REVIEW Laser ablation inductively coupled plasma mass spectrometry: achievements, problems, prospects Steven F. Durrant Laborato ´ rio de Processos de Plasma, DFA, IFGW, Universidade Estadual de Campinas (UNICAMP ), CP 6165, 13083–970, Campinas, SP, Brazil Received 4th March 1999, Accepted 25th May 1999 1 Introduction ferents with isotopes of analytical interest or that are incorpor- ated into polyatomic interferences. 2 Evolution of LA-ICP-MS 3 Laser principles Motivated partly by such di culties, alternative sample introduction methods such as direct sample insertion (DSI ),4 3.1 Ruby 3.2 Nd5YAG electrothermal vaporization ( ETV ),5 spark ablation (SA)6 and laser ablation (LA)7 began to be used with ICP-MS. The 3.3 Excimer 4 Laser ablation systems ability of the ICP to accept vapours and solid aerosols produced by such methods for atomization and ionization was 5 Laser–solid interactions 6 Particle transport already known from optical emission spectrometry (OES ). Indeed, development of these sample introduction techniques 7 Optimization 8 Calibration strategies for ICP-OES continues in parallel with their use in ICP-MS. Ablation of solids using pulses from a laser and carriage of 9 Performance and problems 10 Growth areas the released material to the ICP in a gas flow, usually of argon, is a very attractive alternative to the nebulization of 10.1 Fingerprinting 10.2 Geological microprobe analysis aqueous sample solutions. In addition to the usual analytical advantages of ICP-MS, LA o ers reduced sample preparation, 10.3 Isotope ratios 11 Prospects rapid sample exchange and throughput, reduced spectral inter- ferences and the possibility of in situ spatially resolved analysis. 12 References The key challenge in LA-ICP-MS, evident from the first feasibility study of the technique, which employed pulses from 1 Introduction a ruby laser,7 is to obtain fully quantitative analyses. Houk et al.1 first demonstrated the combination of an argon Developments in both laser systems and spectrometer technol- inductively coupled plasma (ICP) and a quadrupole mass ogy are beginning to meet this challenge. spectrometer for elemental analysis of aqueous sample solu- In this paper, the development, performance and future tions. The technique, now known as inductively coupled prospects of LA-ICP-MS are discussed. The diverse and plasma mass spectrometry (ICP-MS), developed rapidly, successful applications of the technique to biological, geologi- especially after the launch of commercial instruments in cal and metallurgical samples are selectively illustrated from 1983–84, and is now a standard method for multi-elemental the literature, which is now extensive.7–229 Only articles pub- and isotope ratio analysis of diverse biological and geological lished in international journals in English by September 1998 samples.2 Recognized advantages of ICP-MS include direct have been considered. Apologies are given in advance for any analysis of solutions, calibration against aqueous important literature that has escaped my net. It is appropriate standards, pg ml-1 detection limits for many elements, a wide here to refer the reader to some key papers. Fundamental elemental coverage and a linear dynamic range of up to 10 aspects of laser probes, including laser hardware and operation, orders of magnitude. Although the use of aqueous solutions laser–sample interactions and laser ablation combined with is usually both convenient and successful, analytical di culties OES, atomic absorption spectrometry (AAS), atomic fluores- sometimes arise. Matrices such as milk powder or tea leaves cence spectrometry (AFS) and mass spectrometry (MS) are resist dissolution, necessitating the use of potentially hazardous dealt with by Moenke-Blankenburg.230–232 The literature on reagents such as H 2 O 2 and HF. Moreover, any sample hand- laser–solid interactions is also extensively covered by Dittrich ling involves the risk of contamination and the loss of volatile and Wennrich.233 Diverse applications of lasers, including elements. In addition, dissolution procedures are laborious to some early LA-ICP-MS literature, are examined by Darke and develop and even when successful require hours (or days) to Tyson.234 In a narrower review, Radziemski235 discusses ana- perform. Hence sample preparation becomes the limiting factor lytical applications of laser plasmas published between 1987 in the analytical procedure. Another analytical di culty is the and 1994. These papers provide a grounding in the physical presence of spectroscopic or non-spectroscopic interferences principles of laser ablation. in the sample analysis.3 Well known examples of these are the In 1985, Gray reviewed the emergence of ICP-MS in a paper polyatomic species 40Ar35Cl+ , which complicates the determi- entitled ‘The ICP as an Ion Source—Origins, Achievement nation of (monoisotopic ) arsenic at m/z 75, and the suppression and Prospects’,236 hence the title of the present paper. Today, of analyte responses in the presence of a high concentration considering only analytical journals, there are about 2000 of an easily ionized element (e.g., of trace elements in brine). articles dealing with ICP-MS, hence a comprehensive review Dissolution reagents often contribute elements that are inter- of ICP-MS would not fit comfortably within the compass of a single paper. The incentive for the writing of the present paper lay not only in the exciting new developments occurring in LA-ICP-MS but also in the rapidly growing literature, Presented at the 1999 European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999. which is ripe for consolidation. Not every aspect of J. Anal. At. Spectrom., 1999, 14, 1385–1403 1385
Transcript
REVIEW
Laser ablation inductively coupled plasma mass spectrometry:achievements, problems, prospects†
Steven F. Durrant
Laboratorio de Processos de Plasma, DFA, IFGW, Universidade Estadual de Campinas(UNICAMP), CP 6165, 13083–970, Campinas, SP, Brazil
Received 4th March 1999, Accepted 25th May 1999
1 Introduction ferents with isotopes of analytical interest or that are incorpor-ated into polyatomic interferences.2 Evolution of LA-ICP-MS
3 Laser principles Motivated partly by such difficulties, alternative sampleintroduction methods such as direct sample insertion (DSI ),43.1 Ruby
3.2 Nd5YAG electrothermal vaporization (ETV ),5 spark ablation (SA)6 andlaser ablation (LA)7 began to be used with ICP-MS. The3.3 Excimer
4 Laser ablation systems ability of the ICP to accept vapours and solid aerosolsproduced by such methods for atomization and ionization was5 Laser–solid interactions
6 Particle transport already known from optical emission spectrometry (OES).Indeed, development of these sample introduction techniques7 Optimization
8 Calibration strategies for ICP-OES continues in parallel with their use in ICP-MS.Ablation of solids using pulses from a laser and carriage of9 Performance and problems
10 Growth areas the released material to the ICP in a gas flow, usually ofargon, is a very attractive alternative to the nebulization of10.1 Fingerprinting
10.2 Geological microprobe analysis aqueous sample solutions. In addition to the usual analyticaladvantages of ICP-MS, LA offers reduced sample preparation,10.3 Isotope ratios
11 Prospects rapid sample exchange and throughput, reduced spectral inter-ferences and the possibility of in situ spatially resolved analysis.12 ReferencesThe key challenge in LA-ICP-MS, evident from the firstfeasibility study of the technique, which employed pulses from1 Introductiona ruby laser,7 is to obtain fully quantitative analyses.
