Laser-enhanced ionization: recent developments

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Laser-enhanced ionization: recentdevelopmentsDenis Boudreau*, Jean-Franc°ois GravelDepartment of Chemistry, Universiteè Laval, Sante-Foy, Que, Canada G1K 7P4

Initially developed for the analysis of simple aque-ous solutions, laser-enhanced ionization (LEI) is avery sensitive trace analysis technique which isbased on the spectrally selective laser excitationof analyte atoms followed by their collisional ion-ization and detection in a suitable atom reservoir.Its unique versatility, granted by a rich choice ofexcitation wavelengths, and its high ef¢ciency ofcharge creation and collection make LEI one ofthe most sensitive techniques for elemental analy-sis. Despite these attractive features, LEI has notbeen widely used for the chemical analysis of liquidand solid samples having more complex matrices.This article describes the current limitations of LEIand recent achievements in the ongoing develop-ment of this promising technique. z2001 Else-vier Science B.V. All rights reserved.

Keywords: Laser-enhanced ionization; Trace analysis;Ionization techniques using lasers

1. Introduction

Laser-enhanced ionization (LEI ) is, akin to otherlaser-based analysis techniques such as laser-excited atomic £uorescence spectrometry(LEAFS) and resonance ionization spectrometry(RIS), based on the selective excitation of analyteatoms by tunable laser( s ), followed by the ioniza-tion ( in the case of LEI and RIS) and detection of thespecies of interest. However, unlike RIS, the laser inLEI serves only to enhance the ionization rate, sinceionization itself occurs via inelastic collisions ^ mostfrequently in an air /acetylene £ame ^ from thehigh-lying laser-excited state. This increase in theionization rate is in turn detected by an electric ¢eld

applied across the region probed by the laser, usu-ally between a standard burner head and an elec-trode immersed in the £ame (Fig. 1).

LEI has often been described as one of the mostsensitive elemental analysis methods, and this iscertainly true for simple aqueous solutions whereatomic transition(s) have been found that bring theanalyte atoms in suf¢cient proximity to the ioniza-tion threshold to achieve ef¢cient collisional ioniza-tion (vEion 6 1 eV for air /acetylene £ames). Insuch cases, ionization and charge collection ef¢-ciencies are believed to be almost unity [ 1 ], andthe smallest measurable analytical signal is thendetermined by the £uctuations in the current car-ried by the native charges in the £ame. Estimates forthe ultimate limit of detection (LOD) attainable byLEI in a typical experimental setup range from 1 to100 fg / ml [ 2,3 ]. Published experimental LODs areusually higher because of incomplete atomizationof analyte species in the £ame, RFI pickup from thelaser system, high contamination levels from theblanks, or a poor choice of excitation wavelengthsleading to spectral interference or to excited levelslying too far below the ionization threshold.

However, while this technique has been inten-sively researched since the late 70s, both for ana-lytical and non-analytical purposes ( recent exam-ples of the latter include the spatially resolveddetermination of temperatures [ 4 ] and of neutralatomic diffusion and ionêlectron recombinationcoef¢cients [ 5 ] in £ames, and the determinationof supersonic gas velocities in shock tubes [ 6,7 ]),the number of papers published each year on theuse of LEI for chemical analysis has been graduallyleveling off. Moreover, the number of articles pub-lished since its inception in 1976 [ 8 ] seems dispro-portionate to its actual application in working ana-lytical laboratories. There are a number of factors toexplain this:

1. the complexity, high cost and lack of user-friendliness of lasers, while being rapidly van-ishing obstacles, have certainly contributed to

0165-9936/01/$ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 5 - 9 9 3 6 ( 0 0 ) 0 0 0 5 5 - 8

*Corresponding author. Tel.: +1 (418) 565-3287; Fax:+1 (418) 656-7916.E-mail: [email protected]

20 trends in analytical chemistry, vol. 20, no. 1, 2001

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limit the number of participants in the devel-opment of the technique;

