Anal. Chem. 1988, 60, 113R-131R (221) Alybaeva, K. N. Zdarvookhr. Klrg. 1988, (6),22-24. Chem. Abstr. 1987, 106, 208924. (222) Scott, W. J.; Chapoteau, E.; Kamur, A. Clln. Chem. 1988, 32(11), 2119-2120. (223) Oesch, U.; Mallnowska, E.; Simon, W. Anal. Chem. 1987, 59(17), 2131-2135. (224) Maas, A. H. J.; Weisberg, H. F.; Burnett, R. W.; Mueiler-Piathe, 0.;
(240) Swandulla, D. Can. J. Physbl. Pharmacol. 1987, 65(5),898-903. (241) Pucacco, L. R.; Corona, S. K.; Carter, N. W. Anal. Bbchem. 1988, 159(1),43-49. (242)Lee, C. 0.; Im, W. B.; Sonn, J. K. Can. J . Physiol. Pharmacol. 1987, 65(5),954-962. (243) Oakiey, B. Can. J. Physlol. Pharmacoi. 1987, 65(5),1018-1027. (244) Chao, A. C.; Armstrong, W. M. Am. J . Physlol. 1987, 253(2 Pt. l),
65(5),889-893. (229) Ransom, B. R.; Carlini, W. G.; Yamate, C. L. Can. J. Physioi. Pharmacol. 1987, 65(5),894-897. (230) Kraig, R. P.;Cooper, A. J. L. Can. J. Physlol. Pharmacol. 1987, 8515). 1099-1 104. (23ij’-Ammann, D.; Chao, P.; Simon, W. Neurosci. Lett. 1987, 74(2), 221-226. (232) Stokols, M.; Corona, S. K.; Pucacco, L. R.; Jacobson, H. R.; Carter, N. W. A d . Blochem. 1987. 163(2),530-534. (233) Khalil, S. A. H.; Moody, G. J.; Thomas, J. D. R.; Lima, J. L. F. C. Analyst 1988, 111(6),611-617. (234) Oyama, N.; Hlrokawa, T.; Yamaguchi, S.; Ushlzawa, N.; Shlmomura, T. Anal. Chem. 1987, 59(2),258-262. (235) Oesch, U.; Ammann, D.; Simon, W. Can. J. Physiol. Pharmacol. 1987, 65(5),885-688. (236) Schlue, W. R.; Deitmer. J. W. Can. J. Physlol. Pharmacol. 1987, 65(5)978-985. (237) Thomas, R. C. Can. J. Physiol. Pharmacol. 1887, 65(5),1001-1005. (238) Feile, H.; Bertl, A. J . Exp. Bot. 1988. 37(182, 1416-1428. (239) LeFurgey, A.; Liu, S.; Lieberman, M.; Ingram. P. Microbeam. Anal. 1988, 2 1 S t , 205-208.
(245) Lyall, V.; Croxton, T. L.; Armstrong, W. M. Biochlm. Blophys . Acta 1987, 903(1),56-67. (246) Yamaguchi, H. Cell Calcium 1986, 7(4),203-219. (247) Ammann, D.; Buehrer, T.; Schefer, U.; Muelier, M.; Simon, W. Pfluegers Arch. 1987, 409(2), 223-228. Chem. Abstr. 1987, 107,35847. (248) Morris, M. E.; MacDonald, J. F.; Friedllch, J. J.; Szekelyhldi I.Can. J. Physiol. Pharmacol. 1987, 65(5),926-933. (249) Toth, K.; Fucsko, J.; Lindner, E.; Feher, Z.; Pungor, E. Anal. Chlm. Acta 1988, 179,359-370. (250) Iicheva. L.; Trojanowlcz, M.; Krawczynski Krawczyk, T. Fres. 2. Anal. Chem. 1987, 328(1-2),27-32. (251) Christopouios, T. K.; Dlamandis, E. P. Analyst 1987, 112(9), 1293-1298. (252) Fan. W.; Fan, L. Fenxl Huaxue 1986, 14(5), 387-390. Chem. Abstr. 1988, 105,128201. (253) Petak, P.; Stulik, K. Anal. Chlm. Acta 1988, 185,171-178. (254) Frenzel, W.; Braetter, P. Anal. Chlm. Acta 1988, 185,127-136. (255) Frenzel, W.; Braetter, P. Anal. Chim. Acta 1988, 187,1-16. (256) Cardwell, T. J.; Cattrall, R. W.; Hauser, P. C.; Hamilton, I.C. Anal. Chem. 1987, 59(1),206-208. (257)Van Staden. J. F. Anal. Chlm. Acta 1988, 179,407-414. (256) Van Staden, J. F. Fres. 2. Anal. Chem. 1987, 328(1-2),68-70. (259) Ilcheva, L.; Cammann, K. Res. 2. Anal. Chem. W88, 325(11), 11-14. (260)Lockridge, J. E.; F&r, N. E.; Schmuckler, G.; Fritz, J. S. Anal. Chlm. Acta 1987; 192(1),41-48. (261) Martin, G. B.; Meyerhoff, M. E. Anal. Chlm. Acta 1988, 186, 71-80. (262) Chang, Q.;Meyerhoff, M. E. Anal. Chim. Acta 1988, 186, 81-90.