Houk et al.1 first demonstrated the combination of an argon Developments in both laser systems and spectrometer technol-inductively coupled plasma (ICP) and a quadrupole mass ogy are beginning to meet this challenge.spectrometer for elemental analysis of aqueous sample solu- In this paper, the development, performance and futuretions. The technique, now known as inductively coupled prospects of LA-ICP-MS are discussed. The diverse andplasma mass spectrometry (ICP-MS), developed rapidly, successful applications of the technique to biological, geologi-especially after the launch of commercial instruments in cal and metallurgical samples are selectively illustrated from1983–84, and is now a standard method for multi-elemental the literature, which is now extensive.7–229 Only articles pub-and isotope ratio analysis of diverse biological and geological lished in international journals in English by September 1998samples.2 Recognized advantages of ICP-MS include direct have been considered. Apologies are given in advance for anyanalysis of solutions, calibration against aqueous important literature that has escaped my net. It is appropriatestandards, pg ml−1 detection limits for many elements, a wide here to refer the reader to some key papers. Fundamentalelemental coverage and a linear dynamic range of up to 10 aspects of laser probes, including laser hardware and operation,orders of magnitude. Although the use of aqueous solutions laser–sample interactions and laser ablation combined withis usually both convenient and successful, analytical difficulties OES, atomic absorption spectrometry (AAS), atomic fluores-sometimes arise. Matrices such as milk powder or tea leaves cence spectrometry (AFS) and mass spectrometry (MS) areresist dissolution, necessitating the use of potentially hazardous dealt with by Moenke-Blankenburg.230–232 The literature onreagents such as H2O2 and HF. Moreover, any sample hand- laser–solid interactions is also extensively covered by Dittrichling involves the risk of contamination and the loss of volatile and Wennrich.233 Diverse applications of lasers, includingelements. In addition, dissolution procedures are laborious to some early LA-ICP-MS literature, are examined by Darke anddevelop and even when successful require hours (or days) to Tyson.234 In a narrower review, Radziemski235 discusses ana-perform. Hence sample preparation becomes the limiting factor lytical applications of laser plasmas published between 1987in the analytical procedure. Another analytical difficulty is the and 1994. These papers provide a grounding in the physicalpresence of spectroscopic or non-spectroscopic interferences principles of laser ablation.in the sample analysis.3 Well known examples of these are the In 1985, Gray reviewed the emergence of ICP-MS in a paperpolyatomic species 40Ar35Cl+, which complicates the determi- entitled ‘The ICP as an Ion Source—Origins, Achievementnation of (monoisotopic) arsenic at m/z 75, and the suppression and Prospects’,236 hence the title of the present paper. Today,of analyte responses in the presence of a high concentration considering only analytical journals, there are about 2000of an easily ionized element (e.g., of trace elements in brine). articles dealing with ICP-MS, hence a comprehensive reviewDissolution reagents often contribute elements that are inter- of ICP-MS would not fit comfortably within the compass of
a single paper. The incentive for the writing of the presentpaper lay not only in the exciting new developments occurringin LA-ICP-MS but also in the rapidly growing literature,†Presented at the 1999 European Winter Conference on Plasma
Spectrochemistry, Pau, France, January 10–15, 1999. which is ripe for consolidation. Not every aspect of
J. Anal. At. Spectrom., 1999, 14, 1385–1403 1385
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LA-ICP-MS can be addressed here, but wherever possible the was only classified in the Glass category if its analysis was themain feature. Many papers involve some analysis of glassreader is directed to the relevant literature for greater details.fusions or calibration based on glass standards, but these fallunder Geological.)2 Evolution of LA-ICP-MS Some specific applications are dealt with in Sections 9and 10.The roots of LA-ICP-MS lie in part with the logical extension
of previous experiences with LA-ICP-OES,237,238 and in part3 Laser principleswith the anticipated analytical advantages indicated above.
Fig. 1 shows the evolution of the publications dealing with or By late 1953, a source of coherent microwave radiation hadcontaining LA-ICP-MS analyses since 1985. The number has been constructed at Columbia University.239 This was the firstincreased each year and there are now more than 200. (Note maser, an acronym for microwave amplification by stimulatedthat the data for 1998 are for 9 months only and the yearly emission of radiation. Its operation relies on the existence oftotal is expected to be greater than that of 1997.) The subjects two energy states of the ammonia molecule (NH3) and theseof the publications have been classified into the following depend upon the position of the nitrogen relative to thecategories: geological; biological; metal; polymer; glass; cer- hydrogen atoms. The three hydrogen atoms of the ammoniaamic; and fundamental. For simplicity, each paper was classi- molecule may be considered as occupying the three apexes offied into one category only. In the category Geological fall an equilateral triangle, while the nitrogen atom lies eitheranalyses of soils, minerals, mineral inclusions, prepared above or below the centre of the triangle. Transition of thefusions, etc. Papers classified as Fundamental include studies molecule from the high to the low energy state results in theof sample transport to the ICP, chemical fractionation, and emission of a photon of wavelength 1.25 cm. When a popu-so on. Although such classification is somewhat subjective, lation inversion is produced (more than 50% of the moleculessome useful observations may be made: are in the higher energy state), incident photons of this
(1) geological applications of LA-ICP-MS have always been wavelength (i.e., of energy equal to the energy differenceof interest, typically accounting for 30–50% of all between the upper and lower states) will stimulate furtherLA-ICP-MS papers; downward transitions and produce an avalanche of photons
of the same wavelength and direction of propagation. If these(2) in the last few years, biological applications have receivedphotons are retained in a reservoir, their release produces aincreasing attention;very intense beam of coherent radiation.(3) as reflected in the number of recent papers categorized
as Fundamental, the ablation process and its relation to3.1 Rubyquantification is beginning to receive the interest it warrants;
(4) analyses of plastics, glass and ceramics have received With a suitable active medium, the principle of the masercould be applied to produce coherent radiation in the visiblelow but continued interest in recent years. (Note that a paper
Fig. 1 Annual number of papers concerning LA-ICP-MS published in analytical journals since 1985. The numbers of publications in sevenapplication areas are also given. See text for more details.
region of the spectrum. This was achieved by Maiman240 ofthe Hughes Research Laboratory using a ruby crystal as theactive medium.
Despite the age of this technology, it is of relevance herebecause ruby lasers were first used to ablate solids for introduc-tion to ICP-MS.7,13 A typical ruby laser is shown in Fig. 2.Generally, the end surfaces of the crystal are highly polishedand silvered, one end fully and the other partially. Rubyconsists of aluminium oxide, Al2O3 , to which has been addeda small proportion (about 0.05% by mass) of Cr2O3 . Stimulatedemission is triggered between energy levels of chromium ions,Cr3+ . Fig. 3 shows a simplified energy-level diagram.Chromium ions in the ground state are excited to an upperenergy band in the process known as optical pumping, pro-duced by an intense flash of white light from the flash tube.
Fig. 4 Temporal forms of the emitted laser power of free-running (NExcitation is followed by a rapid non-radiative transition to mode) and Giant (Q mode) pulses. Reproduced with permissionthe middle metastable level. The relatively long lifetime of the from ref. 247.metastable state allows its population to be increased at theexpense of that of the ground state, achieving a population
stopped parallel to the fixed mirror. Owing to the largeinversion. Spontaneous transitions produce photons, whichpopulation inversion, the gain and the available stored energytrigger further emission. Photons travelling perpendicular toin the laser rod are very high, and the round trip gain in thethe ends of the ruby rod accumulate and are released fromlaser cavity is much greater than one. Consequently, the laserthe partially silvered face.oscillation builds up much more rapidly than in normalStimulated emission depletes the upper lasing level muchoperation and will rise very rapidly to a peak power levelfaster than the pumping rate of the flash tube. The light outputmuch above normal. Thus a very short, very high power pulsethus consists of many intense spikes. This normal or free-is produced. Laser pulses of 10–100 ns and peak powers fromrunning mode is illustrated in Fig. 4. Also illustrated is a107 to >109 W are so obtained (ruby and Nd3+ lasers).Q-switched (Q mode) pulse, which is produced by rapidly
Although the production of giant (Q-switched) pulses stillchanging the quality (Q) factor, a measure of the energyrelies on the rapid change in the Q-factor, this is typically nowstorage capacity of the device, by, for example, rotating onedone by Pockels or Kerr cells rather than rotating mirrors.of the end mirrors. During most of the rotation cycle the
mirror is thus tilted at an angle, so that the two mirrors are3.2 Nd5YAGnot facing each other, and the laser is pumped very strongly
with a pulsed flashlamp. Near the end of the pump pulse a Fig. 5 shows the four energy levels of Nd5YAG (Y3Al5O12).very large population inversion will have been built up in the In this system lasing occurs between the metastable levels. Aslaser material, but no lasing action can begin because the the terminal level is essentially empty at room temperature,mirrors are not parallel. At this point the rotating mirror is the population of E1 can be increased by a relatively small
pump power above that of the E3 level. This is a significantadvantage over the ruby system.
