Anal. Chem. 2008, 80, 4455–4486
Inductively Coupled Plasma Mass Spectrometry Diane Beauchemin Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada Review Contents Conferences Books and Reviews Sample Preparation Sample Introduction Nebulizers Spray Chambers Vapor Generation Electrothermal Vaporization (ETV) Laser Ablation Speciation Methods Spectroscopic Interferences Nonspectroscopic Interferences Fundamental Studies Isotope Ratios Isotope Dilution Instrument Performance Literature Cited
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The multielemental ultratrace detection capability of inductively coupled plasma mass spectrometry (ICPMS) makes it a most powerful technique. With each new version of ICPMS instrument, which typically decreased in size, came lower detection limit (DL), more user-friendly software to control it, and better accessibility to its different components for maintenance. These improvements facilitated its use by more people, which in turn increased sales worldwide. However, ICPMS is a black box for many people, which can be dangerous in terms of the reliability of the results obtained by these people if they are not appropriately warned about the remaining shortcomings of the technique (such as its susceptibility to matrix effects). Unless these users can attend workshops, conferences or read the literature, then their sole source of training will be their instrument suppliers, who understandably provide only a general introduction, as they are in the business of selling instruments (and then providing technical support), not teaching people. That is likely why numerous people qualify ICPMS of a mature, routine technique. Since the last fundamental review was published in Analytical Chemistry (1), some 2000 papers containing the subject “inductively coupled plasma mass spectrometry” were published according to SciFinder Scholar 2007, the majority of which were applications. Unfortunately, the number of fundamental studies declined, despite the remaining shortcomings that jeopardize the robustness of ICPMS for chemical analysis. Until these issues are resolved, fundamental reviews will continue to be warranted. The purpose of this paper is to critically review significant developments in ICPMS from October 2005 to October 2007 (exclusively). As it must be limited to a maximum of about 250 references, this review is not meant to be comprehensive. Nonetheless, several references to relevant books and review papers have been included to provide a more comprehensive coverage. 10.1021/ac8006945 CCC: $40.75 2008 American Chemical Society Published on Web 05/20/2008
To ensure that no fundamental paper would be missed, selected peer-reviewed journals were systematically perused: Analyst, Analytical Chemistry, Analytical Chimica Acta, CRC Critical Reviews in Analytical Chemistry, Analytical and Bioanalytical Chemistry, Applied Spectroscopy, Applied Spectroscopy Reviews, International Journal of Mass Spectrometry, Journal of Analytical Atomic Spectrometry, Journal of the American Society for Mass Spectrometry, Microchemical Journal, Microchimica Acta, Rapid Communications in Mass Spectrometry, Spectrochimica Acta, Part B, Talanta, and Trends in Analytical Chemistry. Only the most significant (in my humble opinion) or representative papers were selected. Unfortunately, the originality and significance of many papers is marginal at best, a problem that was identified during the last review (1), because several authors review only the recent literature. However, each paper should represent a significant advance over what was published both recently and up to 24 years ago, when ICPMS was commercially introduced. The onus is on the referees and the editors to ensure that the literature coverage is appropriate. In fact, the journal editor should not allow a manuscript to go on to referees if, at first glance, the literature review is inadequate. I suspect that this would drastically reduce the frustration of reviewers (such as me!) who find it very annoying to have to do the authors’ homework. Although, it is acceptable to have missed a reference or two, especially if they are in less accessible journals, something is clearly wrong when several key references are missing. Journal editors should also select reviewers carefully. Not only should they be knowledgeable in the area but they should also be critical without being “picky”. Given the wide variability of papers in terms of their significance, it is evidently not a trivial task! Conferences. Of the numerous conferences being held, only one remains that is mostly on ICPMS: the Winter Conference on Plasma Spectrochemistry, which is held in the USA and Asia on even years and in Europe on odd years. For example, it was in Temecula, California, in January 2008 and will be in Tsukuba, Ibaraki, Japan, on November 16-21, 2008 (http://envsun. chem.chuo-u.ac.jp/plasma/2008apwc.htm), and in Graz, Austria, February 15-20, 2009 (http://www.winterplasmagraz.at). This is the only conference where all ICPMS vendors showcase their latest product. Unfortunately, the format of the conference, at least that held in the USA and organized by Ramon M. Barnes, is degrading. Indeed, it used to only have a single session so that nothing could be missed. The latest one in Temecula had poster sessions in parallel with oral presentations on several days (people noted that the attendance at the poster session was lower than on days when there was nothing going on in parallel). It also started at 8 a.m. everyday and ran until 6:30 p.m. (excluding social Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
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Table 1. Books Related to ICPMS title Encyclopedia of Analytical Science, 2nd ed. Practical Inductively Coupled Plasma Spectroscopy Handbook of Elemental Speciation II: Species in the Environment, Food, Medicine and Occupational Health Inductively Coupled Plasma Mass Spectrometry Handbook
authors and/or editors Worsfold, P., Townshend, A., Poole, C., Eds. Dean, J. R.
year
reviewer
ref (2) (3) (4)
2005
John Wiley and Sons
2005
Butcher, D. J. Fetzer, J. C. Butcher, D. J. Rybarczyk, J. P.
(5)
Cornelis, R. Caruso, J., Crews, H., Heumann, K., Eds.
John Wiley and Sons
2005
Lobinski, R.
(6)
Nelms, S. M., Ed.
Blackwell Publishing
2005
Boulyga, S. F. Sturgeon, R. Beauchemin, D.