2. the use of electrodes for signal collectionmakes LEI vulnerable to the presence of easilyionizable elements (EIE) in the sample matrix.The charges created by the thermal ionizationof EIE tend to increase the signal backgroundnoise, to depress the electric ¢eld gradientbetween the electrodes, and to degrade LODs;

3. the feature of the technique that makes it sosensitive, i.e. non-selective, universal chargedetection, is unfortunately also the source ofspectral interference in cases where the exci-tation wavelength(s ) for the analyte inducesthe LEI or photoionization of species from the£ame or the sample matrix. The absence in LEIof a selective detection step ( like in LEAFS)increases the risk of matrix interference andrepresents the most serious impediment to itsapplication to the routine analysis of sampleshaving complex matrices;

4. the temperature of the air /acetylene £ame isinsuf¢cient to completely atomize elementsthat tend to form refractory species (Cr, W,most rare earths, etc. ), and this decreases theconcentration of the analyte in the £ame whileincreasing the risks of spectral interference.

These resilient obstacles, combined with thearrival on the market of ICP^MS ^ which providesexcellent LODs, multielemental detection capabil-ities and higher atomization ef¢ciencies in an inertatmosphere ^ are responsible for the limited use ofLEI for trace elemental analysis and to the current

con¢nement of LEI development to a relativelysmall number of laboratories.

2. Recent developments

There is still, however, an ongoing research effortto overcome the aforementioned obstacles, moti-vated by the potential bene¢ts of LEI as a very sen-sitive and selective analysis technique. The aim ofthe present article is not to cover the ¢eld of LEI in itsentirety, since a number of papers [ 9^11 ] and, inparticular, the book edited in 1996 by LEI pioneerworkers Travis and Turk [ 12 ] have done thisalready. Rather, papers published between 1996and the present time that best illustrate the recentachievements and future trends in the developmentof the technique will be reviewed.

2.1. Novel atomization, excitation or ionizationstrategies

Because it involves the transduction of analyteconcentration into a commensurate change in elec-trical conductivity, LEI is a close cousin of laseroptogalvanic spectroscopy (LOGS) [ 13 ]. The typi-cally low collisional activity of the electrical dis-charges used in LOGS, however, makes the latterbetter suited to the isotope-selective determinationof species from pure substances or present at sig-ni¢cant concentration levels [ 14,15 ]. The muchhigher temperature of the £ames used in LEI, onthe other hand, is well suited to the desolvation ofnebulized aqueous samples, and the higher colli-

Fig. 1. Principle of LEI and typical £ame assembly.

trends in analytical chemistry, vol. 20, no. 1, 2001 21

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sional rates ensure high ionization ef¢ciencies anddetection sensitivities. However, unlike LOGS, inwhich ionization often results from non-thermal,level-speci¢c processes such as charge-transfer orPenning mechanisms, the ionization process in LEI£ames is primarily of a thermal nature, and thedetection sensitivity depends strongly on the prox-imity of the excited level to the ionization threshold^ and hence, on the excitation scheme chosen for agiven element.

As was previously stated, the temperature in anair /acetylene £ame (V2200³C) requires, for anef¢cient collisional ionization of the analyteatoms, that these are excited to a level lying lessthan 1 eV from the ionization threshold. While sev-eral elements can be ef¢ciently excited to high-lying excited states using single step excitationschemes, many others bene¢t from a two-stepapproach, either because one-step schemes fromthe ground state suffer from poor transition proba-bilities or because they leave the atoms in excitedstates that lie too far below the ionization threshold.In both cases, selection of the excitation transi-tion(s) has an important impact on the ionizationef¢ciency (and hence, on the sensitivity of the ana-lyte signal ), with the added bene¢t, for two-stepstrategies, of an enhanced spectral selectivity. Forexample, in a recent paper by Riter et al. [ 16 ] on thestudy of several two-step excitation schemes of Pb,decreasing the energy gap vEion between the top-most excited level and the ionization thresholdfrom 0.97 eV (second transition 7s 3P1³C8p 3D2,600.193 nm) to 0.60 eV (7s 3P1³C9p 3P1, 509.001nm) resulted in an 8-fold improvement in sensitiv-ity. The use of a two-step excitation strategy, how-ever, usually requires a more complex experimen-

tal setup and more strenuous alignmentprocedures. Martin Paquet et al. [ 17 ] used a fortu-itous proximity in the wavelengths of the transitionsimplicated in a two-step excitation scheme for chro-mium to ef¢ciently populate the 3d44s5s 7D4