Following the initial success of the ruby laser forLA-ICP-MS, Nd5YAG systems were widely used since theyare relatively simple and cheap, and were incorporated intoseveral commercial LA systems. Ablation using pulses, usuallyof 10–300 mJ at 1064 nm, became very common. Over the last5 years, frequency doubling, tripling and quadrupling haveFig. 2 Layout of a ruby laser. Reproduced with permission from C.been investigated to produce wavelengths of 532, 355 orC. Eaglesfield, Laser Light: Fundamentals and Optical Communication,266 nm and employed for analysis, especially in geologicalMacmillan, London, 1967.applications, where laser–sample coupling is often very poor
Energy
Pumping
Rapiddecay
Lasing
Fig. 3 Ruby laser energy-level diagrams. Reproduced with permissionFig. 5 Nd5YAG laser energy-level diagram. Reproduced with per-from M. J. Beesley, Lasers and Their Applications, 2nd edn., Taylor
532 and 355XeCl 308 Excimer, Lambda-Physik EMG 102 MSC 71ArF 193 Excimer 71Nd5YAG 532 (Green) Modified PE 320 89
266 (UV ) 89Nd5YAG 213 Frequency quintupled, Quantel (now 228
Continuum, Santa Clara, CA, USA)YG-660–10
for matrices such as apatite, quartz, fluorite and calcite. The trated for A=Kr and B=F, where the following are essential:first report of a frequency quintupled Nd5YAG laser applied
Kr*+F2�KrF*+F (1)to LA-ICP-MS is now available.228 These developments inlaser technology are illustrated in Table 1. e−+F2�F−+F (2)
F−+ Kr++M�KrF*+M (3)3.3 Excimer where M is a third body, usually an Ar or He atom, whose
presence is required for conservation of momentum andThis type of laser is beginning to be used in LA-ICP-MS. Itsenergy.operation depends on electronic transitions in molecules.
As indicated in Table 1, both XeCl (308 nm) and ArFConsider a diatomic molecule A2 with a ground state which(193 nm) excimer lasers have been used in LA-ICP-MS. Tois repulsive and an excited state as shown in Fig. 6. As thethe author’s knowledge, neither KrF (248 nm) nor XeFground state is repulsive, species A only exist in the monomer(351 nm) has yet been studied for this purpose. The formerform A in the ground state. The potential energy curve forwas employed, however, in LA-ICP-OES.241 Another interes-the excited state, however, possesses a minimum, so thatting possibility is the use of Ce3+-doped LiSrAlF6 , whosespecies A exists in the dimer form A2 in the excited state,output is tunable in the ultraviolet (UV ) region from 280 todesignated A2* and known as an excimer, from excited dimer.320 nm.242If a large number of excimers are created, lasing can occur
between the upper bound state and the lower free state asindicated in Fig. 6. A common type of laser is based on excited 4 Laser ablation systemsstates of a noble gas (A) halide (B) molecule, AB. Since atoms
A typical LA-ICP-MS system is shown in Fig. 7. The laserA and B are distinct, this is an exciplex laser, from excitedmay be mounted vertically or horizontally. In the latter, thestate complex, but is commonly called an excimer laser.beam is deflected on to the sample cell by a 45° mirror,Pumping mechanisms for such systems are complex as illus-
Fig. 7 Schematic diagram of a typical laser ablation inductivelyFig. 6 Excimer energy-level diagram. Reproduced with permissionfrom Principles of Lasers, ed. O. Svelto and D. C. Hanna, 2nd edn., coupled plasma mass spectrometry (LA-ICP-MS) system. Adapted
with permission from ref. 40.Plenum Press, New York, 1982.
typically incorporated into a viewing microscope. Optical and not usually possible, in practice some empirical investigationsof the influence of key parameters are necessary.visual focusing are adjusted to coincide. Often the sample
surface may be viewed remotely via a camera. Numerous publications exhibit electron micrographs of thepits produced in different matrices by laser pulses, thusThe sample sits within a glass ablation cell, the upper face
of which may be slanted at 45° to the vertical to reduce back revealing the nature of the ablation process. The matrixdependence of the ablation process is illustrated, for example,reflection. For use with ultraviolet lasers, a fused silica window
must be installed in the cell.97 A platform or turntable, which by images of laser pits produced in gold bullion and crystallinenatural gold,69 copper,71 pyrite (FeS2) and chalcopyriteis usually under computer control, allows positioning of the
sample in the X, Y and Z directions. Displacements as small (CuFeS2),101 steel,106 brass,107 seashell,118 aluminium,149 sili-con glass and scheelite (CaWO4),154 nickel-base alloy247 andas a few microns may be employed and the laser firing arranged
to occur only between sample movements. According to the fluid inclusions in quartz.219Part of the material that erupts from the ablation crater ispattern of laser shots, individual features such as mineral
grains in rocks may be analysed, or depth profiling, line in the form of particles. These may be fragments but manyare spherical droplets. The number density of particles typicallyprofiling, area or bulk analyses are possible.
The cell is fed an inert gas for scavenging of the ablated increases with decreasing radius as shown in Fig. 8 for particlesproduced by the ablation of Mo using 180 mJ Q-mode pulsesmaterial; this is the equivalent of the nebulizer flow in conven-
tional solution nebulization ICP-MS. Argon or occasionally from an Nd5YAG laser (1064 nm). Since, for spherical par-ticles, the particle mass depends on r3 , where r is the particlehelium is employed; the latter is reported to improve scaveng-
ing.205 The sample remains at atmospheric pressure and this radius, larger particles contribute greatly to the total mass.is an advantage over many other analytical techniques, suchas X-ray photoelectron spectroscopy, where the sample must 6 Particle transportbe under vacuum to permit analysis. As air must not enter theICP, an arrangement for cell purging to atmosphere must be The transport efficiency to the ICP is typically low for small
diameter particles <5 nm, which tend to be lost by diffusion,included, usually by a three-way valve in the cell to ICPtransport line. and is also low for large particles >3 mm, which settle out
owing to gravity. Particles with diameters between theseextremes are carried with an efficiency of >80%.
As pointed out by Arrowsmith and Hughes,14 the overall5 Laser–solid interactionsefficiency of the system may be defined as the fraction of
The interactions between the incoming radiation and the solid ablated material that is ionized in the ICP. This depends onsample depend upon numerous variables related to the laser, the convolution of the particle size distribution, the transportthe sample and the atmosphere above the sample. Among function of the cell and transfer tube and the response functionthese are the wavelength, energy, spatial and temporal form of the ICP. (For simplicity the proportion by mass of sampleof the laser beam and the heat capacity, heat of vaporization vaporized to that in the form of particles is consideredand thermal conductivity of the sample.230,233 negligible.) Clearly, inter-shot variation, for example in laser
The incident beam is partially reflected by the sample surface energy, can influence the particle size distribution and henceto a degree that depends on the nature of the surface and the transport efficiency (and ultimately the system response).which decreases as the temperature of the sample surface is Alteration of the cell gas flow rate may alter particleincreased. An advantage of using giant pulses is the very high scavenging and hence the particle distribution. An additionalrate at which energy is deposited at the surface, which so complication in this case is that the cell flow also determinesdecreases reflection that it can be neglected. Absorption of the optimum plasma to cone-orifice sampling depth.incoming photons produces photoelectrons; ions may also be Readers interested in further literature are directed to aemitted. The conversion of incoming energy to heat is very recent simple and clear review of laser ablation sampling forrapid, leading first to melting, then boiling, over the area of chemical analysis, which treats particle transport, mass load-impact. Evolution of this process depends on the parameters ing, chemical fractionation, sensitivity enhancements, optimiz-indicated above. Specifically, the time required for the sample ation and calibration.243to be raised to its vaporization temperature is given by
tv=pKrC(Tv−T0)2/4P2 (4)
where K is the thermal conductivity, r the mass density of thesample, C the heat capacity, T0 the initial temperature, Tv thevaporization temperature of the sample and P the laser powerdensity. As K, C and Tv depend on the composition of thesample, tv varies with composition. Hence different elementswill vaporize at different rates. Moreover, at the periphery ofthe crater high temperature gradients exist, allowing segre-gation of elements of high and low boiling-point.Consequently, the ablated material, in the form of dropletsand vapour, may not have a composition wholly representativeof the original sample.
The nature of the scavenging gas is a further complicatingfactor because in the presence of photoelectrons and ionsincident photons can be absorbed by the inverse bremsstrah-lung process. In infrared laser ablation, for example, complexplasmas form above the sample and are thought to be partlyresponsible for removal of sample material.71 Indeed, theablation process is so complex that case studies are rec-ommended to determine appropriate ablation conditions for Fig. 8 Particle size distribution obtained by laser ablation of molyb-
denum metal. Adapted with permission from ref. 14.each sample type, and although exhaustive investigations are
7 OptimizationBoth laser and mass spectrometer parameters may be optim-ized for maximum response or response-to-background ratios.Key laser parameters include the wavelength, mode, focusing,energy and number of shots. A few comments on the influenceof these parameters may be made. The laser wavelength hasbeen mentioned in previous sections and is determined by thetype of laser. Commercial ablation systems have traditionallybeen based on Nd5YAG lasers of fundamental wavelength1064 nm. In the last few years, methods to produce harmonicfrequencies have been widely employed. These allow lowerwavelengths to be used, resulting in better spatial resolution,lower fractionation effects and improved analytical precisions.Once the laser wavelength has been chosen, however, it isfixed for any particular application. This will remain so untiltuneable lasers are employed for ablation.