(7) (8) (9)
events in the evening) with barely an hour for lunch, depending on how strict with time the Chair of the morning session was. Short courses immediately preceding the conference were held during the mornings, afternoons, and evenings (until 11p.m.)! In addition to being exhausting (even for people from the same time zone), this format is totally inconsiderate for people who traveled a long way and suffer from jet lag. The most civilized format that I have experienced was during the 1989 European Winter Conference, which was held in Reutte, Tyrol. There was only a single session each day where only invited speakers made oral presentations, all other presentations being posters. Furthermore, these oral presentations were made in blocks of three at the most, with long breaks in between, so that people could go rest, ski, work, etc. There was 2 hours reserved for lunch so that people had time to walk to the various restaurants nearby and enjoy good meals without being rushed. It was a most enjoyable and memorable experience. I therefore urge conference organizers to try this format! In North America, the 35th annual meeting of the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS), which has a broader focus that includes molecular spectroscopy, electrochemistry, etc., in addition to ICPMS, will be held in Reno, Nevada, September 28-October 2, 2008 (see http://facss.org). The 54th International Conference on Analytical Sciences and Spectroscopy (ICASS), which is similar to FACSS in content but is smaller in size, will be held in Montreal, Quebec, August 3-6, 2008 (see http://www.icass.ca). Books and Reviews. Table 1 lists books that were reviewed by experts and contain significant information on ICPMS or where ICPMS plays a major role. (Other books were published during this review period, which have not been included because, as of the end of September 2007, they had not been reviewed yet.) Several general review articles were published (for specific topics, the reader is referred to the following sections). Some review papers included significant developments in instrumentation, methodology, and the understanding of the fundamentals of ICPMS (10, 11). Others focused on specific applications of ICP quadrupole MS (ICP-QMS), with or without collision/reaction cell (C/RC) or dynamic reaction cell (DRC), ICP high-resolution MS (ICP-HRMS), and ICP time-of-flight MS (ICP-TOFMS), such as the determination of •elements in industrial products including metals, chemicals (organic, inorganic, and petroleum products), and advanced materials (polymers, composites, glasses, ceramics, catalysts, etc.) (12–16) 4456
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•elements and their speciation in environmental samples (air, water, soil, plants, geological materials, etc.) (17–21) •elements and their speciation in clinical and biological materials, food, and beverages (22–26) •transuranium elements (27, 28) These determinations are most often carried out by external calibration (EC), usually with internal standardization (IS), but the method of standard additions (MSA) and isotope dilution (ID) analysis are valuable alternative calibration strategies that are often required to obtain the most accurate results. SAMPLE PREPARATION Several review articles were published on different aspects of sample preparation. For instance, classical methods of digestion or dissolution (dry vs wet, with or without the assistance of microwave energy) were reviewed as well as their recent applications to soils, sediments, food, cosmetics, oils, and coal (29). Another review article focused on combustion reactions in closed vessels, which can offer significant advantages over wet digestion methods for the analysis of hard-to-dissolve samples such as coal, and were in fact recommended for the subsequent determination of halogens, Hg or S in coal, rubber, plastics, or carbonaceous materials (30). The use of tetramethylammonium hydroxide (TMAH), water-soluble tertiary amines, and strongly alkaline reagents for analyte extraction, sample digestion, or slurry preparation was also reviewed and identified as a good alternative to more conventional reagents, although it often induces matrix effects and hence requires calibration by the MSA (31). The application of ultrasonic energy to enhance analyte extraction was reviewed and identified as an efficient way of improving the analytical performance of various procedures provided that experimental conditions were carefully controlled, as uncontrolled ultrasonic irradiation can induce decomposition of analyte species (32, 33). The growing use of flow injection analysis (FIA) over the past 25 years, to perform online sample pretreatment for instance, was recently reviewed (34). Indeed, as everything is performed in a closed system and only small sample volumes are injected, contamination and memory effect are minimized. Furthermore, kinetic discrimination (where, for example, the analyte reacts faster than potential sources of interference) can be used to minimize interference (while maximizing sample throughput), in contrast to batch methods that depend on equilibrium being reached. Readers who want more information on the multiple possibilities of FIA can get the third edition of a CD-ROM tutorial
on FIA by Jaromir Ruzicka, one of the two coinventors of FIA, free of charge from www.flowinjection.com (35). This tutorial also includes the development of sequential injection analysis and laboratory-on-valve, i.e., respectively, the second and third generations of FIA, which have resulted in further downscaling. In any case, it is written in such a way so as to appeal to anyone with an interest in FIA, from the newcomer to the experienced user. In addition to allowing online pretreatment to ICPMS, FIA can also be used for the discrete introduction of samples that would induce clogging of the sampling interface if nebulized continuously. For example, the determination of ultratrace elements in coal was facilitated by FIA, which allowed the analysis of a less diluted digest (36). Indeed, although microwave digestion with HNO3 alone (at up to 250 °C and up to 7.5 MPa) was demonstrated to be sufficient to decompose most of the coal organic matrix, the resulting digest nonetheless contained concentrated inorganic components whose continuous nebulization might have been problematic (36). In any case, an evaporation step (to maximize the removal of volatile organic components) should be avoided if Hg is being determined, as it can be partially lost in the process (36). Care should be similarly taken for the determination of any volatile element. Another example is that of Os, which forms volatile OsO4 (boiling point 105 °C) and is readily lost during conventional digestion of the Te precipate that is formed following redissolution