(vEion=0.97 eV) state through a two-step /one-color near-resonant excitation mechanism. Thisprocess is believed to result from the combinationof resonant excitation of one step and off-wingexcitation of the other laser-broadened transition(Fig. 2). The authors were unable to determinewhether the off-wing excitation process occursthrough off-resonant absorption in the wings oflaser-broadened excited states or by absorption atthe transition resonant wavelength of photons fromthe far wings of the laser spectral pro¢le. Neverthe-less, this excitation strategy was shown to providethe lowest LOD to date (0.5 ng / ml in aqueous sol-utions) as well as the instrumental simplicity of asingle dye laser setup, while retaining the spectralselectivity of a two-step excitation approach.

For some elements with higher ionization ener-gies, both one- and two-step excitation schemesmay be dif¢cult to implement, because either oneor both excitation transitions may lie in the UVregion, where the background signal from the mul-tiphoton ionization (MPI) of species such as O2, COand particularly NO degrades the signal-to-noiseratio. In addition, excitation wavelengths below220 nm pose a special problem because they aredif¢cult to reach with standard tunable dye lasers.Elwood et al. [ 18 ] have recently described a newstrategy for the LEI measurement of a number ofelements having high ionization potentials (Cu,7.72 eV; Sb, 8.64 eV; Se, 9.75 eV; As, 9.81 eV). Inorder to reach the far-UV region where the resonanttransition lines of some of these elements arelocated (Se, 196.026 nm; As, 197.197 nm), theyused an anti-Stokes Raman shifting technique toextend the output of a dye laser frequency-doubledin a BBO crystal. Moreover, in order to minimize therisk of spectral interference from the MPI of NO, theauthors used a 20/80 mixture of oxygen and argon,instead of air as the oxidant mixture. They reportedfor all the elements listed above LODs that comparefavorably with previously reported results. In par-ticular, they reported LODs of 0.5 and 30 ng / ml forSe and As, respectively, despite the fact that theseelements are known to be only fractionally atom-ized in the acetylene £ame (atomization ef¢cien-cies of 1% and 0.01% for Se and As, respectively). Itis worthwhile to note that there were no previous

Fig. 2. Excitation^collisional ionization pathways for the two-step /one-color near-resonant excitation of chromium [ 17 ].


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reports of Se measurements by LEI and only onereport for As. The authors also reported that detec-tion sensitivity was 2^6 times higher (depending onthe element ) and the ionization background onethird lower with the use of the O2 /Ar mixture.

Apart from being non-selective, the use of elec-trodes for signal collection has other disadvantages.Immersed electrodes can decrease the £ame tem-perature (and hence the atomization ef¢ciency)and are very susceptible to RFI noise pickup (ema-nating from pulsed laser sources ). Moreover, elec-trode designs all suffer from the large differencebetween the £ame volume effectively probed bythe laser beam(s) and the volume from whichnative £ame charges are swept by the electric¢eld and collected. In an effort to avoid these lim-itations, Matveev et al. [ 19 ] have reported on thepossibility to indirectly measure analyte ions pro-duced by resonance ionization or LEI in a neon-¢lled gas cell, by measuring the light emitted byneon atoms excited by collisions with the analyteions (Hg�, in this case) accelerated in a strong elec-tric ¢eld. The avalanche ampli¢cation obtainedunder certain experimental conditions of voltageand pressure results in a very large enhancementin sensitivity, with the result that ultra-trace levels ofthe analyte can be detected without the use of elec-trodes inserted into the gas medium. The authorspredicted that, with further improvement in theexperimental setup, LODs for this techniquecould reach a single ionêlectron pair.