Fixed-Q (free-running) pulses typically produce greaterablated mass than Q-switched pulses of the same energy. Free-running pulses have been successfully used in appli-cations,43,57,76 but Q-switched pulses are more frequentlyemployed, especially in recent studies.
Optimum sensitivity and precision are often obtained with Fig. 9 Response as a function of carrier gas flow rate for (&) 40Ar2+ ,focusing displaced from the sample surface. For example, for (+) Li+ , ($) La+ and (#) Pb+ when a powdered JB1 sample was
ablated with N mode pulses of 0.81 J. Adapted with permissionablation at 1064 nm using Q-mode 0.18 J shots (pulse widthfrom ref. 19.15 ns) at a repetition rate of 4 Hz, Abell21 found that for the
ablation of a nickel alloy optimum sensitivity and precisionwere obtained when the focusing was about 12 mm below thesample surface. The optimum defocusing for sensitivity was
the responses of 40Ar2+ , Li+ , La+ and Pb+ produced by thealways close to the optimum defocusing for precision, but wasablation of powdered Geological Survey of Japan JB1 (basalt)material dependent, being 4 mm for glass, 10 mm for siliconby 0.81 J free-running pulses (1064 nm).19and 0 mm for quartz.21
The rapid scanning ability of the quadrupole spectrometersTypically, greater laser energies produce greater ablatedis essential for capturing the transient or rapidly fluctuatingmasses, greater transport to the ICP and hence greaterresponses typically obtained with laser ablation. Spectrometerresponses for a constant analyte concentration. (Of course, weparameters to be selected by the operator include the numbermust remember that the transport efficiency depends on theof elements, quadrupole settling time, dwell time, sweeps perparticle size distribution, which may be a function of laserreading, readings per replicate, number of replicates and pointsenergy.) Sufficient response for good quantification (accuracyper spectral peak. In a very useful Interlaboratory Note,and precision) is required without saturation of mass peaks atLongerich et al.139 examine the choice of these parameters form/z values of interest. Saturation of individual peaks may beLA-ICP-MS analysis. Several important recommendations aretolerable if other elements present at trace concentrations aregiven: (1) temporal responses should be observed to obtainthe only analytical interest. Excess ablated material, however,an overview of the ablation process; (2) a background countwill have three deleterious results: memory effects, cone block-should be acquired before the sample analysis; (3) for instru-age and plasma disequilibrium. Particulate matter depositedments with a settling time of ca. 1.5 ms, a dwell time isin the cell and cell-to-ICP transfer line is likely to berecommended that is about six times the quadrupole settlingre-suspended in the cell flow in subsequent analyses, leadingtime and is a multiple of the mains frequency; and (4) oneto memory effects. As the cell will accumulate sample debrispoint per mass peak should be used.even in normal operation, regular cleaning of the cell and
Recommendation (1) derives from the observation thatreplacement of the transfer line are recommended.temporal separation of elemental responses may indicateCone blockage and plasma disequilibrium are more seriousspecific processes such as the ablation of bulk rock and theand must be avoided. Deposition at the sampling-cone orificevaporization of an inclusion.or on the skimmer leads to erratic responses and eventually
Estimation of detection limits is necessary but there is noshuts down the ICP. Plasma disequilibrium results from excessagreed method of calculation. Line equivalent backgroundsample loading, which the ICP is unable to volatilize andconcentrations,43,57 or calculations based on the standardionize, preventing quantification.deviation of the blank response at a single mass56 or theA single laser shot produces a transient response19 similarconventional standard deviation of the blank (3s) approach32in temporal form to that observed in flow injection. Multiplehave been used.shots at repetition rates of a few Hz produce quasi-stable
Considering recommendation (3), note that the settling timeresponses, which are more useful for quantification. In someis the time taken for the detector response to stabilize followingcases,21 pre-ablation of the sample under optimized lasera peak hop in the selected m/z value and that the dwell timeconditions for several minutes prior to the analysis improvesis the integrating time per selected m/z. The relation dwellthe analytical precision.time=6×settling time ensures that only about 15% of theSpectrometer parameters such as the forward rf power andtotal time is lost (not used for measuring).gas flow rates also need to be optimized as they would in
Recommendation (4) is very interesting and has a strongsolution nebulization (SN )-ICP-MS. Choice of carrier gasrationale.139 One point per peak yields the following advan-(cell ) flow rate is important for maximum analytical perform-tages: maximum response; minimum change in response withance but in multi-elemental analysis a compromise is inevitableany change in mass calibration; maximum counting efficiency;since maximum responses do not necessarily occur at the same
flow rate. This difficulty is illustrated in Fig. 9, which shows and best abundance sensitivity.
8 Calibration strategiesVarious strategies are employed to obtain elemental concen-trations in solids by LA-ICP-MS. Quantification using a single-point calibration, based on the sensitivity obtained by analysisof a sample containing an element of known concentration, issimple but generally inaccurate. A more sophisticatedapproach is to use elemental response factors determined bySN-ICP-MS and modified based on element-dependent volatil-ization efficiencies.15 This method, derived from a thermalmodel of the ablation process, requires either the knowledgeof one analyte concentration and the ablation temperature TAor two or more analyte concentrations from which TA can bedetermined. Although external standards are not necessary,the obvious disadvantage is that the concentration data arenot fully quantitative.
From sensitivities obtained from analysis of a single multi-elemental standard, a response–mass curve may be con-structed, allowing elements other than those of known concen-tration in the standard to be determined. Fig. 10 shows sucha response curve obtained by the analysis of an artificial
Fig. 11 Concentrations of a range of elements in geological carbonateCaCO3 standard39 containing Li, Mg, Mn, In, Ba, Pb and Ureference materials determined by LA-ICP-MS as a function ofat 100 mg g−1 . Curves (a) and (b) were obtained without andaccepted values. Reproduced with permission from ref. 39.with correction by the Saha factor, which accounts for differ-
ences in the degree of ionization in the ICP. Curve (b) showsa more consistent fit than curve (a) and more closely resembles drying and pressing,39,78 80 the production of glass fusions26,32those obtained by a similar procedure employed in and the production of sintered compacts.150SN-ICP-MS. The agreement between the determined and Although the US National Institute of Standards andaccepted concentrations of Mg, Mn, Fe, Cu, Zn, Sr, Ba and Technology (NIST) produces a number of well known glassU in two limestone reference materials is shown in Fig. 11. standards (SRM 610, 611, 612 and 613), the wide use of such
Element-for-element calibration against sensitivities standards for calibration in LA-ICP-MS has led to additionalobtained by analysis of an external well characterized matrix analyses and compilations of concentration data as an aid tohas been used for the multi-elemental analysis of diverse the analytical community.244,245 Glasses of known compositionbiological41,76 and geological samples.43,57 Numerous diffi- have also been prepared using the coprecipitated gelculties, however, are associated with this approach: (i) the technique.246samples and standard need to be matrix-matched, (ii) the In-house polymer standards have been produced by addingstandard must be well characterized and must contain all the metal stearates to a range of polymeric materials via theelements of interest at concentrations sufficient to give good melt.110sensitivity and (iii) the standards must be readily available Geochemical reference standards of zircon have been fabri-and cheap. In addition, if response–concentration curves for cated by finely pulverizing zircon and sintering it with a loweach element are to be produced for maximum accuracy and melting temperature phase, such as albite, at high temperatureprecision, a complete set of standards is required. and pressure to produce a sintered zircon compact.150
Much effort has been expended in finding ways to produce A novel procedure was devised for the analysis of particulatemulti-element solid standards cheaply and rapidly. Approaches matter trapped on a cellulose nitrate membrane filter.220include the addition of individual compounds to a powdered Known elemental masses were added to the filter in solutionmatrix, mixing and pressing with (or without) a binder,75 the to act as standards. The concentrations of Al, Ti, V, Cr, Mn,addition of liquid standard solutions to a powdered matrix, Fe, Co, Ni, Cu, Zn, As, Se, Cd, Sb and Pb showed good
agreement with instrumental neutron activation analysis(INAA) and X-ray fluorescence ( XRF) data. Precisions(RSDs) were typically less than 10%.