of a NiS button to preconcentrate the platinum group elements (PGEs) (37). However, this extraction could be quantitatively carried out using refrigerated reagents in an ice-water bath with ultrasonication (37). In fact, a temperature as high as 30 °C could then be used without significant loss of Os, which precluded the requirement for ID analysis (37). Indeed, an EC was sufficient, provided that all standards were submitted to the same NiS fire assay, Te precipitation, and ultrasonic dissolution as the sample to ensure that the Os was in the same valence state, as the ICPMS sensitivity for Os(VIII) was observed to be 30-40 times higher than that for Os(IV) (37). A mild extraction at room temperature with 0.1% (v/v) HCl, 0.1% (v/v) 2-mercapoethanol, and 0.15% (m/v) KCl in an incubator shaker overnight was developed for the quantitative determination of Hg (total and species) in biological materials (38). This extract, which preserved Hg speciation, could be directly injected, i.e., without pH adjustment, onto the chromatographic column of a high performance liquid chromatography (HPLC) system using a mobile phase containing 5% (v/v) methanol, 0.1% (v/v) 2-mercaptoethanol, and 0.06 M ammonium acetate. Furthermore, the notorious memory effect that is typically observed with aqueous solutions of Hg was completely eliminated, presumably because the Hg species formed a stable complex with 2-mercapoethanol, which effectively prevented them from sticking to the sample introduction system (38). A 0.18% (w/v) L-cysteine solution (in 2% HNO3) was shown to be as effective as 2-mercaptoethanol at eliminating the memory effect and nonlinear calibration curves typically observed for Hg, without having the odor or the toxicity to the central nervous system of 2-mercaptoethanol (39). Achieving total dissolution is usually the goal of digestion methods, such as the microwave-assisted digestion of airborne particulate matter collected on filters, where both matter and filter were digested with 4 mL of HNO3, 2 mL of H2O2, and 0.2 mL HF (40). This approach was found to also completely dissolve quartz
filters but not Teflon and Zeflour filters (40). An assessment of the availability of metals from particulate matter was also made by performing a 5 min, 100 W microwave-assisted extraction (MAE) with ultrapure water (40). A partial digestion may also prove advantageous if the analytes are preferentially extracted. Indeed, for the determination of Ag and Cd in geological and environmental materials (soils, sediment, basalt, etc.), open-vessel extraction with aqua regia provided more accurate results than closed-vessel approaches because the latter solubilized more Mo, Nb, Zr, which are sources of polyatomic interferences on these analytes (41). Similarly, milder conditions will help preserve analyte speciation, thus allowing access to information that is increasingly in demand, as toxicity of a sample depends on the species present in it. For instance, two different procedures were developed to quickly extract arsenic species from hair: one based on ultrasound probe sonication and the other on pressurized liquid extraction (42). Using 0.5% performic acid with 5 min sonication or enzymatic hydrolysis with lipase and sonication for 10 min extracted available As species much faster than conventional methods that required 6 h (42). A multivariate optimization of the temperature, pressure, and extraction time of pressurized liquid extraction gave conditions (two 5 min cycles at 125 °C, 7 MPa) that released slightly more arsenic (42). However, because the methylated As species were not included in the optimization process (as they were present in very small concentration in the samples), about half of the monomethylarsonic acid (MMA) was converted into As(III), as the amount of MMA extracted was about half-that released by the two sonication methods. Nonetheless, multivariate techniques are extremely valuable for the optimization of interdependent experimental conditions, as the resulting models allow identification of the true optimum (43). However, a careful selection of the response(s) to optimize is required, such as the signal of all As species if quantitation of all As species is sought. Sample pretreatment can also aim at removing the bulk of the matrix to eliminate sources of spectroscopic and nonspectroscopic interference, hence greatly facilitating calibration. An example is the analysis of impurities in high purity materials, such as photoresist and SiO2 slurries that are used for polishing in the electronics industry. A simple method was developed for that purpose: 200 mg samples in Teflon beakers were placed in a pool composed of 1 mL HF and 9 mL HNO3 in a vessel, to which microwaves were applied in steps such as to ramp the pressure up to 240 psi (44). Under these conditions, the acid vapors reacted with the bulk of the matrix, thus volatilizing carbon as oxide and Si as fluoride, and leaving a clear solution in the beakers containing the trace analytes, which could then be analyzed by EC (44). Similarly, an open vessel wet digestion procedure was developed for the accurate determination of total As in dried marine fish by a simple EC, with IS using Te (45). This determination is normally plagued by spectroscopic interference from ArCl+ and from nonspectroscopic interference from carbon, whose presence can lead to enhancement of the As signal. In fact, the ICPMS response for a solution of As, present as arsenobetaine (AsB) in an otherwise carbon-free matrix, was 9% higher than that for a solution of As(V) (45), suggesting that the tiny amount of carbon from the AsB molecule itself was sufficient to enhance the As signal (45). To eliminate these problems, digestion was carried out in borosilicate glass tubes with nitric acid and sulfuric Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