Wlmberley, P. D.; Zijlstra, W. 0.; Durst, R. A.; Siggaard-Andersen, 0. J. Clln. Chem. Clln. Blochem. 1987, 25(4),261-289. (225) Kobos, R. K.; Abbott, S. D.; Levin, H. W.; Kllkson, H.; Peterson, D. R.; Dlcklnson, J. W. Clin. Chem. 1987, 33(1),153-158. (226) Djamgoz, M. B. A.; Dawson, J. J. Biochem. Blophys. Methods 1986, 13(1),9-21. (227) Yamaguchi, H. Can. J . Physlol. Pharmacol. 1987, 65(5). 1006-1008. (228) Carllnl, W. G.;Ransom, B. R. Can. J. Physlol. Pharmacol. 1987,
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C3A3-C347 . .. .
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Atomic Mass Spectrometry David W . Koppenaal Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713
INTRODUCTION AND SCOPE This inaugural fundamental review assimilates and summarizes recent developments in atomic mass spectrometry, as reported in the scientific literature published from November 1985 to late 1987. Atomic mass spectrometry is defined as the mass spectrometric measurement of atomic (as opposed to molecular or polyatomic) ions, for the primary purpose of elemental and/or isotopic composition determinations. The ”atomic mass Spectrometry” terminology is preferred for this review over the more commonly used ‘inorganic mass spectrometry” term since it better reflecta the elemental and isotopic emphasis of the review and also precludes the coverage of mass spectrometric investi ations involving organometalliccompounds, chelates, metal cfuster ions, and m u s metal chemistry, which are not reviewed here (but whict have been reviewed elsewhere (1-3)). Consistent with the aim of this journal, atomic mass spectrometry methods development, evaluation, and application studies placing emphasis on quantitative analysis were considered most pertinent in the preparation of this review. Atomic mass spectrometry is not a new field indeed it was the original application for mass spectrometry (MS) when developed for the isotopic compositional analysis of the elements. Com letion of the isotopic characterization of the elements a n x application of the MS technique to organic analysis in the early 1940s led to a diminished interest in atomic mass spectrometry, although thermal ionization and stable isotope ratio (TIMSand SIRMS, respectively) methods were clearly being refined at that time. Subsequent interest in solids analysis by mass spectrometry led to the development of the well-known spark-source mass spectrometry (SSMS) technique in the late 1950s. This technique dominated atomic spectrometry interest until the late 1960s,when rapid investigation and development of modern techniques began. Research and development activity during the last 15 years 0003-2700/88/0360-113R$06.50/0
has resulted in the availability of secondary ion, laser microprobe, glow-discharge, inductively coupled plasma, resonance ionization, and accelerator mass spectrometry techniques (acronyms SIMS,LMMS, GDMS, ICPMS, RIMS,and AMs,respectively). The present review covers these and other developing atomic mass spectrometric methodologies. This review is based on a computerized Chemical Abstracts search of titles, keywords, and abstracts of literature published in the last 2 years. Selected literature from earlier publications is cited where appropriate (i.e., general technique reviews). Government reports and conference proceedings are cited only when other corresponding literature references were not available. Foreign journals are likewise represented only in the absence of other available literature; however, for several subject areas foreign contributions were dominant and had to be included. Accession numbers from Chemical Abstracts are provided with foreign literature citations. Over 2500 literature citations were examined; this review is a result of a somewhat selective and critical evaluation of these search citations. Oversight or omission of certain contributions is inevitable; an advance apology is offered for such cases. Applications of analytical techniques are the ultimate justification for new technique development and are consequently included in this review, typically in an abbreviated tabular format due to space constraints. Reference to these applications is encouraged for pragmatic technique use and efficacy evaluation. Detailed review of such applications is deferred to the alternate-year application reviews of this journal.