The preparation of solid standards is time consuming, butmuch less so than many commonly used dissolution pro-cedures, and yields good quantification. Contamination duringthe production of solid standards is not usually observed,although problems occasionally arise. For example, tungstenfrom the lining of a tungsten mill has been observed tocontaminate sediments during ball-milling.43
One area where calibration is easier is metals analysis, forwhich more standards exist. Moreover, complete matrix-matching of metals is sometimes unnecessary. For example,Fe, Ni, Cu, Zn, Pd, Pt, Pb and Bi have been determined ingold, silver and gold–silver alloys using either gold or silverreference materials as standards.70 The ease of ablation andthe relatively homogeneous distribution of elements in metalssimplify metals analysis.
An alternative to solid standards is so-called ‘solid–liquidFig. 10 Response curve as a function of m/z obtained by analysis ofcalibration’ in which a dual flow system allows simultaneousa 100 mg g−1 multi-element standard: (a) uncorrected; (b) responsesintroduction of laser ablated solids and nebulized aqueouscorrected by the Saha factor. Units of response, normalized counts s−1 .
Adapted with permission from ref. 39. solutions. As both forms of material are present in the ICP,
calibration against the sensitivities obtained by analyzing ment (MTM) cell and employing a UV/VIS spectrophotometerwas constructed by Watling.197 Variations in the integratedaqueous standard solutions is possible.59 An obvious disadvan-
tage, which will be discussed in more detail in Section 9, is the total scattering effect of the ablated particles, obtained bysumming the sub-peak areas of the intensities produced bypresence of oxide and hydroxide interferences associated with
the ‘wet’ plasma. this scattering, were used to normalize the analytical results.Accuracies were improved and precisions (RSDs) loweredIn the last few years, hardware and procedures for the
introduction of dry aerosols for calibration have been devel- from 25 to about 5% using this normalization in the determi-nation of Co, Ni, Cu, Zn, Ga, Ge, Mo, Pd, Ag, Cd, Sn, Sb,oped. These seek to retain the advantages of working with
standard solutions but avoid the associated interferences by Te, Pt, Au, Hg, Tl, Pb, Bi, Th and U in arsenopyrite.‘drying’ the aerosol before it is introduced to the ICP.145,158One development is the use of an internal standard vapour 9 Performance and problemsgenerator (ISVG), which is essentially an electrothermal vapo-rizer adapted to generate vapour of selected elements con- Factors of analytical interest include laser–sample coupling,
fractionation, accuracy, precision, detection limits, linearstantly for mixing with the laser ablation cell flow prior tointroduction to the ICP.145 To achieve this, metal filaments of dynamic range, interferences, throughput, and so on. These
cannot easily be summarized since they depend on the elementsthe internal standard elements Mo and Re were heated closeto their melting-points (2627 and 3180 °C, respectively) and or isotope ratios to be determined, the sample type and the
laser and spectrometer parameters. A survey of interestingthe resulting vapour was removed by an argon flow of0.2 l min−1. A response stability of <1% RSD was obtained applications in environmental analysis including of
metals,27,84,163 geological samples,24,26,31,32,45,75,86,87,105,161over 15 min.A second development is to desolvate multi-element stan- biological materials,67,76,84 ceramics,141 and airborne particu-
lates83,160,220 is given elsewhere.247 Analytical performance anddard solutions for mixing with the outflow from the laserablation cell.158 Gas flow rates and the desolvator temperature difficulties are illustrated here by a few specific applications.
Chen et al.161 used Q-switched pulses from a Nd5YAG laserwere systematically optimized. The formation of oxides wasgreatly reduced, as indicated by CeO+/Ce+ values of <0.3%. (1064 nm) for microprobe analysis of apatite, monazite, chro-
mite, olivine and silicate rock. Sensitivities obtained by theAnother advantage of this method is the ease of optimizationof the voltages on the ion optics and of gas flows. Good analysis of NIST SRM 610 and 612 silicate glasses were used
for calibration. Internal standard elemental responses (Ca, Ce,accuracy but large RSDs were obtained for the determinationof rare earth elements (REE) in IAEA Soil 7. The poor Si or Mn) were employed to correct for response drift,
differences in transport efficiency and sampling yields forprecisions, however, were related to instability of the desolv-ation unit and are currently being improved. different geological materials. Only a selection of the analyses
performed are covered here.Isotope dilution analysis, in which samples are spiked withknown masses of specific isotopes, allows full quantification Table 2 shows the results of the analysis of apatite. Calcium,
whose concentration was determined by electron probe micro-and may be used in LA-ICP-MS as in SN-ICP-MS. Forexample, the precisions of determinations of Ni, Cu, Cd, Ba analysis (EPMA), was the IS.
Following conventional practice, chondrite-normalized REEand Pb through the addition of 61Ni, 65Cu, 106Cd, 135Ba, and204Pb were between 0.07 and 0.10% in most cases in the patterns, obtained by dividing the elemental concentrations
obtained for the sample by those of a standard meteorite,analysis of leaf material; with the exception of Cd, excellentaccuracy was achieved compared with SN-ICP-MS.137 were constructed for LA-ICP-MS and SN-ICP-MS concen-
tration data, yielding the curves shown in Fig. 12. The rela-Owing to variations, for example in laser energy outputbetween shots, sequential responses obtained under the same tively smooth curves (with the exception of Eu) indicate
geologically consistent results. The analytical precisions, how-conditions are not identical. To compensate for such fluctu-ations, internal standardization (IS) is almost always necessary ever, are poor, reflecting the uneven ablation of this material.
The latter is evident from scanning electron micrographs ofin LA-ICP-MS. This is conveniently done using the responsesof a minor isotope of a major element, which may be expected the laser craters produced.
Table 3 shows the results of the analysis of silicate glassto be relatively uniformly distributed in the matrix. Improvedprecision results from IS on a set of replicate analyses; IS buttons produced by fusion of Syenite SY-2 or Andesite
AGV-1 with lithium metaborate (LiBO2). Agreement betweenfrom standard to sample should also improve accuracy and ispossible when the concentration of at least one suitable elementis known. In addition to being homogeneously distributed, an Table 2 Comparison of LAM-ICP-MS and solution nebulization (SN)IS element should behave in the ablation process and in the ICP-MS analysis of apatite (concentrations in mg g−1). ReproducedICP in a manner identical with that of the analytes. In practice, with permission from ref. 161therefore, an IS for specific groups of elements such as REE,
LAM-ICP-MS SNplatinum group metals (PGM ), etc., are required to obtainElement (n=14) RSD (%) (n=3)accurate and precise concentration data.
An alternative to IS is to normalize elemental responses to Y 332 30 460the amplitude of the acoustic wave generated by laser ablation. La 2838 19 3200This improves the precisions of concentration data obtained Ce 4310 7 4200
Pr 310 7 340by LA-ICP-OES28 but the suggestion by the authors of thisNd 822 19 990research that this approach could be applied to advantage inSm 101 28 130LA-ICP-MS does not appear to have been taken up.Eu 17 39 20Compensation for variation in transported mass may also Gd 84 37 120
be made using light-scattering measurements obtained using a Tb 12 47 20photomultiplier tube (PMT) mounted above the cell to ICP Dy 58 39 76
Ho 14 58 15transport line perpendicular to the beam of a He–Ne laser.218Er 37 62 36Although not as effective in improving precisions as the useTm 5.6 66 5.3of IS, this method requires no previous knowledge of theYb 27 67 20sample homogeneity or elemental composition. Lu 7.6 93 4.3
A similar system denoted an in-line mass transport measure-
LA-ICP-MS competes favourably with other state-of-the-artsolid state techniques for these analyses.
Detection limits depend on the analytical conditions, par-ticularly the ablated mass, which depends in turn on the laserpower density. Jeffries et al.100 compared the performance ofinfrared (1064 nm) and ultraviolet (266 nm) lasers for themicroprobe analysis of olivine, garnet, plogopite, magnetite,apatite, calcite, quartz and feldspar. The detection limits(Fig. 13) indicate the superior performance of the UV laser.This laser also provides excellent spatial resolution. Forexample, at low power a lateral resolution of about 4 mm isobtained for most minerals.
