such matrix e†ects arisebecause the conditions of analyte atom ionization areinextricably tied to the nature of the matrix that sur-rounds the atom at the time of ionization. Glow dis-charges achieve a decoupling of the atomization andionization processes to yield a decreased dependence ofthe analytical signal on the analyte matrix.

A schematic diagram of an analytical glow dischargeis shown in Fig. 2. The enlargement provides details ofthe atomization processes occurring at the surface of thesample cathode and of the ionization processesoccurring in the negative glow region of the plasma. Toappreciate fully the analytical capabilities of a glow dis-charge, it is necessary to understand these processes bywhich analytical signal carriers are generated.

The glow discharge ion source employs a modiÐedparticle-induced desorption approach to atomization.Inherent to the maintenance of the glow discharge is thecathodic sputtering process.27 The application of powerto the glow discharge results in the partial ionization ofthe support gas through collisions with energeticprimary electrons, viz. in electrical breakdown. Uponformation, the positively charged discharge gas ionsbegin to accelerate towards the negatively biasedcathode. Impact of these primary ions on the cathodesurface results in the deposition of their kinetic energyinto the solid lattice. This increase in the energy of thelattice is sufficient to release near-surface atoms, clustersand secondary electrons. The sputter atomizationprocess in the glow discharge is similar to the release ofsample species in SIMS and fast atom bombardmentmass spectrometry (FABMS).28

In SIMS and FABMS, the analytical signal carriersare that small fraction of sample species that are sput-tered as secondary ions. This is not the case for GDMS


Figure 2. Schematic GDMS probe tip. Inset : atomization and ionization processes. Bombardment of the sample cathode by argon ions(Ar½) formed in the negative glow results in the release of sample atoms, M0, via sputtering. Any sputtered secondary ions redeposit on thesample surface owing to the electric field. M0 escape into the negative glow region where collisions with electrons, eÉ, and metastableargon atoms, Ar*, result in their ionization. The ionized sample atoms, M½, are then available for mass spectrometric monitoring.

because the strong negative electric bias applied to thetarget cathode rapidly returns secondary ions to thesample surface. Only neutral species sputtered from thecathode can di†use out into the plasma of the glow dis-charge. The release of neutral species in the cathodicsputtering process is dependent principally on thenature of the majority composition of the cathode, theÑux of primary discharge gas ions, the kinetic energy ofprimary discharge gas ions and the discharge gas pres-sure. Any di†erential sputtering among major cathodecomponents is o†set by enrichment of the cathodesurface in the less sputtered component. A rapid equili-bration between surface enrichment and sputter prefer-ence results in the generation of an atom population inthe plasma that is representative of the cathode com-position.

A key analytical advantage, the freedom from exces-sive matrix e†ects, arises because ionization in glow dis-charge ion sources occurs independently of sampleatomization. The population of neutral atoms sputteredfrom the sample cathode is subject to a variety of col-lisional processes in the negative glow region of the dis-charge plasma (Fig. 3). The principal mechanismresponsible for the ionization of sputtered atoms, M, inr.f.-powered glow discharges involves collisions withmetastable discharge gas atoms, Arm, Penning ioniza-tion [Fig. 3(b)] :29

M ] Arm] M`] Ar ] e~

Penning ionization a†ords an added advantage whenthe working gas is argon. The metastable excited statesof argon, Arm, lie at 11.55 and 11.72 eV. This energy isconveniently sufficient to ionize the bulk of the elementsin the Periodic Table without ionizing atmosphericimpurities in the discharge gas. Investigations of ioniza-tion in steady state d.c.-powered glow discharges indi-

Figure 3. Principal collisional ion formation processes in GDplasmas. Electron ionization (a) and Penning ionization (b) areresponsible for the generation of more than 90% of the sampleions, M½. Resonant (d) and non-resonant (e) charge exchangeaccount for a small fraction of the total ion population. Althoughassociative ionization (c) and three-body association (f) contrib-ute marginally to the total ionization in the plasma, they contributemost of the interfering metal argide ions, MAr½.