Clevenger et al. have demonstrated the use of asimilar avalanche ampli¢cation technique for thedetection of traces of mercury vapor £owingthrough a gas cell [ 20 ]. The analyte atoms werecarried into the gas cell ( resistively heated to 550K) by a £ow of argon or P-10 gas (10% methane inargon). The laser beams entered via a hole boredthrough the cell, at which point the Hg atoms wereselectively excited into a Rydberg state by a three-step excitation scheme [ 21,22 ], collisionally ion-ized and the resulting charges detected by a pairof electrodes. They reported an ampli¢cation fac-tor, due to avalanche ionization, of nearly threeorders of magnitude, and estimated an ampli¢ernoise-limited LOD of 15 atoms per laser pulse.Arguably, signal averaging over a larger numberof pulses would result in a substantial increase inS /N ratio. The actual LOD was determined by MPIof the buffer gas at high laser irradiance, and by abackground signal of 103^104 mercury atoms in theinteraction region. According to the authors, this

method could ultimately be used for the determina-tion of mercury at ultra-low concentrations in watersamples using a cold vapor generation technique.

Finally, in a related article [ 23 ], the same researchgroup has demonstrated that it is possible, in certainconditions, to deconvolve the photoionization andcollisional ionization components from the time-resolved ionization signal waveform of mercury.The experimental conditions must be such that(1) the duration of the laser pulse is shorter thanthe collisional ionization process and (2) the transittime of the photoionized electrons is shorter thanthe collisional ionization process. For nanosecondlaser pulses, it was shown that this strategy could beimplemented at source pressures lower than a fewtorr; otherwise, the increase in photoelectrontransit time makes it impossible to distinguish thetwo components from each other. This methodcould be used to provide greater analyte selectivityto LEI measurements in low to medium pressureenvironments, in cases where there is a signi¢cantcontribution to the overall recorded current transi-ent signal from charged species created by the non-selective laser photoionization of atoms and mole-cules originating from the sample matrix.

2.2. Analyte preconcentration / interferent removal

As was stated in Section 1, the exceptional detec-tion sensitivity and spectral selectivity of LEI havebeen amply demonstrated, with sub-ng / ml LODshaving been reported for many elements [ 3 ]. How-ever, a majority of the latter have been determinedfrom synthetic aqueous samples with simple matrixcompositions, since even the increased spectralselectivity afforded by two-step excitation schemescannot always compensate for the non-selectivenature of the charge collection process and correctfor spectral interference from easily ionized matri-ces, through excitation in the wings of absorptionlines belonging to the matrix species. While sometypes of real samples can be analyzed without priortreatment, others ( i.e. seawater, biological £uids,dissolved mineral samples, etc. ) cannot. In caseswhen simply diluting the samples may bring theconcentration of an analyte below the LOD, oneobvious course of action involves the preconcen-tration or separation of analytes from interfering EIEupstream of the LEI £ame.

For example, Ke et al. [ 24 ] have combined £owinjection analysis and LEI to determine Pb at lowpg / ml levels in aqueous samples and in the pres-


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ence of alkali EIE at concentrations as high as 5000Wg / ml. Pb was preconcentrated online by sorptionon a C18 micro-column as the Pb^DPPA (diethyldithiophosphate ammonium) complex, achievinga 48-fold enrichment factor. Interferents Na and Kfrom the sample matrix, being only weakly retainedon the column, were removed by rinsing withdeionized water. The Pb^DPPA complex was theneluted with methanol and detected by two-step LEI.For initial aqueous aliquots of 15 ml, the authorsreported a 1c LOD of 3 pg / ml in aqueous solutions.They also demonstrated the applicability of theirmethod to the analysis of Pb in a seawater CRM,achieving a result (11 pg / ml) consistent with thecerti¢ed value.