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acid, using a heating block, with a homemade glass condenser filled with solid carbon dioxide placed two-thirds of the way up the digestion tube to prevent loss of As (45). The use of sulfuric acid allowed a digestion temperature of 350 °C for 2.5 h, which ensured complete mineralization of the matrix as well as of AsB to As(V), and removal of chlorine through evaporation (45). Monitoring of carbon (13C+) signal, which should be stable throughout the analysis, is strongly recommended to ensure that a memory effect from previously analyzed carbon-containing samples does not cause a spurious enhancement (45). The combination of two separation procedures was proposed to eliminate both spectroscopic and nonspectroscopic interferences on the determination of Cd in geological materials. Indeed, spectroscopic interferences from oxides of Zr and Mo and from isobars of Sn plagued the determination of Cd as, even in high mass resolution, the isobaric interference of Sn on several Cd isotopes (especially the most abundant) cannot be resolved. A liquid-liquid extraction of soil digests with a 10% mono-(2ethylhexyl)-2-ethylhexyl phosphonate-heptane solution left the Cd (and other cations) in the aqueous phase while Mo, Zr, and Sn were essentially completely extracted into the organic phase (46). The aqueous phase was then injected onto a resin prepared from Cyanex 923 (mixture of tertiary octyl and hexyl phosphine oxides) that retained Cd but not matrix elements, and Cd was eluted with 1% HNO3 (46). Separation of the analytes from the matrix and their preconcentration can be simultaneously carried out online to ICPMS by simply pumping the sample through a minicolumn of material that selectively retains the analytes while the matrix goes to waste, washing it to remove leftover matrix and then eluting the analytes. If the elution volume is smaller than the sample volume then a preconcentration is simultaneously achieved. The selectivity of the column material can be adjusted through proper selection of ion-selective ligands that are covalently linked to it. This was demonstrated for newly synthesized functionalized Amberlite XAD-4 copolymer resins, whose retention of trace elements (Mn, Co, Ni, Cu, Zn, Cd, Pb, and U) was unaffected by the salt levels found in seawater (47). Unfortunately, with the small preconcentration factor used (about 7), most of the analytes in open-ocean seawater remained below the DL. However, if the blank is not limiting, then a larger sample volume might be used to achieve a greater preconcentration factor, albeit at the cost of a reduction in sample throughput. An alternative to minicolumns, which totally avoids back-pressure from resin swelling or compaction of column material by unidirectional flow, is a knotted polytetrafluoroethylene (PTFE) reactor. Complexing agents can indeed be adsorbed on the inner wall of the reactor without hindering the flow and retain analytes through complexation, which are later released with a suitable elution solution. This approach (with EDTA as the complexing agent) was preferred for the online determination of Mn, Co, Ni, Cu, Zn, Cd, and Pb in the microdialysate of extracellular fluid from the brains of anesthetized rats (48). If greater selectivity is desired, then separation of the analyte from the matrix can be performed by displacement. For example, only Pd2+ in rock samples displaced Ag+ from a sorbed Ag-APDC (ammoniun pyrrolidine dithiocarbamate) complex because most other heavy metals (with the exception of Hg2+) form a weaker complex than Ag+ with APDC and thus could not displace it (49). 4458
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For fingerprinting of 239Pu and 240Pu in environmental samples containing large amounts of U, the samples were first ashed in a muffle furnace at 500 °C for 30 min to decompose organic matter (50). A MAE of the residue with 8 M HNO3 for 90 min at up to 220 °C was then carried out to extract Pu while leaving much of the matrix (containing cations and anions that can compete with Pu in subsequent steps) behind. L-Ascorbic acid was then added to the extract to convert all Pu into Pu(IV), which could then be separated from U on a TEVA column online to ICPMS, using a desolvation system to minimize U hydride spectroscopic interference (50). Several other examples of sample preparation are included in the following sections (for instance, see Tables 2–5). SAMPLE INTRODUCTION Todoli and Mermet reviewed the different devices that are available for the introduction of liquid microsamples into the ICP (51). They also covered fundamental studies on aerosol generation and transport (including solvent evaporation and droplet coalescence) at less than 100 µL/min, as well as its ultimate effect on the plasma and, thus, on the resulting analyte signal (51). This review article also includes a very useful flowchart on how to select a micronebulizer and aerosol transport device for a given application (51). Another review article focused on the preparation of slurries of inorganic materials for their analysis by ICPMS, with special critical consideration of the grinding step, selection of dispersing agent to stabilize the slurry, and of the calibration strategy (52). It also discussed the advantages and disadvantages of slurry nebulization for typical applications (52). An apparatus was designed for the direct puff-by-puff determination of trace metals in cigarette smoke by ICPMS (53). It combined a single-port smoking machine and an automated cigarette smoke collection and injection apparatus (consisting of a series of solenoid valves) to an ultrasonic nebulizer (USN) with desolvation system for the simultaneous introduction of In internal standard (53). Air was used as the carrier gas at a flow rate of 40 mL/min, as it was the maximum that could be tolerated by the plasma (>50 mL/min would occasionally extinguish it), with an Ar makeup gas flow of 0.8 L/min (53). This ensured a constant plasma composition, as the injection of air into an all-Ar plasma induced a momentary change in the plasma conditions, which resulted in an enhancement of 115In+ and 40Ar2+ but not the analyte (53). However, the resulting mixed-gas plasma, combined with the introduction of dry aerosol, made the plasma hotter, which required the use of Pt sampler and skimmer. Nonetheless, the approach revealed a dependence of the temporal profile on the analyte. For instance, Hg gave a symmetrical peak similar to that of Xe, an intrinsic component of ambient air, whereas Cd and Pb resulted in asymmetrical peak profiles with a sharp leading edge and significant peak tailing. This indicated that Hg was in the gaseous form whereas Cd and Pb were mainly present in the particulate phase of cigarette smoke (53). Despite the variations from cigarette to cigarette and from puff to puff, semiquantitative analysis could be performed using reference cigarettes as standards (53). A HF-resistant multimode sample introduction system (MSIS) was developed, which simply combined a modified commercially available HF-resistant cyclonic spray chamber to a Mira-mist parallel path nebulizer, to allow nebulization, vapor generation, or both simultaneously (54). One modification consisted in placing
Table 2. Selected Vapor Generation Quantitation Methods sample matrix
vaporization method
Hg
analytes
river water CRM
online cold VG with 0.5% w/v SnCl2 in 1.0% HCl with or without trapping on Pt-Au foil followed by thermal release
ELEMENT 2 ICP-HRMS (R ) 300) compared with Optimass 8000 ICP-TOFMS
ID with enriched 201 Hg spike to water sample
Ge, As, and Se
Ni-based alloys CRM
ELAN 6000 DRC ICP-QMS; DRC reduced Cl-containing and Ar2+ ions
MSA, ID (for Ge)
As, Bi, Cd, Co, Cu, Ni, Pb, Sb, and Zn
vegetation, water, and human hair CRMs.