Figure 4. Block diagram of typical GDMS system components. The glow discharge atomization/ionization source, the ion optical systemand mass analyzer are all housed in a differentially pumped vacuum chamber. Operating parameters and data acquisition are controlled viasoftware that also provides data processing, storage and display.

cate that Penning ionization accounts for 40È80% ofsputtered atom ionization.30 A study of the variation ofsensitivity as a function of analyte element indicatesthat the cross-sections for ionization by the Penningprocess vary by only a factor of ten across the PeriodicTable.31 This, in part, leads to the small range of rela-tive sensitivities observed in GDMS compared with thatobserved in SIMS.

Metastable atoms can only be formed by radiativerecombination of discharge gas ions with thermal elec-

trons. The production of these species is favored in thenegative glow region of the discharge. Therefore, mostsample atom ionization occurs in the negative glowregion. In steady-state glow discharge plasmas theremainder of ionization occurs via electron collisions ofcharge-exchange processes [Fig. 3(a), (d) and (e)].Deconvolution of these processes is a†orded by coup-ling pulsed power operation of the discharge with time-resolved GDMS.32,33 This approach takes fulladvantage of the Penning process by monitoring the

Figure 5. The most common GD ion source geometries. The coaxial cathode or pin sample source was the first commercial GDMS ionsource. Its design is simple and straightforward but amenable only to samples that can be formed into pin-shaped cathodes. The planarsample source is akin to the Grimm source employed in optical emission spectrometry and has been developed for the examination of flatsamples or samples not amenable to the formation of pins.


plasma under conditions when the only available ion-ization agents are the metastable argon atoms.

INSTRUMENTATION

The block diagram shown in Fig. 4 outlines the essen-tial components of a typical GDMS system, with a PCacting as the central controller for all aspects of theinstrument. Although individual systems may varyslightly with regard to the ion source, mass analyzerand ion detection system type, they all follow thisgeneral outline. Because the glow discharge ion sourceoperates at pressures of approximately 1 Torr (1Torr \ 133.3 Pa), the vacuum system of a glow dis-charge mass spectrometer must employ di†erentialpumping to maintain a mass analyzer operating pres-sure less than 10~5 Torr. Most systems are equippedwith a high Ñow rate pump on the region between theion source and mass analyzer, where the operating pres-sure is typically on the order of 10~5 Torr. A slightlysmaller pump then suffices to maintain the massanalyzer in the 10~7 Torr range.

Glow discharges employed for the mass spectrometricdetermination of trace elements in solid-state materialscan be operated with d.c. or r.f. power. R.f.-poweredglow discharges allow the direct analysis of non-conducting solid samples,34 whereas such materialsmust be compacted with a conducting host matrix priorto analysis employed a d.c.-powered glow discharge.35Although commercially available GDMS systems exclu-sively employ the simpler d.c.-powered glow dischargeat present it is expected that r.f.-GDMS systems will bemade available to meet demands for the analysis ofnon-conducting materials.

Two glow discharge ion source geometries, shown inFig. 5, Ðnd wide application in elemental mass spec-trometry : (i) the coaxial cathode or pin samplegeometry and (ii) the planar sample or Grimm-typegeometry. Commercial systems allow for the rapidinterconversion of these two geometries, acknowledgingthat this provides the greatest versatility with respect tosample type. For both geometries samples can be intro-duced into the glow discharge ion source chamber bymeans of a direct insertion probe similar to thoseemployed in traditional organic mass spectrometry.36,37Such an approach allows faster sample turnaroundbecause the discharge chamber vacuum is not brokenbetween analyses.

The coaxial cathode geometry is most suitable tosamples that are readily converted into pins of 1È2 mmin diameter and 7È10 mm in length. This includespowder samples compacted in a die-press assemblysimilar to that used to form electrodes for spark sourcemass spectrometry. The sample in this type of source ismounted at the tip of a direct insertion probe. An insu-lating cover conÐnes the discharge to the samplecathode. These pin-type sources are the most commonlyemployed GDMS ion sources.