Martin Paquet et al. [ 17 ] combined ion chroma-tography and LEI to achieve the speciation of chro-mium at trace level in aqueous solutions. Chro-mium species were separated in a pyridine-2,6-dicarboxylate (PDCA)-based eluent as anions, i.e.Cr( III ) as the Cr(PDCA)2

3 complex and Cr(VI ) asthe chromate ion CrO4

23. The column ef£uent wasnebulized into a standard LEI £ame, where chro-mium atoms were detected using two-photon exci-tation at 427.387 nm (3d5 4s 7S3C3d4 4s 5s 7D4 ).Chromatographic 3c LODs of 5 and 4 ng / ml wereobtained for Cr( III ) and Cr(VI), respectively, usinga 100 Wl injection volume. This method also pro-vided adequate chromatographic separation ofboth chromium species from sodium present inthe sample matrix at concentrations as high as 50Wg / ml (Fig. 3), as the latter was shown to generatea LEI signal at the chosen chromium excitationwavelength (427.387 nm), through a bound-to-bound transition at 427.364 nm, from the thermally

populated state 3p 2P1=2³ to the Rydberg states 10d2D3=2;5=2. While the data shown suggested thathigher sodium concentrations in the sample matrixmight interfere with chromium peaks, changing theexcitation scheme to the two-step ladder 427.481nm+427.293 nm (3d5 4s 7S3C3d5 4p 7P3³C3d4

4s 5s 7D4 ) would provide greater immunity fromsodium interference, at no cost in detection sensi-tivity.

LEI was also evaluated by Elwood et al. as adetection technique for the speciation of arsenicand selenium at trace level in aqueous samples[ 18 ]. Selective hydride generation procedurestogether with cryogenic trapping were used forthe preconcentration and speciation of As( III ),As(V), Se( IV) and Se(VI) species in the 5^30 ng /ml range. While LODs were not calculated becauseof the non-linearity of the calibration curvesobtained, their preliminary results indicated a tan-gible increase in detection sensitivity over continu-ous solution nebulization, and demonstrated thepotential bene¢ts and immunity to EIE interferenceof hydride generation LEI for speciation-speci¢ctrace analysis.

2.3. Micro-analysis

The extremely high sensitivity intrinsic to LEI iswell suited for micro-analysis at the trace level, inparticular when the limited amount of sample mate-rial and low analyte concentrations exclude sampledilution and analyte preconcentration procedures.Unfortunately, the £ame atomizer has the followingshortcomings with regards to micro-analysis: ( i )dilution of the atomized sample in the considerablevolume of the typical slot burner £ame; ( ii ) the lowef¢ciency of the nebulization process ( typicallyless than 10% of the sample reaches the £ame);and ( iii ) the limited energy available to vaporizethe sample during its passage in a typical LEI£ame, and the inability of the latter to directlyvaporize microgram-sized solid samples.

Despite these drawbacks, however, the £ameatomizer currently remains an integral componentof the LEI technique, primarily because its high col-lisional activity grants it an unsurpassed ionizationef¢ciency, and also because in situ LEI detection inalternate atom reservoirs, such as the inductivelycoupled plasma and the graphite furnace, has hadvery limited success [ 25,26 ]. Moreover, couplingLEI detection with external sample vaporizationmethods maintains the high ionization ef¢ciency

Fig. 3. Ion chromatograms of Cr( III ), Cr(VI) and Na,obtained with LEI detection at 427.387 nm [ 17 ].


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of the £ame reservoir while allowing the independ-ent control of both vaporization and detection pro-cesses. For example, the early work of Chekalin etal. with the `rod-in-£ame' approach [ 27 ], where thesample was deposited and vaporized on a resis-tively heated graphite rod immersed in an airâce-tylene £ame, resulted in a 1 pg /g LOD for indium insolid micro-samples. Shortly after, Smith et al. [ 28 ]reported on an alternate approach in which aque-ous samples vaporized in a longitudinally heatedgraphite furnace were transferred in an argon car-rier gas to a miniature airâcetylene £ame. Thesmaller £ame resulted in a decrease in sample dilu-tion and a better overlap between the laser beam(s)and the £ame. They obtained 3c LODs rangingfrom 1 to 260 fg for Mg, Tl and In. These earlyattempts at coupling LEI with electrothermal vapor-ization (ETV) techniques hinted at the analyticalperformances that could be obtained by using LEIdetection for micro-analysis.