When simultaneous counting and current detection areemployed, a linear dynamic range of at least six orders ofmagnitude may be obtained in LA-ICP-MS.
In routine operation, 60–70 samples per day can be analyzedby SN-ICP-MS, and this is easily equalled in LA-ICP-MSFig. 12 Mean chondrite-normalized data for apatite. Data obtainedbecause of the rapid sample exchange and cell purging.using LA-ICP-MS and solution nebulization ICP-MS. Reproduced
A principal source of analytical difficulties in SN-ICP-MSwith permission from ref. 161.is the presence of water-related spectral interferences. Theseare greatly reduced in LA-ICP-MS as illustrated in Table 6,taken from ref. 41. Although interferences may arise in thethese data and literature values is reasonable, with the RSDsample analysis from the presence of refractory elements atfor most elements being <20%. As is to be expected,57 poorermajor concentrations, this is rarely a practical limitation. Forprecisions are obtained for those elements whose concen-example, the reduction in oxide interferences, MO+, is advan-trations are close to the detection limits. These data show thetageous for the determination of REE which have high oxidepromise and limitations of ablation of complex matrices usingbinding energies. In the determination of REE and Ba inradiation of wavelength 1064 nm.ferromanganese crusts,87 no significant barium-related inter-Table 4 shows concentration data obtained in the analysisferences were detected, despite the high concentration of Ba.of BCR-2G glass by SN-ICP-MS, LA-ICP-MS and, whereTable 7 shows the elements determined and the (unobserved)possible, proton-induced X-ray emission (PIXE).217 Thepossible interferences.samples were ablated using 1–2 mJ pulses at 266 nm and a
A serious problem associated with LA is non-representa-repetition rate of 4 Hz. The data for SN-ICP-MS andtive sub-sampling, or fractional ablation.71,97,99,107,116,LA-ICP-MS agree within the combined analytical uncertainty.129,144,153,165,204,205,206,248 This phenomenon is generally dif-The largest discrepancy is for Cu, which may be the result officult to correct since it causes increases or decreases inthe interference of 48TiOH on 65Cu in the solution analysis, arelative analytical responses and sensitivities.solution blank problem, or contamination from the saw blade
Chenery et al.248 examined the material produced byused to slice the sample. Inaccuracy may also result fromablation of pyrite (FeS2) and olivine [(Mg, Fe)2SiO4 ] usingerrors in the calibration concentration data.N- and Q-mode pulses from a ruby laser at 347 nm, and fromBecker et al.195 determined Zn, B, Si, Ge, Sn, Sb, P, S, Sean Nd5YAG laser at 532 nm. Scanning electron microscopyand Te in a synthetic GaAs standard using secondary ion mass(SEM) and energy-dispersive X-ray (EDX ) spectrometry werespectrometry (SIMS), SN-ICP-MS, ICP-OES, spark sourceemployed for morphological and chemical characterization.mass spectrometry (SSMS), radiofrequency glow dischargeAblation of pyrite by N-mode pulses (694 nm) of 1 J producedmass spectrometry (GDMS) and LA-ICP-MS. Table 5 showsspherical particles with compositions principally in the rangethe concentration data obtained. For the LA-ICP-MS analysesFeS1.2–FeS1.7 . This results from the relatively long N-modeablation at 20 Hz using 10 ns pulses (266 nm) was used withpulses (500 ms). The X-ray emission ratio of S5Fe was fairlydetection by a double-focusing spectrometer in the reverse
Nier–Johnson geometry. Inspection of Table 5 reveals that constant as a function of particle diameter (although without
Table 3 Rare earth element composition of fused standard reference material powders determined by LAM-ICP-MS (concentrations in mg g−1).Adapted with permission from ref. 161
Table 4 Comparison of solution ICP-MS, laser ablation (LA) ICP-MS and proton microprobe (PIXE) analyses of glass standard BCR-2G. Alldata in mg g−1 except for K and Ti in wt.%. Reproduced with permission from ref. 217
Table 5 Results of determination of dopants by different analytical methods and relative sensitivity coefficients (RSC ) in spark source massspectrometry (SSMS), rf GDMS and LA-ICP-MS in synthetic GaAs laboratory standard (concentrations in mg g−1). Reproduced with permissionfrom ref. 195
aInternal standard element. bPossible inhomogeneity.
a large spread), implying that the variable loss of S occurred lar gas of moderate volatility could suffer enhancements dueto very high transport efficiency or losses due to condensationbefore the particles were formed from the liquid.
Semi-Q-switched pulses caused more intense heating of the or reaction with interior surfaces of the ablation system,or both.target and a greater loss of S from the droplets. Greater
thermal and mechanical shock were also produced, resulting A later study of the ablation of a Cu target as a functionof numerous parameters including the laser wavelength andin a separate population of fragments with a composition of
roughly FeS1.9 . energy produced more definite conclusions.71 Ablation usingpulses from a UV laser ( XeCl excimer, 308 nm) was found toThe overall pattern of results is not easily summarized. The
authors concluded that matter differentiated into components be far superior to IR ablation. Reproducibility, spatial reso-lution, quantification, sensitivity and fractionation effects wereof low volatility at ambient temperature will be transported
with an efficiency governed by such properties as particle size improved with the UV laser. These effects were attributed toa different ablation process whereby UV ablation is directand density, whereas materials that produce atomic or molecu-
Table 6 Comparison of background integrals obtained by laser ablation and solution nebulization methods. Reproduced with permissionfrom ref. 41
Analyte isotope Ratio of peak integralsm/z Species abundance (%) ( laser5solutiona)
28 14N2+ Si (92.18) 0.0429 14N2H+ Si (4.71) 0.2030 14N16O+ Si (3.12) 0.1931 15N16O+ , 14N16OH+ P (100) 0.00232 16O2+ S (95.02) 0.0133 16O2H+ S (0.75) 0.000334 16O18O+ S (4.22) 0.0254 40Ar14N+ Fe (5.90), Cr (2.38) 0.0756 40Ar16O+ Fe (91.52) 0.0157 40Ar16OH+ Fe (2.25) 0.0176 36Ar40Ar+ Ge (7.76), Se (9.12) 0.4280 40Ar2+ Se (59.96), Kr (2.27) 1.03
aIntegrals for the solution technique were obtained from integration of nebulized 1% v/v HNO3 solution.
transport rates is questionable, however, because the passageof so much undissociated material shows that the ICP isalready working at its analytical limit. Typically, also, ICP-MSis several orders of magnitude more sensitive than ICP-OES.
In a study of brass and steel standards ablated using thefourth harmonic of a Nd5YAG laser (266 nm, 8 ns), it wasobserved that fractionation is reduced at high laser fluenceand with increasing spatial separation of successive laserpulses.107
For geochemical analysis elemental responses for suites oftrace elements—high field strength, light and heavy REE,large ion lithophile and chalo/siderophile—varied with elementand the type of rock analysed, indicating the complexity ofthe analysis of geological materials.99Fig. 13 Detection limits as a function of laser power density and
Fractionation factors of 60 elements obtained by thecrater diameter for (A) Li, (B) Co, (C) Cs, (D) Ce, (E) Lu and (F)U. Sample: NIST SRM 610. Reproduced with permission from ablation of NIST SRM 610 glass by ratioing the integratedref. 100. response obtained for a second 2 min period to that obtained
in the first 2 min integration, normalized to the Ca response,Table 7 Compilation of isotopes selected for analysis and quantifi- were given by Fryer et al.97 No single physical or chemicalcation (*), acquisition parameters and interferences. Possible inter- property of the elements correlated with or explained theferences in the analysis of ferromanganese crusts. Reproduced with observed pattern of fractionation factors. Groups of elementspermission from ref. 87
such as lithophiles (silicate loving), chalcophiles (sulfide loving)and siderophiles (iron loving), however, exhibited similarElement m/z Abundance (%) Interferencesbehaviour.97,116,144
Pt 194 32.9 [178Hf16O]− The studies of fractionation hitherto outlined have one196 25.3 *, [180Hf16O]− major flaw, namely the failure to address the fact that fraction-
Ba 137 11.32 * ation may occur both during ablation and transport to theLa 139 99.91 *
ICP. This fault in previous studies, including some of theirCe 140 88.48 *own, was courageously pointed out by Outridge et al.,153 whoPr 141 100 *
Nd 145 8.29 * went on to quantify these two sources of fractionation. Mass146 17.26 *, [130Ba16O]− transport effects were smaller than ablation effects for most
Sm 147 15.07 *, [130Ba16OH]− elements in the analysis of copper, but greater than ablationEu 151 47.77 *, [134Ba16OH]− , [135Ba16O]− for all but the most volatile elements (Au and Te) in NIST153 52.23 [136Ba16OH]− , [137Ba16O]−
glass. A possible transport fractionation mechanism was ident-Gd 157 15.68 *, [140Ce16OH]− , [141Pr16O]+ified as differential partitioning between vapour and particleTb 159 100 *, [143Nd16O]−
Dy 163 24.97 *, [147Sm16O]− phases as a function of elemental volatility. The importanceHo 165 100 *, [149Sm16O]− of the dual source of fractionation has not been sufficientlyEr 166 33.41 *, [150Nd16O]− , [150Sm16O]+ recognized in some of the subsequent literature. The currentTm 169 100 *, [153Eu16O]−
unanimous agreement among analysts, however, is that overallYb 172 21.82 *, [156Gd16O]− , [156Dy16O]+fractionation effects are far less with ablation using UV ratherLu 175 97.4 *, [159Tb16O]−than IR lasers.