The principal limitation encountered with thesesources arises from the fact that sputtering erodes thesample. Pin erosion and consequent decreases in surfacearea increase current density and discharge operating

voltage for glow discharges operated at constantcurrent. As the operating voltage increases the primaryparticle kinetic energy and discharge power increase,with the result that sputter rates increase. Constantvoltage operation requires constant current densitiesand produces the opposite problem. As the cathodesurface area decreases, the current must be decreased tomaintain the constant current density. This decrease incurrent results in a decrease in ionization within theplasma and a decrease in analytical signal. Becauseoperation in constant current or constant voltagemodes results in these changes, it has been suggestedthat constant power operation would a†ord the moststable GDMS signals for pin cathodes.38

The planar sample cathode geometry is preferred forrefractory samples or when depth proÐling analysis isdesired. This Ñat geometry is also preferred when r.f.-power is employed in the direct analysis of non-conducting samples. The sample is mounted on the endof the direct insertion probe in a fashion that exposes aÑat surface. This surface seals against an O-ring on theion source. A circular area of the Ñat surface is thenexposed as the cathode for the glow discharge. With theplanar geometry the surface area of the sample remainsconstant, so the problem described for the pin sample iscircumvented. By use of a low-mass support gas such asHe or Ne, the planar geometry permits depth proÐlingof the sample. The depth resolution does not approachthat of static SIMS but can be useful for many applica-tions. It must also be considered that sample thicknessinÑuences analyses with r.f.-powered glow dischargesbecause as the thickness of the dielectric sampleincreases the impedance of the r.f. circuit containing thesample cathode changes. As the sample thicknessincreases the analytical signal will decrease if all othervariables remain constant.

Species e†use from the glow discharge source throughan ion exit aperture (slit or oriÐce, depending on massanalyzer employed) into the ion optical region. The ionoptics focus the e†using ions into a tight beam permit-ting their efficient transport into the mass analyzerregion through a sampling aperture. Without such effi-cient transport the analytical signal su†ers from a lossin intensity and a consequent decrease in analyticalsensitivity.

Most GDMS systems sample ions from the anoderegion of the plasma. In those instances where cathodicsampling has been employed, large di†erences in ionkinetic energies have been observed to exist betweendischarge gas and cathode species ions, presumablybecause whereas the cathode atoms are ionized by thePenning process in the negative glow and acceleratedacross the full cathode dark space, discharge gas atomsare ionized by electron impact in the cathode darkspace and are accelerated to a lesser extent dependingon the location of their ionization in the dark space andhence have di†erent kinetic energies. The use of akinetic energy analyzer then a†ords the ability to dis-tinguish electron ionized species from Penning ionizedspecies.39 It has also been determined that there are dif-ferences in the kinetic energy of polyatomic metalargide ions, MAr`, depending on whether they areformed by associative ionization [Fig. 3(c)] or three-body collision [Fig. 3(f )] processes.40


To date, all the common mass analyzers have beenexplored for use in GDMS (Table 1). The Ðrst com-mercially available GDMS systems employed double-focusing mass analysis systems originally employed withthermal ionization for isotope ratio measurements. Thisapproach permitted the acquisition of high-resolutionspectra with great sensitivity. However, modern GDMSbegan with quadrupole-based mass analyzer systems.Such systems were favored by those working on thedevelopment of GDMS and those performing funda-mental research. These systems a†ord a more cost-e†ective alternative to spark source mass spectrometry.The demonstrated utility of quadrupole based GDMShas resulted in the commercial availability of suchsystems from various manufacturers. Some of thesesystems are designed to allow rapid switching betweenglow discharge sources for solids analysis and induc-tively coupled plasma for solutions analysis. Promisingresults have been obtained from ion trap mass spectro-metric systems by investigators at Oak Ridge NationalLaboratory41 and from time-of-Ñight mass spectro-metric systems at Indiana University42 and West Vir-ginia University.28 The use of Fourier transform (FT)MS coupled with a glow discharge ion source wasfound to permit difficult analyses requiring great massresolving power.43 At present, the commercial GDMSsystems available for trace element analysis employ onlydouble-focusing or quadrupole mass analyzers.