A. BOOKS,REVIEWS, AND PERIODICALS New books, general reviews, and periodical literature covering atomic mass spectrometry developments are addressed in this section. Mass spectrometry is a dynamic technique and therefore many books have been published recounting recent developments. Many of the mass spectrometry texts 0 1988 American Chemical Society
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ATOMIC MASS SPECTROMETRY
are devoted to organic or molecular mass spectrometry, however, and do not even mention the use of mass spectrometry for atomic or inorganic analytical purposes (see, for example, ref Al). Fortunately, several exceptionsto this trend now exist with the availability of new mass spectrometry texts published during the review period. White and Wood, in their book titled Mass Spectrometry-Applications in Science and Engineering, give approximately equivalent treatment to atomic and molecular mass spectrometry techniques (A2). Their coverage includes atomic MS ion sources, along with concise discussions of applications in geochemistry, geochronology, material sciences, and environmental pollution. The second edition of the book Mass Spectrometry by Duckworth, Barber, and Venkatasubramanian (A3) also addresses inorganic and atomic MS instrumentation and applications more appropriately than many other MS texts. A new book devoted to nonorganic appications of mass spectrometry is, however, the publication of note (A4). This work, entitled Inorganic Mass Spectrometry and edited by F. Adams, R. Gijbels, and R. Van Grieken of the University of Antwerp (Belgium), is destined to become a classic book for atomic mass spectrometrists. Excellent and authoritative chapters on SSMS, GDMS, SIMS, LMMS, ICPMS, and IDMS are written by the editors and/or noted experts in the respective techniques. In addition, a very interesting historical perspective of solids mass spectrometry is provided as an introductory chapter. This book, scheduled for publication in early 1988, will fill an obvious void in the mass spectrometry literature and is recommended for novice and experienced atomic MS practitioners alike. Literature reviews on the general subject of atomic mass spectrometry include an earlier review by Adams accentuating the rogress in SSMS, SIMS, LMMS, and ICPMS techniques (A5f Heumann has also discussed advances in elemental determinations using mass spectrometry (A6)and presented a detailed discussion of isotope dilution methodology in another review ( A n . The latter technique was stressed as a “definitive analytical method”, usable with electron impact (EI), TI, or ICP ion sources. Several general reviews on trace element analysis of solids included discussions of mass spectrometric contributions to the available state-of-the-art (A8-AI4); these discussions offer an interesting comparison of MS techniques to other analytical methods. During the preparation of the present review, the latest in a series of Specialist Periodical Reports in Mass Spectrometry was published; a new section on quantitative metals analysis by MS was included, reviewing the relevant literature from mid-1984 to mid-1986 (A15). The intent of this review is similar to that of the present one, and it therefore appears that alternate-year reviews for this subject will be forthcoming-in no way unfortunate considering the activity in the field, the paucity of such reviews in the past, and the advantages of differing review perspectives. To conclude this section, the availability of abstracted information in The Mass Spectrometry Bulletin should be pointed out. This monthly publication includes sections on isotopic analysis, age determination, atomic processes, and surface and solid-state studies of interest to atomic mass spectrometrists (A16). Analysts may find this periodical especially useful for locating otherwise obscure material, including conference and symposia presentations and activities.
B. SPARK-SOURCE MASS SPECTROMETRY “No single analytical technique offers as much potential for the complete compositional characterization of inorganic materials as does spark source mass spectrography. The simultaneous and direct determination of nearly all elements a t the nanogram level in widely diverse materials requiring the utilization of only a few mg of sample is indeed an impressive capability” (B1). Although somewhat dated now by competition from other recently developed atomic mass spectrometric methods, this testament to the capabilities of SSMS still stands unrivaled in the eyes of many analysts (B2). The principal problems long associated with the SSMS technique, Le., poor precision and accuracy, are often outweighed by the advantages of sensitive semiquantitative (“panoramic survey”) and bulk/surface analysis capabilities. The technique attained much popularity in the late 1960s and early 1970s, but use of SSMS has declined rapidly since then, although more so in the U.S. than in Europe and Japan. An 114R
ANALYTICAL CHEMISTRY, VOL. 60, NO. 12. JUNE 15, 1988
Table I. Selected Applications of SSMS application semiconductor, electronic materials metals, alloys, etc.