Transport efficiency to the ICP has recently been increasedfrom 25–50% for the ablation of NIST glasses with only Arwhereas in IR ablation shielding of the incoming radiation byin the ablation cell to around 90% for ablation in He.205 Thea plasma above the sample limits simple direct ablation.mechanism is thought to be that the He atmosphere allowsUltraviolet ablation typically produces greater ablated massthe plasma plume to expand further, giving more time forthan IR ablation under the same conditions. The suggestion71material to condense out of the cooling plasma.that this greater ablated mass might be better exploited using
To close this section, it is worthwhile emphasizing theICP-OES since blockage of the sampling-cone orifice is animpediment to analysis by ICP-MS at high sample mass diversity of successful applications of LA-ICP-MS. Some such
applications have already been discussed above. A further tors, for example, lead55 and strontium90 isotope ratios havebeen determined with RSDs of around 0.05% and 0.01%,illustration of the breadth of LA-ICP-MS applications is
provided by Table 8, where the matrices, analytes and details respectively. A more detailed discussion of this important areais given elsewhere.247of the laser used in various studies are listed. Readers interested
in greater details of a specific application are directed to thereferences provided.
11 Prospects10 Growth areas
Traditionally, relatively cheap, robust, compact, fast-scanningApart from the multi-elemental analysis of bulk materials, quadrupole mass spectrometers have been used for ICP-MS.which we have already addressed, several topics are of growing Over the last few years, however, the capabilities of theinterest: fingerprinting, geological microprobe analysis and technique have been extended by the use of double-isotope ratio measurements. focusing,249–251 ion-trap252 and time-of-flight253 instruments.
Double-focusing instruments were first introduced a decade10.1 Fingerprinting ago,249,250 but were very expensive and cumbersome, being
used mainly for the analysis of high purity semiconductorLA-ICP-MS is the technique of choice for the very rapidmaterials. More recently, cheaper instruments giving greateraccumulation of full mass spectra. In many applications theoperational flexibility have become available, and their usesourcing of the sample is possible directly from such spectrahas increased substantially. The principles of these types ofwithout the need for quantitation. Watling and co-workersspectrometer are sketched in a clear and concise review.254 Ahave developed fingerprinting techniques to source the prov-significant advantage of HR-ICP-MS is the separation ofenance of gold,69 diamonds105 and cannabis crops.198 Thespectral interferences. For example, 56Fe+ may be separatedidentification of safes, firearm barrels and crowbars has alsofrom 40Ar16O+ using a resolution of 2502, allowing thebeen demonstrated.157determination of iron at m/z 56. The improved precision ofThese techniques rely on setting up libraries of multi-elementisotope ratio measurements obtained using such instrumentsspectra of samples from different sources, producing plots offitted with multiple Faraday detectors has already beenthe relative responses of groups of elements (often plotted formentioned.three elements in a ternary plot105), or in the production of
Another interesting possibility is the use of a Mattauch–chondrite normalized profiles when REE are of interest.105Herzog configuration with microchannel plate/photodiodeThese approaches have already been applied with success toarray detection, which allows a full mass spectrum to bethe tracing of stolen gold and diamonds in Australia. Inacquired in <10 ms. Nam et al.229 published a preliminaryaddition to tracing diamonds, trace element distributions ininvestigation of this combination with SN and LA. A Heindicator materials for diamond exploration, such as garnetsrather than an Ar ICP was used, offering relatively simpleand chromites, may be established.105 The developed protocolsbackground spectra because of the absence of Ar-relatedalso permitted the differentiation of sources of cannabis inpolyatomic interferences. Owing in part to the presence of aAustralia, and the determination of whether seized cannabisstrong secondary discharge for the He ICP, inferior detectionwas grown at several sites or was imported. These importantlimits were obtained with He compared with an Ar ICP.analyses can be performed easily and quickly without the needNevertheless, detection limits of a few mg g−1 were obtainedfor full quantitation and represent an important growth areaand there is clearly the possibility that further systematicfor LA-ICP-MS.characterization and optimization will improve these figures.
Shuttleworth and Kremser214 assessed the potential of the10.2 Geological microprobe analysisnew generation HR-ICP-MS with ablation by 3.5 mJ pulses
The use of LA for in situ trace element analysis of liquid or (8 ns) from a Nd5YAG laser (266 nm). The Finnigan MATmineral inclusions in rocks has recently received considerable Element ICP-MS is a sector field spectrometer in which theattention.44,52,60,88,91,93,97,113,146 A few specific developments plasma is maintained at ground potential. Ions from the ICPmerit highlighting. Spatially resolved analysis requires good are extracted and accelerated into the mass analyser, a double-definition of the ablated volume and this has been greatly focusing electromagnetic sector instrument with reverse Nier–improved by the use of UV lasers. When craters of only Johnson geometry. Preset resolutions of 300, 4000 and4–30 mm diameter are ablated (compared with the >100 mm 10 000 may be used. Either the accelerating voltage or magneticdiameter typical of earlier LA systems), the very small ablated field strength may be varied for mass control. The field at themass (a few mg) limits analytical responses. electrostatic analyser is proportional to the applied accelerator
Table 9 highlights a few applications of LM-ICP-MS to voltage and corrects for changes in the ion kinetic energy. Iongeological problems. Readers interested in further treatment detection is by a secondary electron multiplier, the ions hit aare referred to the cited references and to the authoritative conversion dynode and the electrons produced are amplifiedreview by Perkins and Pearce.112 in an electron multiplier which can operate in either counting
or analogue modes. Using this system, detection limits for10.3 Isotope ratiosbulk analysis of NIST SRM 612 glass, at a resolution of 300,ranged from 100 pg g−1 (B) to 5 pg g−1 (U ).In isotope ratio analysis using SN-ICP-MS, analytical pre-
cisions are rarely better than about 0.1% RSD. As we have With micro-sampling (crater diameter about 10 mm), excel-lent agreement was obtained between certified and measuredseen, laser ablation of solid samples involves more sources of
response variability and further degrades analytical precisions. values for Ba, La, Ce, Nd, Sm, Eu, Gd, Dy, Er and Y withprecisions of <2% RSD. Detection limits for these elementsThus in the early work of Tye et al.,8 the following data were
obtained for uranium isotope ratios (with RSDs in parentheses) were <10 ng g−1 . This LA-HR-ICP mass spectrometer rep-resents the state-of-the-art in the field for elemental analy-in uranium oxide (U3O8): U2345U238 0.01644 (1.340%),
U2345U236 0.08725 (0.774%) and U2355U236 5.30754 (1.020%). sis.203,214 A recent application of similar instrumentation tothe determination of PGE (Ru, Rh, Pd, Os, Ir, Pt) and Au inGreat improvements in the capability for isotope ratio determi-
nations have been made with the use of UV lasers but especially nickel sulfide buttons obtained from geological reference mate-rials by fire assay196 achieved detection limits in the rangethe introduction of double-focusing magnetic sector instru-
ments. Using such an instrument with multiple Faraday detec- 0.2–7 ng g−1 , an order of magnitude better than those obtained
Geological—AGV-1, BHVO-1, BSK-1 NIST 49 elements; lithium borate N, Nd5YAG, 1064 nm 1042704, GSD-2 fusionsA biotite, a plagiioclase, a 38 elements in 80 mm sections; cf. UV excimer, 308 nm 115cordierite, an astrophyllite solution ICP-MSOlivines, clinopyroxenes 37 elements Pit diameter Q, Nd5YAG 96
Zircons Hf, Y, REE, Th, U; fusions, Q, Nd5YAG, 1064 nm 36pressed pellets
Reference materials 43 major, minor and trace Q, Nd5YAG, 62SY2, SY3, MRG1, BIR1 elements including REE 1064 nmG2, RGM1Soil IAEA Soil 7 REE Q, Nd5YAG, 1064 nm 80
Metallurgical—JSS 002-1 High Purity Steel B, P, Al, Cr, Co, Ni, Mo N,Q Nd5YAG, 1064 nm 40NIST SRM 1265 Si, Ti, V, Cr, Mn, Fe, Ni; Q, Nd5YAG, 38Electrolytic Iron cf. dc arc OES 1064 nmJSS169 and JSS173 Steel Standards B, Al, Ti, V, Cr, Co, Ni, As, Zr, Low Q, ruby, 694 nm 13
Nb, Mo, Sn, Sb1263a Steel Mn, Cr, Si, Ni, V, Al, Sn, Cu, Ta, Q, Nd5YAG, 266 nm 107
Table 9 Selected applications of laser microprobe (LM)-ICP-MS. can be captured per second. Combined with suitable technol-Unless stated otherwise, a Nd5YAG laser operating at 1064 nm in the ogy, such as an ion reflectron, higher resolution than thatQ-switched mode was employed for ablation obtainable using quadrupole instruments can also be achieved.