Trace analysis work requires that the full sensitivityof mass spectrometry be exploited. The demands ofdynamic range (the range of concentrations that can bedetermined in a single measurement) and abundancesensitivity (the ability to determine a trace isotope (ppb)adjacent to a major isotope (%)) dictate detection anddata analysis schemes that di†er signiÐcantly from thoseemployed in standard MS instrumentation, where thefocus is more generally on identiÐcation rather thanquantiÐcation. Elemental MS methods suited to thesedemands, such as peak area integration and peakhopping, have developed as the Ðeld has evolved fromphotoplate detection to electronic detection. The needfor a large dynamic range led to the use of an automa-ted dual detection system in one commercial GDMSsystem. A Faraday cup is employed to measure currents

corresponding to major species (ion beam intensitiesdown to 10~13 A). At a measured current less than10~13 A, the data system automatically begins monitor-ing the ion beam with an electronic detection systembased on a Daly detector, in which ions are convertedinto electrons that impinge upon a phosphorescentscreen monitored with photomultiplier tube.44 The ioncounting signal from the Daly detector is calibratedagainst the ion current on the Faraday detector so thatall results are presented as ion beam currents. Thedynamic range a†orded by this approach is on theorder of 1011, allowing both major components andtrace (sub-ppb level) components to be measured in asingle cycle. More reliable quantitative results areachieved by using peak areas rather than peak inten-sities as the analytical data from which concentration iscalculated. With careful attention to all aspects of themeasurement, detection limits in range of hundreds ofppt can be achieved.

QUANTIFICATION

In element mass spectrometry, a variety of approachescan be taken to achieve quantitative results. Most ofthese methods are identical with those employed withtraditional optical spectrometric determination of ele-ments ; however, mass spectrometry also a†ords newapproaches, such as isotope dilution, not open to thetraditional optical methods of elemental determination.GDMS typically relies on two variations of internalstandard calibration for qualitative and quantitativeanalyses.

Ion beam ratio method (semi-quantitative analysis)

The most common approach to quantiÐcation inGDMS is the ion beam ratio (IBR) method. Theassumption is made that the ratio of ion current for anyone isotope with respect to the total ion current, exceptthe signal arising from the discharge support gas, is rep-resentative of the ratio of the number of atoms of that

Table 1. Mass analyzers for GDMS

Mass analyzer system Advantages Limitations

Double-focusing sector High resolution Cost

Commercial availability Speed

Quadrupole Speed Low resolution

Cost

Commercial availability

FT-ICR Highest resolution Cost

Quantification

Difficult

Quadrupole ion trap Speed Detection limits

Cost Low resolution

Size Dynamic range

Collisional dissociation

Time-of-flight Cost Detection limits

Speed Signal saturation

Resolution


Figure 6. Flow chart of typical GDMS analysis procedure. The initial sample, S, containing elements X, Y, and Z, must first be convertedinto a pin or flat cathode and loaded into the glow discharge ion source. Qualitative mass spectra are acquired to provide guidance as to therough composition of the sample. Knowledge of the general sample composition allows the selection or determination of relative sensitivityfactors (RSFs) to be employed in quantitative analysis. Examination of the qualitative spectrum allows the identification of the elements ofinterest for quantitative evaluation. Quantitative mass spectra are obtained under carefully controlled conditions. Quantitative results areobtained through the correction of analyte signals with the use of RSF values. Accuracies of 10-20% are obtained in quantitative GDMS.

isotope in the sample to the other constituent atoms ofthe sample. When trace analyses are performed on high-purity materials, the ion signal for the matrix isassumed to be large relative to individual trace species ;therefore, the matrix ion current is a good approx-imation to the total ion current and the matrix can beassumed to have a concentration of 100%. The concen-tration, C, of an isotope X is obtained from the productof the ion beam ratio and the concentration of thematrix :

CX \ (IX/IM)CMThis is a variation on internal standard calibrationwhere M, the matrix element, serves as the internalstandard. The power of this approach lies in that factthat any e†ect of plasma instability is normalizedbecause the analytical quantity is the ratio of two sput-tered species ionized by the same processes. The limi-tation to this approach, and the reason why it providesonly semi-quantitative results (accuracies of a factor of2-3 are typical), is that it cannot correct for the varia-tion in analytical sensitivity that occurs between di†er-ent elements.