nonconducting materials geological
miscellaneous
sample various (reviews)
ref B21-B24
GaAs B25 AB B26 B27 Cu f Be-Cu A1 alloys B28 In B29, B33 Ni,Fe alloys B30 Fe,Fe alloys B30 Zircaloy B32 A1203 B34-B36 Allende meteorite B37, B38 coal B39 oil shale B40 B41 sediments (SRMs) zircons B42-B45 beach placers B46 inclusions B47 Na2(FeNO(CN)&2H20 B48
excellent review of the SSMS technique and its applications is available in the new book previously mentioned (B2). Several fundamental studies of the SSMS spark process were published within the review period. The charge distributions of various elements in different matrices were evaluated in an attempt to improve the precision of SSMS analysis (B3). Element-sensitive multiple ionization, differences in recombination rates, ionization energies per particle, and average energies per particle were considered in the study. The effects of spark discharge conditions on sample evaporation and atom ionization were also investigated in the development of a model to describe SSMS and LMS ion production processes (B4). Sparked electrode surfaces and spark-generated particles were examined by using SEM and SIMS; differences in the composition of these samples as compared to the bulk electrode were implicated in the observed different relative sensitivities of the sample constituents (B5). The spark erosion process was also studied by using an electron microprobe to examine particles generated from the analysis of A1 and Zn alloys (B6). General ionization phenomena in solids mass spectrometry were also reviewed and discussed in regard to SSMS and LMS analysis (B7). The discrimination processes in the spark plasma and within the mass spectrometer make the use of relative sensitivity coefficients necessary for quantitative analysis using SSMS; a number of studies were reported that examine the dependenceof the relative sensitivity coefficients on elemental physicochemicalproperties ( B 8 4 1I ) and/or matrix composition (B12-BI4). The potential for quantitative SSMS analysis without use of reference samples was also discussed (B10,B15), based in part on the use of derived relative sensitivity coefficients. Sample presentation techniques, using the traditional graphite electrode and the aluminum pellet techniques, were evaluated; disadvantageswith the aluminum pellet technique include elevated matrix effects and less “universality”to all sample types (B16).The effects of residual organic matter, always a problem in the SSMS analysis of natural samples, were investigated; four preparation procedures designed to eliminate organic matter effects were proposed and evaluated in this study (B17). The use of photographic emulsions as SSMS ion detectors still predominates, and consequently a number of investigations relative to their use were published. These studies included the use of Gez+ ion measurements to determine photoplate intensity-transmittance relationships (B18),and the incorporation of automated (BI9)and computer-assisted (B20)analysis of photographic plates. Very few references to the use of electrical detection methods in SSMS were found. Applications using SSMS were numerous; many of these applications are given in Table I. Use of spark source mass spectrometry is obviously popular in semiconductor, metallurgic, and eologic research. A review of SSMS techniques in serniconcfuctor analyses is available (B21). Several studies using SSMS for lateral and or depth profiling were also published (B22,B24, B27); S MS was used in these studies presumably because of its higher sensitivity relative to other
6
ATOMIC MASS SPECTROMETRY Darld W. KOpp.naal is currently Chist ChemW of me Mlnerai W l a s Laboratory. Bveau 01 E m m i c Gedogy. me unhwshy 01 Texas at AmUn. He receiMd Ms B.S. degee in envkonmemal &emktry and ma* matics horn Souihwest Missouri State Uniuersity (1974) and his Ph.D. in anaiyiical chemistry from the University of Missouri at Cclumbia in 1978. His interests in atomic m a s specbometry began in 1977. when he began use of s m r k source mass spectrometry in Ma ~dissertationresearch. conducted &in A. 0. Sharkey's group at me Pmsburgh E n a g l Technoloay Center. U.S. Department 01 Energy. under an Energy Rested Research Traineeship awarded him by the Oak Ridge Associated UniuwsMes. I n 1978 Dr. Koppenaai pined me Unbnsily of Kentucky where he was associated wilh the Instlute lor Minlng 8 Minerals Research as a Senior Chemlst. He has been at his ~urrentp~snionsince 1984. where he has respansibiiW for inductively coupled plasma and stable i~otopemass Specnometry lnsrmmentatian in his rwearchlsewlce group. His research interests inelude atomlc specmmetric methods 01 analysis (parncularly ICPOES and ICPMS). anai,?icai geochemistry, environmental chemistry, and fossil fwd analysis and converrbn demlstry. He is a member of ACS. SAS. ASTM. and ASMS.