More recently, Riter et al. [ 29 ] reported on thethorough characterization of an ETV^LEI system,based on the original design described above andoptimized for Mg detection. The experimentalparameters involved in the overall detection ef¢-ciency of the technique, including sample vapor-ization and transport from the graphite furnace tothe £ame, analyte atomization and ionization, laserprobing and charge collection were all evaluatedand optimized. For example, methanol was usedas a physical carrier for the vaporized analyte inorder to increase the transport ef¢ciency and theprecision of the technique, particularly for Mg con-centrations below 100 ng / ml. The overall detectionef¢ciency was found to be only 0.0025%, limitedprimarily by the atomization ef¢ciency of magne-sium in the miniature £ame (0.96%), which is sig-ni¢cantly lower than in typical airâcetylene£ames. Other important limiting factors included aprobing ef¢ciency of 2.3% ( inherent in short-pulselasers systems with low repetition rates ), and atransport ef¢ciency, despite the use of methanolas a physical carrier, of only 17%. In the lattercase, it was found that most of the analyte waslost to adhesion to transport tubing walls (8% oflosses) and to diffusion through the graphite tube(24% of losses). Despite this rather poor overalldetection ef¢ciency, the authors reported an instru-mental LOD for Mg of 29 pg / ml (290 fg in a 10 Wlinjection volume), which was limited by RF inter-ference from the excimer pump laser. They calcu-lated that, with adequate shielding of the charge

collection circuitry, the ampli¢er noise-limitedLOD could reach 590 fg / ml (5.9 fg).

Another paper from the same group and pub-lished the same year reported on the use of thistechnique for the determination of extremely lowconcentrations of lead in human blood [ 30 ]. Verylittle sample preparation was required, consistingof a 21:1 dilution of whole blood micro-samplesin ultra-pure water. This work also demonstratedhow ETV can be used to avoid matrix-related inter-ference problems in LEI, by removing potentiallyinterfering species during the drying and ashingstages. Furthermore, the authors believed thatmatrix components remaining after the ashingstage were vaporized along with lead, acting as aphysical carrier and hence improving precision andsensitivity. Using the two-step excitation strategypublished separately [ 16 ] (discussed in Section2.1), the authors reported a 3c LOD of 4.2 pg / ml(42 fg of Pb in a 10 Wl injection volume) in dilutedblood, equivalent to 89 pg / ml (890 fg) in wholeblood prior to dilution. These ¢gures of merit com-pare favorably well with those obtained by othertechniques such as ETVÂAS and ICP^MS.

Laser ablation, as a solid sampling technique,shares with the graphite furnace the ability to ef¢-ciently vaporize small solid samples with little or nosample preparation and reduced contaminationrisks. Furthermore, the small interaction area of afocused laser beam allows the spatially resolvedanalysis of heterogeneous solid samples. However,LEI detection cannot be performed directly in theplasma plume, because the high local temperatureresults in a high background current from the ther-mal ionization of analytes and matrix componentsalike, and also because spectral resolution isseverely degraded by pressure broadening effects[ 31 ]. Hence, laser excitation leading to enhancedionization must be performed outside the plasma.Gorbatenko et al. [ 32 ] reported on the develop-ment of a laser ablation^LEI hybrid technique, inwhich a solid sample was placed directly belowthe combustion zone of a specially designed airâcetylene £ame. Ablation of aluminum alloy sam-ples was performed with the second harmonic of aNd-YAG laser (532 nm), and lithium was detectedby two-step excitation (V1=670.784 nm andV2=610.362 nm). This close coupling aimed to min-imize sample losses during transport to the £ame,while allowing the independent optimization ofboth vaporization and detection processes. No ana-lytical ¢gures of merit were given.