Relative sensitivity factor method (quantitative analysis)

Quantitative results require that di†erences in elementalsensitivities be characterized using standards similar incomposition to the material under study. This charac-terization generates relative sensitivity factors (RSFs)that can then be employed to correct the measured ionbeam ratios. This is the most widely employedapproach to quantiÐcation in GDMS and has histori-

cally been the approach used in other forms of massspectrometry in which trace element analysis is of inter-est, such as SIMS and SSMS.20,45 Relative sensitivityfactors can be deÐned for any element X in a matrix Mas the sensitivity, S, of X with respect to the sensitivityof M, where I is the ion current and C is the concentra-tion of the selected species :

RSFX \ SX/SM\ (IX/CX)STD/(IM/CM)STDRSF values for the elements of interest are calculatedfrom data acquired on standard samples, such as NISTStandard Reference Materials, which closely match thematrix of interest. Quantitative results are obtained foran unknown sample of matrix M containing element Xby dividing the IBR results by the RSF

x:

CX \ [(IX/IM)CM]/RSFXBecause RSF values vary only slightly between matricesof the same general composition, it is not necessary tohave exact matrix matching for the RSF method toyield quantitative results with accuracies of 15È20%.46

Hypothetical GDMS determination

To provide a Ñavor for the GDMS approach to elemen-tal analysis, consider a hypothetical case. Investigator Abrings the analyst a sample of high-purity material, S,containing trace levels of X, Y and Z. Fortuitously, S isconducting, and therefore straightforward d.c.-poweredGDMS can be employed. Unfortunately, S is a powder.Employing compacted sample methods learned fromSSMS, the power can be converted into a pin cathode, 2


Figure 7. Ion injection time approach to quantification for GDMS employing ion trap mass spectrometry systems.41 A practical limitation ofion trap systems is their relatively small dynamic range. Ions of major sample constituents create space charge problems limiting the determi-nation of trace species. The use of selective-ion monitoring and signal integration allows quantification on the basis of injection time.Essentially signal in the desired mass range is accumulated until the trap is filled with a predetermined number of analyte ions. The durationof the ion injection period needed to fill the trap can be related to the concentration of the analyte in the sample. This approach is not idealbecause only a selected range of ions can be measured in any given injection cycle. Reprinted with permission from Analytical Chemistry ,1994, 66, 92. 1994 by American Chemical Society.(

mm in diameter and 7 mm in length. This cathode isthen introduced by direct insertion probe into thesource region of the glow discharge mass spectrometer.A potential is applied to this pin cathode, biasing it byD1.1 kV with respect to the source chamber anode. Thesource chamber is pressurized with argon to 0.75 Torrand a discharge with a current of 2.7 mA is established.Analysis is performed following the steps outlined in theÑow chart shown in Fig. 6.

After sputtering the sample for 15 min to remove anysurface contamination and to permit discharge stabili-zation, mass spectrometric data are acquired for thenext 30 min. In the resulting mass spectrum, the totalion current corresponding to the isotopes of the matrixmetal, S, is 4.3 ] 10~10 A. The ion current for theisotope of X that has a natural abundance of 9.32% is2.0] 10~16 A, the more abundant isotope of X su†ersinterference from SAr` and requires a resolution of12 000 for analytical separation. Element Y is mono-isotopic and free from spectral interference. An ioncurrent of 7.3 ] 10~18 A is observed for Y. The major

isotope of Z is present at only 60.9% but su†ers nospectral interference. This isotope of Z exhibits an ioncurrent of 1.7 ] 10~18 A, that is, just four times greaterthan the standard deviation of the background signal.Employing the ion beam ratio method, a semi-quantitative answer can be given : X is present at D 5.0ppm, Y is present at D17 ppb and Z is present at D6ppb.