,
surface/depth profiling analysis techniques such as SIMS. Isotope dilution (831, B42) and electrical detection (833) modes of analysis/quantitation were used in some applications. SSMS was also used to determine iron isotope ratios in an irradiated nitrosylferrate compound (B4.9). Reviews of these applications indicate that SSMS was most used due to ita sensitive, "complete picture^ characteristics; other specialized applications focused on singular analytes, e.g., H in zircaloys (B32)and S in iron alloys (B31). A sole me of SSMS to determine isotope ratio compositions was found (B48).A standard method for the use and evaluation of SSMS in the analysis of solids is available (849).
C. PLASMA SOURCE MASS SPECTROMETRY The recent development and w e of plasmas as atomic mass spectrometry ionization sources are reviewed in this section. Such plasma sources include inductively coupled plasmas (ICP), microwave induced plasmas (MIP), and the lowpressure glow-discharge (GD) and hollow-cathode (HC) plasmas. The most significant general development with plasma sources has been the rapid commercialization of the ICPMS technique and its subsequent acceptance hy the analytical community; since commercial introduction in 1983, an estimated 130 such intruments have been installed, the majority of these within the last 2 years. While only two commercial ICPMS instruments currently exist (at the time of writing) on the US.instrumentation market, new ICPMS instruments from France and Japan are imminent. A glow discharge mass spectrometer is also commercially available and is apparently taking over the traditional solids mass spectrometry applications from the SSMS technique. Plasma sources clearly possess unique advantages that promise their increased use in mass spectrometry. 1. Inductively Coupled Plasma Mass Spectrometry. Several comprehensive reviews of the ICPMS technique are available (CIGC6). These reviews provide excellent discussion of the historical development of the technique, details of the instrumentation, modes of data acquisition, and illustrations of various analytical applications. These reviews are recommended for the novice; however, certain information (particularly on interface or ion optics design) may he outdated due to recent refinements in instrumentation. Other reviews me also available that discuss ICPMS in a comparative fashion with other trace analytical techniques (C7-CII). An excellent discussion of the ICP as used as an emission and ionization source is also available (CZZ) and is recommended, especially if making an optical-to-mass spectrometry transition in use of the ICP. The first mmmercial ICPMS instruments began appearing in user laboratories during 1984,consequently many inventorand manufacturer-independent ICPMS publications began appearing in the literature during the period of this review. Many of these initial reports concerned the existence and tabulation of interferences, along with their analytical manifestations. An extensive study of the dependence of mono-
positive analyte (M+),analyte oxide (MO+), analyte hydroxide (MOH+),and double charged (M2+)ion species on aerosol gas flow (pressure), plasma power, and plasma sampling depth was reported for one of the available instruments (C23). A companion report presented background spectra and tahulated and assigned observed atomic and polyatomic ion species for several common acid matrix solutions ((214). The general problem of polyatomic ion interferences was shown early to be most significant only under a mass of approximately m / z 80 ( C I S ) . Partial dependence of MO+ and MZf response on ion energy and/or ion energy distribution was shown (C26C28). as well as their dependence on other instrumental parameters ( C I M 2 1 ) . The effect and assessments of such interferences in real-world sample analyses including alloys (C22) and ores (C23) have been published. Hutton showed that reduction of plasma water vapor loading resulted in decreased Mz+and MO+ intensities, among other beneficial effects (C24). Reduction of water vapor loading to the plasma was accomplished with simple water cooling of the spray chamber; this investigation has led to the current consensus that vapor loading of the plasma has greater implications of ICPMS than for ICP-OES. These interference investigations, as well as other presented hut as yet unpublished studies, have shown that polyatomic or molecular ion interferences in ICPMS can be significant. The analyst must therefore be cognizant of isobaric ion interferences, which can be both instrument parameter and sample dependent. Existence of argoo/water/acid polyatomic species is now well documented in the above-referenced works. While these interferences are problematic, their severity has decreased dramatically with improved interface and ion optics designs, the details of which unfortunately remain in the unpublished