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More recently, Gravel et al. [ 33 ] used instead anablation cell separated from the LEI £ame by atransfer line, akin to systems used in laser ablationICP^MS, on the basis that most of the materialejected over the sample cools and aggregates veryrapidly, and that sample losses to condensation inthe transfer line would not be dominant in laserablation^LEI as in hybrid graphite furnace systems.Laser ablation of aluminum and stainless steel sam-ples was performed using an excimer laser at 308nm, and the vaporized sample was carried in anargon carrier gas from the cell into a miniature£ame similar in design to that developed by Smithet al. for ETV^LEI work [ 28 ]. This £ame is welladapted also to laser ablation, since it involves amuch smaller dilution factor than other atom reser-voirs and provides the high sensitivity required bythe few nanograms of material ablated by each lasershot. Fig. 4 shows the LEI signal pro¢le for Pb, usingthe two-step excitation scheme 3d5 4s 7S3C3d5 4p7P3³ (283.306 nm)C3d4 4s 5s 7D4 (600.193 nm),for a single ablation pulse onto an aluminum alloysample containing 170 Wg /g of Pb. Given an esti-mated ejected sample mass of V30 ng, this signalpro¢le represents about 6 pg of Pb ejected from thesample ( the transport ef¢ciency to the miniature£ame was not measured). The authors reported a3c LOD of 0.4 Wg /g of Pb over 100 successive abla-tion pulses (V3.3 s of ablation at a 30 Hz repetitionrate).

3. Summary

LEI provides signi¢cant bene¢ts for trace chem-ical analysis, i.e. exceptional sensitivity and a richchoice of excitation pathways that grant it excellentspectral selectivity. The most serious impediment toa more widespread use of LEI for routine analysis isa marked vulnerability to matrix interference prob-lems. While this limitation has been thoroughlystudied and is now well documented, it is verylikely that LEI will always require some knowledgeof the sample matrix prior to analysis. However, itspotential bene¢ts are such that we believe that itcan become ^ in particular through the develop-ment of hybrid methods in which LEI is used as adetection technique for external vaporization orseparation processes ^ a very powerful techniquefor trace analysis of solid and liquid samples of envi-ronmental, industrial or geological origin.

Acknowledgements

The authors are grateful to the Natural Sciencesand Engineering Research Council of Canada(NSERC) and the `Fonds pour la Formation desChercheurs et l'Aide aé la Recherche' (FCAR) ofQueèbec for ¢nancial support of the research donein the Laser Spectrochemical Analysis Laboratory atLaval University and referenced in this work. J.-F.G.is indebted to the Laval University Foundation for¢nancial support.

References

[ 1 ] N. Omenetto, B.W. Smith, L.P. Hart, Fresenius J. Anal.Chem. 324 (1986) 683.

[ 2 ] O.I. Matveev, N. Omenetto, AIP Conf. Proc. 329 (1995)515.

[ 3 ] G.C. Turk, in: J.C. Travis, G.C. Turk (Eds. ), Laser-Enhanced Ionization Spectrometry, John Wiley andSons, New York, 1996, p. 186.

[ 4 ] C.-B. Ke, K.-C. Lin, Appl. Spectrosc. 52 (1998) 187.[ 5 ] P. Barker, H. Rubinsztein-Dunlop, Spectrochim. Acta

Part B 52B (1997) 459.[ 6 ] P. Barker, A. Bishop, H. Rubinsztein-Dunlop, Appl.

Phys. B Lasers Opt. B64 (1997) 369.[ 7 ] P.F. Barker, A.M. Thomas, T.J. McIntyre, H. Rubinsz-

tein-Dunlop, Aiaa J. 36 (1998) 1055.[ 8 ] R.B. Green, R.A. Keller, P.K. Schenck, J.C. Travis, G.G.

Luther, J. Am. Chem. Soc. 98 (1976) 8517.[ 9 ] G.C. Turk, J. Anal. At. Spectrom. 2 (1987) 573.

[ 10 ] O. Axner, in: R.A. Meyers (Ed. ), Encyclopedia of Ana-

Fig. 4. LEI signal pro¢le for Pb from a single ablation pulseonto an aluminum alloy sample containing 170 Wg /g of lead[ 33 ].