Because the analyst is unfamiliar with the sensitivityof X, Y and Z in samples of matrix S, it is necessary todetermine RSF values for these elements in S to convertthe IBR results into quantitative results. A surrogatepin cathode is produced by doping pure S powder withX, Y and Z at known trace levels close to those of theunknown sample (recall that approximate levels of X, Yand Z in S were determined by the IBR method). Usingthis surrogate, cathode data are acquired from whichRSF values are determined to be 1.1 for X, 0.77 for Yand 1.8 for Z. Based on the data available, a report isprepared in which the amounts of X, Y and Z in S arestated to be 4.5 ppm, 22 ppb and 4 ppb, respectively.


Table 2. Summary of elemental MS methods

RSFa Detection Sample Analysis

Method range limit requirement type

GDMSb 10 ppb Solid or compacted Survey, quant.

Depth profile

ICPMSc N/A pg mlÉ1 Solution Survey, quant.

SIMSd 103 ppm Solid Survey, quant.,

depth profile,

microscopy

LIMSe 103 ppm Solid Survey, quant.

microscopy

SSMSf 10 ppb Solid or compacted Survey, quant.

TIMSg N/A ppb Solution or solid Isotope ratio, quant.

a Relative sensitivity factor.b Glow discharge mass spectrometry.c Inductively coupled plasma mass spectrometry.d Secondary ion mass spectrometry.e Laser ionization mass spectrometry.f Spark source mass spectrometry.g Thermal ionization mass spectrometry.

An alternative approach to quantiÐcation

Recently, an alternative approach to quantiÐcation hasbeen developed for use with ion trap mass spectrometricsystems that are limited in their dynamic range.41Dynamic range limitations arise from space chargee†ects that occur when too many ions are stored in thetrap. The limitation, however, is true only if the ioninjection time is constant and the ion current isemployed as the analytical factor for quantiÐcation.This limitation can be ameliorated by Ðxing the ioncurrent value and varying the injection time, then theinjection time becomes the analytical factor of impor-tance. Selective ion injection procedures allow the trapto be Ðlled with the ion of interest and the time it takes

to Ðll the trap to a predetermined level is relateddirectly to the concentration of the ion. The top part ofFig. 7 shows the change in trapping potential as a func-tion of time. The Ñat regions preceding each scan rampin the regions numbered 2, 3 and 4 correspond to thee†ective injection time for ions of interest. Note in thebottom mass spectrum of Fig. 7 that each element isaccumulated to the same nominal abundance. It is theinjection time required to achieve this accumulationthat is employed as the analytical parameter. The injec-tion time in region 3, for Sn, is the longest because Sn isthe least abundant element. Although this approach toquantiÐcation is unique to ion storage mass spectro-metric systems, it e†ectively solves the problem oflimited dynamic range associated with such systems.

Figure 8. Comparison of molybdenum analyses by GDMS, SIMS and SSMS. The concentrations of 25 of elements in molybdenumsamples are plotted as a function of the analyte element for the three methods. The labels on the x-axis alternate to fit all 25 elements on thechart. Typically all three methods yield similar results.


Figure 9. Low-gain, low-mass range spectra from an aluminum sample.48 The ICP mass spectrum (a) shows a contribution from atmo-spheric species, and and the solution matrix, 0.6 M HCl, aspirated into the plasma. The GD mass spectrum (b) contains the analyteN

2O

2,

matrix ions, Al½, and discharge gas ions. Reprinted with permission from Applied Spectroscopy, 1994, 9 , 823. 1994 by the Society of(Applied Spectroscopy.

Unfortunately, problems with the efficiency of injectionof GD-generated ions into the ion trap has resulted inpoor detection limits of only 10 ppm by this method.

COMPARISONS OF METHODS INELEMENTAL MASS SPECTROMETRY

In Table 2, various methods of elemental mass spec-trometry are summarized in terms of the range of rela-tive sensitivity factor values, the reported detectionlimits, sample requirements and types of informationa†orded by each technique. No one elemental massspectrometric method is a panacea for all trace elementdeterminations. Each technique possesses uniqueadvantages and limitations making it suitable to partic-ular determinations.