digest adjusted to 1% v/v HCl (prereductant) and 2% m/v L-cysteine (masking agent); 200 µL FI-HG with 0.2% m/v NaBH4 in 0.1 M NaOH water or digest adjusted to 0.1 M thiourea (prereducing and masking agent); simultaneous nebulization and HG with MSIS
ELAN 5000 ICP-QMS
EC
Pb
plant and water CRMs, spiked seawater
ELAN 5000 ICP-QMS
ID (with 204Pb spike) or EC
sulfide
natural waters, sea sediments
Agilent 7500c ICPQMS with octopole reaction cell
MSA
sum of As(III), As(V), MMA(V), MMA(III), DMA(V), and DMA(III)
urine
PlasmaQuad 3 ICP-QMS; Scott-type double-pass spray chamber used as gas/liquid separator
EC with As(III) standards
digest adjusted to 0.1 M HNO3 and 0.28 M H2O2; nebulization or HG with 1.5% m/v NaBH4 in 0.1 M NaOH using MSIS hydrogen sulfide generation
urine adjusted to 0.2 M HCl and 50 mM L-cysteine; continuous HG with1.0% m/v NaBH4 in 50 mM NaOH
inserts in the drain outlet and in the outlet to the ICP, which faced each other, with a gap of 1.7 mm between them, to, respectively, allow introduction of sample solution and reagent for vapor generation (VG). In the VG mode, the solution inlet of the nebulizer had to be capped so that it only introduced Ar carrier gas in the chamber. The other modification included the addition of two new lines (made of PEEK) for transport to the ICP and the drain (54). The latter had to be capped and the inserts removed when only nebulization was carried out. In any case, when used in dual mode, it provided similar (for nonhydrideforming elements) or improved DL and sensitivity compared to conventional sample introduction. To eliminate peristaltic pump noise, a N2-pressurized carrier bottle was connected to a FI valve and provided 160 µL/min to a PFA-ST nebulizer, while 500 µL aliquots were rapidly loaded at
instrument
calibration strategy
comments
ref
TOFMS provided more precise peak area isotope ratios than HRMS; poorer precision of transient signal from the trapping step degraded DL HG required to eliminate sources of spectroscopic interferences (oxides) that could not be reduced by DRC
(74)
multivariate optimization of compromise conditions to enhance As, Bi, and Sb signal 33-77-fold without degrading that of other elements ID gave lower DL and more accurate results than EC because it compensated for pH and matrix effects on VG VG to separate analyte from matrix; multivariate optimization of C/RC with He and H2 to further decrease background conditions optimized for similar HG efficiency for each species; 16-fold enhancement in sensitivity compared to nebulization; HG minimized ArCl+ interference
(75)
(76)
(77)
(78)
(79)
400 µL/s using a vacuum pump (55). Under these conditions (i.e., 9 s loading, 12 s uptake time, with a microflow nebulizer), a signal plateau was obtained for each injection, followed by a 9 s washout that afforded a 500-fold washout of all elements, and allowed a sample throughput of 24/h (55). A 1000-fold washout would be possible in 9 s with a smaller volume spray chamber. With the worldwide move toward micro- and nanosample analysis, which is increasingly being performed on laboratoryon-a-chip devices, sample introduction systems that can interface these chips to ICPMS would add a very sensitive detector to microfluidic analyzers. Such an interface was developed using a PTFE block (5.3 cm × 5.0 cm × 4.5 cm) with a 2.5 cm channel on top to hold the microchip and another channel through the center for the nebulizer gas (56). One outside edge of this latter channel contained a screw-in HPLC finger-tight fitting, while the other end Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
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had a wider cone-shaped opening to allow for expansion of the sample aerosol. This latter side of the PTFE block had a groove for a 2.2 cm i.d. O-ring for gastight connection to an evaporation chamber. It also had a 2 cm long, 50 mm i.d. PEEK capillary (with a 38 nL volume) that went from a 3.6 cm hole drilled from the channel on top of the PTFE block (directly under the reservoir at the end of the liquid channel of the microchip) into the coneshaped opening of the gas channel. As this capillary was perpendicular to the gas flow channel, a microcross-flow nebulizer resulted. To optimize nebulization, the vertical position of the tip of this capillary could be altered by adding spacers under the microchip. A 250 mm i.d. PEEK horizontal capillary allowed 3-5 bar delivery of gas to the nebulizer at a flow rate of 0.8-1.0 L/min. The position of this gas capillary was adjusted using a HPLC screw-tight fitting with a locking nut (56). An evaporation chamber was made in-house using 2.2 cm i.d., 6.0 cm long quartz tubing with one end tapered and sealed to a glass ball joint (to fit to the ICP torch) and the other end fitting the O-ring of the nebulizer outlet of the PTFE block. This chamber was designed by multivariate optimization of its length (4-8 cm) and of the liquid delivery rate (5-30 µL/min), so as to provide maximum analyte signal intensity (56). Because recondensation was observed beyond 20 µL/min, even with the longest chamber investigated, a liquid delivery rate of 5-20 µL/min was used with a 6 cm evaporation chamber, which provided a 4-fold increase in signal intensity as a result of an apparently 100% sample introduction efficiency (56). With the use of this approach, as little as 42 nL injections of a 150 mg/L Cr solution (i.e., 6.3 ng of Cr) could be detected (56). The great detection power of ICPMS can also be used to get quantitative, online, time-dependent information on the corrosion process. A microcapillary system was indeed developed for this purpose (57). A 600 µm polypropylene capillary filled with 0.1 M NaCl or 0.1 M HCl was mounted orthogonal to the sample surface on a 3-D micromanipulator that allowed readings to 0.1 mm or 10 µM, depending on the axis. A peristaltic pump continuously circulated the corrosive solution within the capillary onto the sample, which was mounted on a XY-axis stage orthogonal to the capillary. A peristaltic pump flow rate of 500 mL/min provided a laminar flow of the corrosive solution while avoiding the formation of air bubbles within the system (57). The setup not only allowed monitoring of the main alloy elements, for which different phases resulted in different dissolution rates, but also of secondary elements (57). Comparison of the dissolution rate of the latter with those of main elements may provide insight into corrosion processes. Nebulizers. To facilitate the analysis of microsamples without requiring an expensive and relatively fragile direct injection nebulizer (DIN) or direct injection high efficiency nebulizer (DIHEN), Westphal and Montaser shortened a standard demountable torch from 12.2 to 5.9 cm (58). This then allowed the use of a conventional micronebulizer as a DIN, without any spray/ evaporation chamber, which significantly reduced the dead volume of the sample introduction system, thus making it suitable for microscale FIA. Because the distance between the plasma gas inlet and the end of the torch intermediate tube is smaller than in a conventional torch, a lower outer gas flow rate and no nebulizer gas flow rate were required during plasma ignition to 4460