or proprietary d o main at the time of witing. The appearance of sample matrix plyatomic and/or doubly charged ions may he less apparent to the unwary analyst. Descriptions of many such interferences and their consequent problems are given in many of the application references cited below and should he consulted for illustration (C22, C23, C72, C77). Sample matrix-induced ionization suppression enhance ment effects in ICPMS have also been discussed (CzS-CZSi and appear to pose a more serious problem a t present. Both enhancement and suppression of analyte signals by concomalthough the latter itant elements have been reported (C26), effect appears to predominate, especially in the presence of easily ionized elements. The effect is mass dependent, with the greater atomic mass concomitants resulting in greater analyte suppression, and low mass analytes are more susceptible to such suppression than are higher mass analytes. The interference effect was found to he more serious for easily ionized elements (Le., Na > Mg > I > Br > CI) and was postulated to he due to analyte ionization equilibria shift effects (C25). Thompson and Houk have reported the partially successful use of internal standardization to compensate for such ionization interferences; use of internal standard elements close in mass and ionization energy to the analyte(s) was recommended (C29). Effects of instrumental operating parameters on these nonspectroscopic interferences have also been reported (C30)for one of the available instruments. Tan and Horlick have recently published a well-illustrated study on matrix induced effects on analyte intensity (C32). These workers observed that the matrix-induced effects were more severe at high nebulizer flow rates and were more pronounced for low-ionization-energyanalytes. Minimization of the matrix effect by dilution appears to mitigate the problem somewhat; these workers showed that the effect was related to the ahsolute concentration of the matrix element rather than to the relative concentration compared to an analyte element. Matrix-induced ionization interferences are, in general, less obvious than their isobaric ion interference counterparts. Their subtle occurrence makes their identification and/or correction more difficult, and it appears that close matrixmatching of standards and samples, judicious use of internal standard elements, and proper control of instrumental parameters are the only currently available remedies. Recent work ((231)indicates that these interferences may be artifacts of the ICPMS ion extraction and/or ion focusing processes; future detailed studies may provide explanations and allow mitigation of this problem. Several studies reported various efforts in optimizing ICPMS analyte ion signals (C32-C36);the successful appliANALYTICAL CHEMISTRY, VOL. BO. NO. 12. JUNE 15, 1988
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Table 11. Selected ICPMS Applicationsn sample or application
elements, isotopes, or ratios determined
novel aspects of application
ref
Concentration Determinations geological geological geological geological
REE 17 trace
marine sediments
10 trace 11 trace Sn 20 trace various trace various trace various trace Cd, Pb, Mo, V various trace 12 trace Sb
ID, IS quant w/fire-assay
Mo, W PGE
C65-C68 C68 C70 C71
preconcn
marine ref. matls. river water ref. matl. acid rain, precipitation environmental,water organic, solvents food nuclear samples, U archeological samples H3P04 semiconductor matls powders (sediments, coal, ores, metals) human feces
ID quant ID quant ID, SA quant
c72 c73 c74
c75 C76, C77
C78-C80 C81-C83 C84
cu
various trace Fe, Cu, Zn
liq-liq extn. (C88)
C85-C88, C109
ID quant
C89 c90 c91
w/arc nebuliz
c53
ID quant
C92
Isotope Ratio Determinations Pb; Pb, T1
geological geological
c93; c94
os
geological
B
milk powder human feces human feces, plasma blood plasma
Pb Fe
OSOl
generation ion-exchange preconcn ID quant
67Zn/68Zn,70Zn/BBZn Zn. Se
c57, c95 C96 C97, C98 c99 C92, ClOO
ClOl
c102 C103 Fe in vitro ID %iI7Li blood, urine C104 powders w/arc nebuliz c53 Ti, Cu, Rb, Dy, Nd Key: REE, rare-earth elements; PGE, platinum group elements; ID, isotope dilution; IS, internal standardization;SA, standard addition. blood
0