In 1987, a comparison between SSMS, SIMS andGDMS for the analysis of sintered tungsten and molyb-denum samples was published.47 The results of this

comparison did not indicate a clear choice solely on thebasis of analytical capability. The chart in Fig. 8 showsthe concentrations of trace elements in molybdenumdetermined by the three methods. The results for allmethods typically fall within a factor of three from oneanother. Other factors that must be considered whencomparing these techniques are the ability to performion microscopy with SIMS and so obtain regio-speciÐcinformation from a sample. All methods claim an abilityfor depth proÐling, yet the ability in SIMS to controlthe Ñux of primary ions makes this technique bettersuited to high-resolution depth proÐle analyses. Thestrength of GDMS lies in an ability to provide rapidsurvey scans and quantitative analysis of bulk materials.

The interest in GDMS by atomic spectrometrists fol-lowed the development of inductively coupled plasmamass spectrometry (ICPMS) into a successful methodfor trace analyses of solution samples. In a recent reportthe two methods were compared for the determinationof trace elements in aluminum samples.48 The spectra


shown in Fig. 9 illustrate the advantage of GDMS oper-ation in an inert, reduced pressure environment com-pared with ICPMS with aqueous samples in an ambientenvironment. GDMS precludes the spectral clutter orig-inating from the aqueous and atmospheric contribu-tions to the ICPMS spectrum. In the region of thefourth-row transition metals, the spectra show inter-ferences arising from oxide ions in ICPMS [Fig. 10(a)]but not in GDMS [Fig. 10(b)]. The authors pointed outthat the detection limits obtained with both methodsare comparable. For GDMS the detection limits werefound to be of the order of 1È10 ng g~1 (ppb). WithICPMS, solutions containing 1 mg ml~1 of sampleyielded detection limits of 1È10 pg ml~1, which thenconverts to the same 1È10 ng g~1 range reported forGDMS. As the authors pointed out, the need to dis-solve the sample for ICPMS not only increases theanalysis time but also introduces the possibility of con-tamination. Although cathode fabrication can lead tosurface contamination, this is removed in the pre-

sputtering or “burn-inÏ of the cathode prior to analysis.A key point in this particular case was that the alumin-um alloy containing higher concentrations of Si was notanalyzed by ICPMS because of difficulty associatedwith dissolving this material.

Thermal ionization mass spectrometry (TIMS) is anelemental mass spectrometric method that bridges thegap between solid sampling and solution samplingmethods.49 In TIMS the sample is applied as a solution,slurry or resin bead to a wire Ðlament. The passage ofelectrical current through the Ðlament results in resistiveheating sufficient to volatilize the sample and ionize afraction of those volatile species in accordance with theSahaÈLangmuir equation.50 In some thermal ionizationsources, a secondary Ðlament serves to ionize speciesvolatilized by the primary Ðlament. TIMS is well knownfor its ability to yield isotope ratio data of extreme pre-cision and accuracy.51 It can be coupled with isotopedilution calibration to a†ord a very accurate method forthe determination of trace elements using small

Figure 10. High-gain, fourth-row transition metal mass range spectra from an aluminum sample.48 The ICP mass spectrum (a) showsinterferences from oxides species that are absent in the GD mass spectrum (b). Detection limits by both methods were found to be in the1–10 ppb range. Reprinted with permission from Applied Spectroscopy, 1994, 9 , 823. 1994 by the Society of Applied Spectroscopy.(


Table 3. GDMS determination of trace ele-ments in forest soil

Element GDMSa LA-ICPMSb ICP-AESc

Mg (%) 0.341 0.440 0.390

Al (%) 7.03 5.32 4.89

Si (%) 33.9 35.8 15.4

Ca (%) 0.298 0.310 0.330

Ti (%) 0.779 n.a. n.a.

Fe (%) 3.50 3.10 2.70

Sr (ppm) 119 57 n.a.