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lytical Chemistry: Applications, Theory and Instrumen-tation, Wiley, London ( in press ).

[ 11 ] J. Sneddon, T.L. Thiem, Y.-I. Lee, Lasers in AnalyticalAtomic Spectrometry, Wiley, New York, 1997.

[ 12 ] J.C. Travis, G.C. Turk, Laser-Enhanced Ionization Spec-trometry, John Wiley and Sons, New York, 1996.

[ 13 ] D.L. Monts, Abhilasha, S. Qian, D. Kumar, X. Yao, S.P.McGlynn, J. Thermophys. Heat Transf. 12 (1998)66.

[ 14 ] J.P. Young, R.W. Shaw, C.M. Barshick, J.M. Ramsey,J. Alloys Compd. 62 (1998) 271.

[ 15 ] X. Yao, S.P. McGlynn, R.C. Mohanty, Microchem. J. 61(1999) 223.

[ 16 ] K.L. Riter, O.I. Matveev, B.C. Castle, B.W. Smith, J.D.Winefordner, AIP Conf. Proc. 388 (1997) 431.

[ 17 ] P. Martin Paquet, J.-F. Gravel, P. Nobert, D. Boudreau,Spectrochim. Acta Part B 53B (1998) 1907.

[ 18 ] S.A. Elwood, N.V. Chekalin, M. Ezer, H.L. Pacquette,D.J. Swart, J.B. Simeonsson, Appl. Spectrosc. 53(1999) 1237.

[ 19 ] O. Matveev, L. Mordoh, W. Clevenger, B. Smith, J.Winefordner, Appl. Spectrosc. 51 (1997) 798.

[ 20 ] W.L. Clevenger, O.I. Matveev, S. Cabredo, N. Ome-netto, B.W. Smith, J.D. Winefordner, Anal. Chem. 69(1997) 2232.

[ 21 ] O.I. Matveev, B.W. Smith, N. Omenetto, J.D. Wineford-ner, Spectrochim. Acta B 51 (1996) 563.

[ 22 ] W.L. Clevenger, O.I. Matveev, N. Omenetto, B.W.Smith, J.D. Winefordner, Spectrochim. Acta Part B52B (1997) 1139.

[ 23 ] W.L. Clevenger, L.S. Mordoh, O.I. Matveev, N. Ome-netto, B.W. Smith, J.D. Winefordner, Spectrochim.Acta Part B 52B (1997) 295.

[ 24 ] C.B. Ke, K.C. Lin, Anal. Chem. 71 (1999) 1561.[ 25 ] K.C. Ng, M.J. Angebranndt, J.D. Winefordner, Anal.

Chem. 62 (1990) 2506.[ 26 ] D.J. Butcher, R.L. Irwin, S. Sjostrom, A.P. Walton, R.G.

Michel, Spectrochim. Acta B 46 (1991) 9.[ 27 ] N.V. Chekalin, V.I. Pavlutskaya, I.I. Vlason, Spectro-

chim. Acta Part B 46B (1991) 1701.[ 28 ] B.W. Smith, G.A. Petrucci, R.G. Badini, J.D. Wineford-

ner, Anal. Chem. 65 (1993) 118.[ 29 ] K.L. Riter, W.L. Clevenger, L.S. Mordoh, B.W. Smith,

O.I. Matveev, J.D. Winefordner, J. Anal. At. Spectrom.11 (1996) 393.

[ 30 ] K.L. Riter, O.I. Matveev, B.W. Smith, J.D. Winefordner,Anal. Chim. Acta 333 (1996) 187.

[ 31 ] H.M. Pang, E.S. Yeung, Anal. Chem. 61 (1989) 2546.[ 32 ] A.A. Gorbatenko, N.B. Zorov, Y.Y. Kuzyakov, A.R.

Murtazin, J. Anal. Chem. 52 (1997) 490.[ 33 ] J.F. Gravel, D. Boudreau, Direct Analysis of Solid Sam-

ples by Laser Ablation and Laser-Enhanced Ionization,presented at the 25th FACSS Conference, Austin, TX,1998.


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