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avoid significant gas turbulence. The outer gas flow rate then had to be increased to 18 L/min shortly after ignition to prevent damage to the torch. Although, a lower outer gas flow rate, which reduced turbulence, resulted in higher signal intensities, the torch could be damaged over time (>2 h). Compared to the DIHEN, the optimal position of the micronebulizer tip was 3 mm further back below the end of the intermediate tube, which reduced possible damage to the nebulizer tip. On the other hand, like the DIHEN, maximum sensitivities were obtained at 1.5 kW, i.e.,the maximum rf power available (58). In fact, sensitivity was, on average, 2.4 times higher than that with the DIHEN. However, despite generally comparable % RSD (relative standard deviation) values, the relative DLs were degraded by a factor of 1.7, on average, compared to the DIHEN. Nonetheless, these results are very encouraging for this simple proof of concept, and optimization of the short demountable torch design is likely to result in significant improvements in analytical figures of merit. Even in this initial state, accurate results were obtained by MSA for the continuous nebulization analysis of several certified reference materials (CRMs) (urine, apple leaves, bovine liver, spinach leaves) as well as the µFIA of Cr-DNA adducts (58). A relatively new nebulizer, the Vulkan DIN, which is mechanically and thermally more robust than the DIHEN because of its thicker-wall sample capillary that terminates 0.7-0.8 mm from the DIN nozzle tip, was studied and compared to the DIHEN in terms of its aerosol and noise characteristics (59, 60). For the comparison to be valid, analyte concentration was selected so that precision would be flicker-noise limited and, hence, the signalto-noise ratio (S/N) would be essentially independent of signal intensity. A higher white noise level was observed for the Vulkan DIN than for the DIHEN (59), which was likely a result of the larger droplet size (Sauter mean diameter of 30.2 mm vs 11.1 mm for the DIHEN) and broader droplet size distribution than with the DIHEN (60). In fact, with the Vulkan DIN, more than 95% of the total sampled aerosol droplets had a diameter larger than 10 µm, as opposed to 62% of the total sampled volume with the DIHEN (59). On the other hand, the mean axial velocity distribution of the aerosol produced by the Vulkan DIN was smaller than that with the DIHEN (10.0 vs 17.5 m/s with DIHEN), which translated into a longer residence time of the aerosol in the plasma for the Vulkan DIN (59). This partly counteracted the negative effects of its larger droplets, but the remaining higher number density of incompletely desolvated droplets in the plasma likely resulted in the higher white noise observed with the Vulkan DIN. The Vulkan DIN was also found to be less sensitive than the DIHEN to fluctuations in liquid flow rate, as no low-frequency pump interference noise was detected, in contrast to with the DIHEN (59). For both nebulizers, and especially the Vulkan DIN, signal intensity did not increase linearly with liquid flow rate, which indicated that the plasma did not have enough energy to efficiently desolvate the aerosol as well as atomize and ionize the analyte. In any case, with a 3 s integration time, better precision was obtained on ion count rate with the DIN and the DIHEN than that achieved with a conventional sample introduction system (cross-flow nebulizer with double-pass spray chamber) (59). Characteristics of the aerosol produced by a high-efficiency nebulizer (HEN) combined with a Scott-type double-pass spray chamber, using helium and argon as nebulizer gas, were mea-
sured to assess the adequacy of the HEN for sample introduction into He ICPMS (61). Although a larger Sauter mean diameter was obtained with He, especially below 0.5 L/min, a similar value (of around 6.5 µm) to that measured with Ar was obtained at the optimum nebulizer gas flow rate of 1 L/min (61). Nonetheless, even at this optimum gas flow rate, the primary aerosol generated with He was slightly coarser than that with Ar. Furthermore, the resulting droplets were slower and with a narrower velocity distribution compared to Ar, which would enhance coalescence into larger droplets that eventually drained out, and thus translated into a 4-fold loss in aerosol transport efficiency compared to Ar. The lower density and viscosity of He compared to Ar also makes it a less efficient carrier gas. On the other hand, because the thermal conductivity of He is about 8 times larger than that of Ar, a more efficient evaporation and transport of the aerosol was observed upon heating the aerosol in a heated cyclonic spray chamber followed by desolvation in a Peltier-cooled multipass condenser. At 0.2 mL/min, this resulted in similar DLs and slightly improved precision compared to those obtained with an USN with desolvation at 2 mL/min (61). When a desolvation system is used, care should be taken that volatile analyte species are not removed along with the solvent. The extent of these losses will depend on physical properties of the analyte and chemical reactions that may occur during the heating and desolvation (i.e., condensation and/or removal through a permeable membrane) stages. Such loss was observed for bromide ions when using an USN with membrane desolvation as a second desolvation stage following heating and condensation (62). Indeed, at Br concentrations higher than 10 µg/L, the calibration curve was no longer linear. Yet, with a cross-flow nebulizer combined to a Scott-type spray chamber, no such nonlinearity was observed over the same Br concentration range (62). Furthermore, the fact that this nonlinearity was worsened by the addition of acid indicates that Br was most likely lost in the form of volatile HBr (boiling point ) -66.8 °C) during desolvation (62). For solutions of NH4Br, linearity could be improved by adding 1 mg/L Na to the analyte solutions, but such compensation was not possible in 2% HNO3 solutions (62). The removal efficiency of a desolvation system also depends on the solvent. For instance, the analysis of fuel ethanol by continuous ultrasonic nebulization with desolvation by heating at 140 °C and condensing at -3 °C led to carbon deposits on the cones’ surfaces, which could be minimized by using FIA to make 250 µL injections of diluted samples (25% v/v ethanol) (63). By an increase in the robustness of the plasma through a decrease in the carrier gas flow rate, which was optimized so that sensitivity in 25% v/v ethanol was the same as in aqueous solution, an EC could be carried out with aqueous solutions and provided a sample throughput of 60/h (63). This is clearly superior to the 24 samples/h that was possible with electrothermal vaporization (ETV) ICPMS, although significantly lower DLs (by up to 40-fold) were obtained with the latter (63). To enhance the sample throughput in the analysis of crude oil samples (64) and gas condensates (65), a microflow nebulizer was modified and combined to a low dead-volume (8 mL) singlepass spray chamber without drain. Indeed, increasing the internal diameter of its capillary from 50 to 75 µm i.d. increased its maximum uptake rate to 30 µL/min (above which condensation