Ba (ppm) 175 322 290

U (ppm) 3 8 n.a.

a Glow discharge mass spectrometry.b Laser ablation inductively coupled plasma massspectrometry.c Inductively coupled plasma atomic emissionspectroscopy.

amounts of sample.52 This report compares52 TIMSand GDMS results for the determination of impuritiesin electronic-grade titanium. In this comparison TIMSusing isotope dilution calibration surpasses GDMSusing RSF-corrected ion beam ratio calibration. Theauthors also pointed out that TIMS is not a convenientmulti-element method, rather it is intended for determi-nations of speciÐc isotopes of a particular element. Thesuggestion is made that TIMS be used to certify refer-ence materials that may in turn be used to providebetter calibration of GDMS and other element massspectrometric methods.

SOME APPLICATIONS OF GDMS

Analytical GDMS has expanded well beyond its initialutility for the trace determination of elements in con-ducting solid samples. A few examples of variousGDMS applications are worth noting. These exampleswere selected to illustrate the variety of applicationareas covered and are not meant to exemplify typicalGDMS applications. As mentioned in the Introduction,recent technical reviews and the current literatureprovide the best details of GDMS applications.

For atmospheric monitoring, a glow discharge wascoupled with an ion trap mass spectrometry system.7The glow discharge operated at a reduced pressure of0.2È0.8 Torr and sampled the ambient atmosphere at arate of 5 ml s~1. Successful detection of TNT at the ppblevel was reported for the atmospheric sampling glowdischarge ion trap mass spectrometric system. Thisapplication of GDMS is unique because it combines thepioneering technology of ion injection for ion trap massspectrometry with the ability of a glow discharge togenerate analytically useful negative ions of 2,4-dinitro-toluene.

Glow discharges have been explored for use inorganic and biological mass spectrometry, particularlyfor applications involving the interfacing of liquid chro-matography with mass spectrometry.8 A cited advan-tage of this plasma ionization is its compatibility withoxygen-containing solvents that lead to the oxidationand failure of Ðlaments employed in conventional ion

Figure 11. R.f.-GD mass spectrum from a standard firebrick material.55 Direct determination of trace elements in a non-conducting samplerequires the use of an r.f.-powered glow discharge. This spectrum clearly shows the presence of major, minor and trace constituents offirebrick. Reprinted with permission from Analytical Chemistry , 1995, 67, 1026. 1995 by American Chemical Society.(


sources. The early work demonstrated that a glow dis-charge source yielded spectra that were similar to thoseobtained by chemical ionization for a variety of aminoacids. In the most recent work, an atmospheric pressurehelium glow discharge yielded picogram detection limitsfor tyramine present in a nebulized liquid sample.8 Thisplasma ionization approach is found to be very “softÏ,producing high abundances of the protonated molecu-lar ion with minimal fragmentation.

The application of GDMS to the elemental analysisof non-conducting materials is an area of current inter-est particularly with regard to the examination ofenvironmental samples. Recent work has been con-ducted with d.c.-powered GDMS for the analysis of soilsamples.53 Employing a variety of standard soils andclays compacted in a copper host matrix, linear cali-bration graphs were constructed and RSF values deter-mined. These results from the standard materials wereemployed to obtain quantitative results for Ðeld-

collected soils and clays. The GDMS results comparedfavourably with determinations made by laser ablationICPMS and ICP optical emission spectrometry (Table3). In this application the detection limits were found tobe in the single ppm range for samples diluted to 1 : 5 ina conducting host matrix.

Another approach to GDMS of non-conductingsamples uses r.f.-powered glow discharges to permitdirect sampling of the material.54 Figure 11 shows arecently reported mass spectrum obtained for a stan-dard Ðrebrick material examined by r.f.-poweredGDMS.55 The various expansions in scale clearly showthe minor components in the Ðrebrick sample. Semi-quantitative ion beam ratio results yield concentrationsthat are within a factor of two of the certiÐed values.This level of accuracy is consistent with the range ofrelative sensitivity factors expected for such material.The detection limits for r.f.-GDMS of non-conductingsamples are reported to be in the single ppm range.10

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