Moreover, there is no compromise between sensitivity andApplication Ref.mass coverage. Hence TOF spectrometers are ideal for the
32Major and trace elements determined in carbonates, zircon, analysis of transient responses.253olivine, feldspars. Ablation diameter 20–30 mm In a feasibility study of LA-ICP-TOF-MS with a ruby laser,
25 elements from Mg to U determined in stalactite and fossil 42 a detection limit of 10 ng g−1 was obtained for Pb in a castshell. Nd5YAG, N mode, 0.95 J. Internal standard 43Ca iron standard calculated by integrating 0.3 s transient responses207Pb:206Pb ages determined in single zircon grains, ablation 52
generated by a single laser pulse.133 By similarly acquiring anddiameter 30–60 mm. Precision of 207Pb:206Pb 0.5–6% (1s),ratioing the responses from two Zn isotopes, the substantialNd5YAG with modified optics used
In situ U–Pb geochronology for the direct dating of single 60 pulse-to-pulse power fluctuations from the laser were virtuallypitchblende and zircon grains eliminated. Improved performance is expected from such
Measurement of mineral/leucosome trace-element partitioning 72 systems using UV lasers and a flexible data acquisition systemin a peraluminous migmatite such as an integrating transient recorder (ITR).Pb/Pb dating of zircon. Accuracy <1%. REE elements 93
A brief report209 has also been given of LA for solid sampledetermined in amphibole and clinopyroxene magacrystsintroduction into a TOF-MS system in the axial configurationSc, V, Ni, Ga, Y, Zr, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, 113
Tm, Yb, Lu and Hf determined in garnet. Internal as opposed to the orthogonal configuration used by Mahoneystandards 29Si and 43Ca et al.133 Ablation of NIST steels at 20 Hz using pulses from a
207Pb:206Pb ages determined for zircon and monazite. 146 Nd5YAG laser frequency quadrupled to 266 nm allowed thePrecisions 1–10% for grains >80 mm
production of linear response–concentration curves for Cr,Zr, Hf, Nb, Ta, Y and REE determined in individual silicate 155Mo and V. Full assessment of LA-TOF-ICP-MS awaits furthermelt inclusions in phenocrysts. l=266 nm, pit ablationdetailed reports.diameter about 10 mm
B, Mg, Sr, Ba and U determined in corals. Beam area 213 An ion trap (IT ) can capture ions from a continuously600×20 mm. Standard was CaSiO3 glass. Accuracy 3.8% operating source, such as an ICP, and store them, if necessary(B), 31% (U ), precisions 1–3.7%. Ablation was with an for long periods. Techniques exist for the sequential expulsionArF excimer laser at l=193 nm
of stored ions from the trap on the basis of m/z, so that theycan be detected by a suitable ion multiplier. An extensivereview of the ion trap is available.255by a similar analysis using IR ablation and ICP-quadrupole
An IT has been successfully combined with an ICP ionMS.102source.256–258 The trap offers excellent sensitivity and abun-A novel approach to improved quantitation was reporteddance sensitivity, the removal of potential interferences byby Allen et al.,92,162 who employed an ion beam splitter andcollisionally induced dissociation (CID), the shifting of inter-two quadrupoles to detect the resultant beams. The motivationference (or analyte) m/z ratios via ion–molecule reactions andfor this was to eliminate flicker noise from the ablation process.the repeated integration of low responses from specific isotopesUnder optimum conditions,162 52Cr+ :53Cr+ was measuredof low abundance or low sensitivity, or both. To the author’swith an RSD of 0.058%, which approaches the countingknowledge, no LA-ICP-IT-MS system has been constructed,(Poisson) statistics limit.but the analytical potential of such a system is great.Time-of-flight (TOF) mass spectrometers rely on the separa-Anticipated difficulties lie with the treatment of transienttion of monoenergetic ions released into a field-free region. Ifresponses. Perhaps the laser–quadrupole synchronizationthe ions are of kinetic energy K thenmethods developed for laser ionization mass spectrometry
K=mv2/2 (5) (LIMS), where a laser is used for removal and ionization ofthe sample, with the ions being analysed by a quadrupolewhere m is the particle mass and v its velocity. To travel amass spectrometer,259 can be adapted for use with LA-ICP-fixed distance L will accordingly take a time t, whereIT-MS.
t=L/v=L (m/2K )1/2 (6) In addition to the use of alternative spectrometers, noveluses of LA-ICP-MS can be expected. Examples include LAHence, the time-dependent output of a detector placed at thefor the analysis of solutions65 and for speciation studies.207 Inexit of the field-free region represents a complete massthe former, Q-mode pulses (532 nm) were used to vaporizespectrum.sample solution spread on a graphite wheel. This approachTime-of-flight mass spectrometers offer simultaneous multi-allows concentrations in the range 5 ng ml−1 to mg ml−1 to beelemental analysis and a high ion-utilization efficiency (a highdetermined. No sample solution is wasted and only 200 ml areproportion of the ions produced are detected). As the rep-required. Good agreement was obtained for the determinedetition rate of these instruments is governed only by the flight
time of the heaviest ion, typically <60 ms, >16 000 spectra and certified concentrations of Pb, V, Co, Cu, Mn, Mo, Ni
and Cr in NIST SRM 160b Stainless Steel, Cr–Ni–Mo (AIST elemental and isotope ratio analysis can be expected, togetherwith an ever greater diversity of applications. It is beyond the316). Condensation of excess vapour in the cell limits the
maximum usable ablation energy. The technique is similar to scope of this paper to compare LA-ICP-MS in detail withother analytical methods but we may note that in applicationselectrothermal vaporization, producing a transient response
although a steady-state response can be produced by multiple it competes with state-of-the-art techniques such as FANES,83XRF and electron microprobe analysis,84 ICP-OES,135,202laser pulses.
In a speciation study,207 crossed immunoelectrophoresis was TIMS,130 proton microprobe analysis128 and INAA.128,220Note added in proof. The reader’s attention is drawn to aused in combination with LA-ICP-MS for the identification
and quantification of metal binding proteins in blood serum. recently published review of laser (and arc/spark) ablation forsolid sample introduction to ICP-MS,260 which gives specialHuman serum enriched with Co was subjected to electrophor-
esis and the agarose gels corresponding to the first and second attention to geological applications and therefore forms anexcellent complement to the present paper.dimensions were analyzed by LA-ICP-MS (Q-mode pulses,
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