of xylene occurred in the spray chamber), which in combination with µFIA (using 2.5 µL sample loop) provided a sample throughput of 60/h (65). The increase in capillary diameter also virtually eliminated clogging problems, pressure variations, and memory effects (64). Aerosol generation was optimized by changing the position of the capillary tip within the nebulizer orifice. In fact, a multivariate optimization of the tip position, sampling depth, and carrier gas flow rate was carried out (64). To prevent carbon deposition on the cones, oxygen (8% of the carrier gas) was mixed with the Ar carrier gas via a T-connection prior to nebulization. Higher amounts of oxygen increased oxide formation and the plasma reflected power and decreased the lifetime of the cones (64, 65). Under optimized conditions, stable nebulization at 20-30 µL/min was achieved, with a RSD of 1.5%. This sample introduction system allowed the accurate analysis of a 1:10 diluted crude oil sample by the MSA (64) and the determination of Hg in undiluted gas condensates samples by EC using standards in toluene (65), without any memory effect or clogging of the nebulizer. Spray Chambers. The standard waste removal line of a 50 mL internal volume water-jacketed cyclonic spray chamber was modified by mounting the barrel of a 5 mm o.d. Hg pen lamp in the ground glass fitting of the waste line using epoxy resin, such that it extended along the vertical central axis of the spray chamber and did not impede nebulization with a concentric glass nebulizer (66). This modification was done because ultraviolet irradiation may induce radical reactions in solutions of lowmolecular weight (LMW) organic acids (such as formic, acetic, and propionic acids), yielding volatile analyte products. Under conventional operating conditions, the presence of these acids suppressed sensitivity (5-50-fold, depending on the element) compared to that achieved with nitric acid, in part because the operating conditions of the instrument were optimized using nitric acid solutions (66). However, improvements in sample introduction efficiency by 10–40-fold resulted upon UV irradiation for a number of elements (Se, Bi, I, Hg, Pb, Sb, and Sn), but no single LMW organic acid provided the best enhancement for all analytes. However, as only a 1 and 5% concentration of each reagent was tested, a different concentration of a given reagent may have a more universal effect. In that respect, acetic acid appears to be most promising, as it provided the best enhancement for the most analytes. This simple approach is thus very promising and warrants a detailed study of the effect of reagent concentration and the intensity of the UV field, as only a 6 W lamp was used, to further enhance VG. As well, an optimization of the spray chamber is warranted to increase sample throughput by minimizing wetting of its internal walls and, thus, the delayed release of volatile analyte species from the liquid phase. Vapor Generation. As a result of papers such as the one above, which demonstrated that other elements than the usual hydride-forming ones (As, Se, Sb, Bi, Sn, Ge, Te, Pb) and Hg could readily form volatile species, a resurgence of interest for chemical VG was apparent over this reviewing period. This is not surprising, as this approach usually provides a high transport efficiency into the plasma and a separation of the analytes from the matrix if a phase separator is used. A number of reviews have been written on the subject, including a historical review of techniques (including electrochemical ones) that have been used for VG, Analytical Chemistry, Vol. 80, No. 12, June 15, 2008
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which demonstrated that many of the current limitations have been known for a long time (67). The evolution of gas-phase separators to nowadays was also discussed, along with means of alleviating interferences in the liquid phase on the VG process (67). Another article focused on the fundamentals, interferences, and applications of electrochemical VG, which is performed in an electrolytic cell (68). It offers some advantages over chemical VG, such as a reduction in cost and contamination by avoiding the use of a hydride generation (HG) reagent, simplicity, as a highpurity acid medium can be used for generation of all hydrideforming elements, and independence of the efficiency of HG on the oxidation state of the analyte, when using cathode materials with high hydrogen overpotential (68). On the other hand, interferences can be significant, especially from inorganic species such as strong oxidants, transition metal ions and noble metals, and hydride-forming elements (68). Other review articles focused on recent developments in the chemical VG of noble metals (Au, Ag, Pd, Pt, Rd, Rh, and Os) (69, 70) and transition metals (Co, Cr, Cu, Fe, Mn, Ni, Zn, Ti, Tl) (70) species by reaction with tetrahydroborate in acidic medium, as a means of separating them from the matrix. The approach has the potential to become a valuable technique for the determination of noble metals (69). However, it requires further work to reconcile the complexity of the reaction and separation processes with the high instability of the volatile species (that are mostly unknown), which often results in memory effects and insufficient enhancement of the analytical signal, and to alleviate chemical interferences in the liquid phase (69, 70). Indeed, the chemical VG efficiency does not exceed 30% (70). In fact, the way in which the efficiency is measured can lead to an overestimation of the efficiency. For instance, when doing it by difference of the amount of analyte left in the drained liquid of the phase separator, the amount remaining in the system is neglected, which was demonstrated to be significant (70). Better approaches involve direct measurement of the analyte in the gas phase or comparison of the analyte signal with that obtained using a different sample introduction system with the same detector (70). Speciation analysis of the analyte in the gas phase can also be accomplished by collecting the vapor on a gas chromatography (GC) column that is cooled with liquid nitrogen, followed by heating of the column to release and separate the volatile species, which are then detected by ICPMS (71). Recent advances in photochemical VG were reviewed, which involves UV irradiation to initiate reactions resulting in the formation of volatile analyte species (72). This greener and more cost-effective approach than chemical VG, which retains its advantages, also offers additional advantages, such as simplicity of the reactions involved, and applicability to a greater number of elements with less interference and to speciation analysis, with or without prior species separation (72). The approach is, however, still young and many challenges remain, such as understanding the mechanisms involved and identifying the photogenerated species (72). Nonetheless, another review article demonstrated that UV photochemical VG with ICPMS detection using HCOOH as a reductant was better than chemical VG for the determination of ultratrace mercury, as it provided better DLs, freedom from interference in the liquid phase, and was an overall greener approach (73). 4462
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Table 2 summarizes some examples of VG approaches that have been applied to quantify selected analytes in various matrixes. Except when the MSIS was used in dual mode (i.e., simultaneous nebulization and VG), chemical VG was used to separate the analyte from the matrix, which was a source of spectroscopic or nonspectroscopic interference, and enhance its response. Indeed, essentially 100% sample introduction efficiency can be achieved with VG, which is in contrast to the typical 2% provided by conventional sample introduction systems. Furthermore, by injecting a vapor into the ICP, less of its energy is required for desolvation and vaporization, leaving more available for atomization and ionization, which hence increases the degree of ionization of elements with a high first ionization potential. These features make VG techniques extremely valuable when high-precision isotope ratios are desired in combination with MCICPMS (multicollector ICPMS). For instance, Foucher and Hintelmann used cold VG to separate Hg from sediment matrixes and continuously feed its atomic vapor to MC-ICPMS (80). Similarly, a system was designed to provide stable HG to enhance the measurement of Se isotope ratios through the combination of several approaches (81). First, combining the sample and NaBH4 reagent within the HG chamber using a mixing manifold instead of using a mixing coil prior to the HG gas/liquid separator greatly improved stability, as it minimized pulsation of the hydride gas into the ICP torch. Second, instead of continuously pumping the waste out of the gas/liquid separator, removing the waste liquid as a batch at the end of sample analysis further improved hydride stability. Finally, a modified Scott double-pass spray chamber was used as an interface between the gas/liquid separator and the ICP to dampen pulsation induced by the peristaltic pump (81). No microporous PTFE membrane was used to avoid degrading hydride stability and increasing memory effects. In one application (79), HG-ICPMS was used as a fast screening technique to determine the total level of toxic arsenic species in urine because arsenobetaine (AsB) did not form hydride whereas As(III), As(V), monomethylarsonic acid (MMA(V)), monomethylarsonous acid (MMA(III)), dimethylarsinic acid (DMA(V)), and dimethylarsinous acid (DMA(III)) reacted with NaBH4 to form hydrides. Although this approach is fine as a screening technique, verification by chromatography coupled to ICPMS would be required for accurate analysis, as highly polar and major arsenosugar compounds occurring in biological samples as well as organic thioarsenicals have been shown to be HG-active (71). Electrothermal Vaporization (ETV). An article reviewed recent applications of ETV-ICPMS to biological, environmental, refractory, and high-purity materials (82). It also focused on vaporization mechanisms of analytes in ETV, chemical modification, direct solid sample analysis, and preconcentration/separation techniques (82). Table 3 illustrates several of the features of ETV through selected applications. One common feature is the simplicity of the methods compared to time-consuming sample pretreatment procedures that would otherwise be required for sample introduction into the ICP by nebulization. This probably explains why, since the last review, there has been an increase in interest for ETV, despite the fact that none of the ICPMS manufacturers currently offers an ETV accessory (although several did in the
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sample matrix
fluorocarbon polymers
spinach leaves, tomato leaves, typical diet, rye grass CRMs
ancient porcelain
tap and lake waters; human serum; peach leaves CRM
coal fly ash, vehicle exhaust particulates CRMs; atmospheric particulates coal CRMs
Cr, Cu, Fe, K, Mn, Pb, and Zn
B
rare earth elements
C, Pb
Ti, V, Cr, Zr, Mo, and La
polystyrene beads, polyvinyl chloride beads
tomato leaves and dogfish muscle CRMs; swordfish; tea
Cr, Cd, and Pb
Fe, Co, Ni, Cu, and Zn
As, Ge, Hg, Pb, Sb, Se, and Sn
photoresist
hydrated and anhydrous fuel alcohols
quartz and talc CRMs
Mn, Zn, Cu, and Pb
Cu, Mn, Cr, V, Li, Pb, Sn, Mg, U, Ba, Sr, Zn, Ce, Rb, and Sb Ag, Cd, Cu, Pb, and Tl
analytes
EC (As, Ge, Sb, Se, and Sn) with acidic standard solutions or ID (Hg, Pb, Se, and Sn) MSA or ID
ELAN 6000 ICP-QMS; HGA 600 MS ETV unit
ELAN 6100 DRC ICP-QMS; HGA 600 MS ETV unit; USS-100
2 mL of acidic sample slurry, after standing 15-24 h, mixed with 3 mL of 3% m/v NaBH4 in 1% m/v NaOH; VG into Ir-treated graphite tube 10 µL USSe of slurry (1% m/v plastic powder, 1.5% m/v NH4NO3, 1% v/v HNO3, 0.5% v/v Triton X-100) into graphite furnace 20 µL USS of slurry (1% m/v sample, 2% v/v HNO3) onto Pd-pretreated platform in graphite furnace
MSA
EC
Agilent 7500a ICP-QMS; modified WF-4C ETV unit
ELAN 6100 DRC ICP-QMS; HGA 600 MS ETV unit; USS-100
EC with standard solutions subjected to SDME
Agilent 7500a ICP-QMS; WF-4C ETV unit
15 min 4 µL SDMEb of 1 mL sample/digest with 0.1 M 8-HQc in CHCl3; injection of the drop into graphite furnace 10 µL of slurry in 4.0% m/v PVDFd pipetted into graphite furnace
EC with standards prepared in 6.0% m/v PTFE
ID
ELAN 5000 ICP-QMS; HGA 600 MS ETV unit
Agilent 7500a ICP-QMS; modified WF-4C ETV unit
EC with Pd IS
MSA
ELAN 5000 ICP-QMS; HGA 600 MS ETV unit ELAN 5000 ICP-QMS; HGA 600 MS ETV unit
ID
ELAN 6000 ICP-QMS; HGA 600 MS ETV unit
calibration strategy MSA with In IS
instrumentation HP-4500 ICP-QMS; in-house W-coil ETV unit
sample ground to