cation of the ICPMS technique is obviously dependent on a wide range of plasma, mass spectrometer, and ion optics operatin variables. The singular published com arison of ICPMb with ICP-OES (C37) concludes that ICPM! possesses the advantages of superior sensitivity for most elements and comparative simplicity of the ICPMS spectra (only 211 lines for all naturally occurring elements). Disadvantages cited for ICPMS included oorer precision, less freedom from matrix interferences, an a low instrument duty time. This admittedly brief study used an early ICPMS interface design; a more comprehensive comparison using currently optimized instrument designs is therefore merited. The subject of ion energies in the ICP ion source has been one of active debate and is one of the differentiating factors between the commercial instruments. Olivares and Houk described a useful method for the measurement of ion energies that uses retardation plots (C38). Douglas and French described a novel method of eliminatingthe pinch discharge that plagued early workers in the ICPMS field (C17);their solution involved a center-tapped load coil to reduce the undesired capacitive coupling of rf voltage between the induction coil and the plasma, thereby eliminating the discharge and reducing ion energies (C39). Gray also exhibited reduction of ion energy with various changes in load coil geometry (C16). Plasma potential measurements have been made directly by using Langmuir probes for both the conventional endgrounded (C40) and center-grounded (C41) load coil configurations, confirming the essential elimination of a discharge in the latter case. A novel vertically oriented ICPMS instrument of an independent design has been described (C42) and characterized with regard to spectral and physical interferences (C43). The construction and operating characteristics of a Japanese-built instrument have also been reported (C44). One of the major limitations in ICPMS is the performance of the typical continuous dynode electron multiplier at high count rates (C45); the use of a scintillation ion detector in an effort to improve
B
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988
the ion-countingperformance of the ICPMS system has been described (C46). The scintillation detector reportedly exhibited superior precision, linear range, signal stability, and fatigue characteristics. Novel methods of sample introduction or use of the ICPMS as a multielement detector have been actively reported. Solid sample analysis using electrothermal vaporization (ETVICPMS) has been reported and applied by Park and coworkers (C47-C50). Laser ablation (C51) and arc nebulization ((252, C53) have also been incorporated with the ICP-MS technique for the direct analysis of solids. A less expensive approach to solids analysis, using simple slurry nebulization, resulted in impressive data; no exacerbated cone blocking problems were encountered (C54). Wire loop direct insertion (C55),hydride generation (C56),and osmium tetroxide vapor generation (C57) devices were also demonstrated as ICPMS sample introduction alternatives. Finally, the utilization of ICPMS as a detector for gas chromatography (C58, CN), liquid chromatography (C60),gel chromatography (C6I),and flow injection analysis (C60, C62) indicates that the technique will be a powerful, albeit expensive, detector for chromatographic and flow injection monitoring. Although the argon ICP has been the standard ionization source for ICPMS, a helium ICP has been reported as a possible su plemental or alternative ionization source (C63). A helium IEP, if viable, would have the advantages of a higher plasma ionization energy (and presumably be a more efficient ionization source for high ionization energy elements) and a less complex background mass spectrum. Use of ICPMS as a plasma diagnostic tool has also been illustrated with the determination of ionization temperatures using CsI as the diagnostic species (C64). Applications of the ICPMS technique involved both concentration and isotope ratio determinations, as presented in Table 11. These citations include the first user-published applications of the technique and reflect the present use of ICPMS primarily for the determination of "problem" elements
ATOMIC MASS SPECTROMETRY
(e.g., Pb, Mo, W, Sb, REE, PGE), for the determination of trace and ultratrace concentrations and for its isotope dilution analysis capability. It is interesting to note that the wide multielement capability of the technique has yet to be fully exploited, as only a few published applications involve the determination of more than 10-15 elements. The technique possesses good credibility, however, as evidenced by its use in several reference material and standard sample analyses, with excellent com arisons of data acquired from ICPMS and other techniques ($72, C73, C105). Isotope ratio investigations using ICPMS focused on geological and biomedical applications. Use of ICPMS isotope ratio information in the geological field, somewhat limited by the 1% RSD best ratio precision obtainable, centered on Pb, Os, or B isotope ratio determinations (Table 11). ICPMS determination of isotopes or isotope ratios has also been useful in the biomedical and nutrition fields, where increased use of stable isotope tracers has occurred. An excellent discussion of ICPMS isotope ratio capabilities, limitations, and problems is available (C45). The ability to perform isotope ratio determinations also makes the unequivocal isotope dilution analysis technique more available to the analyst; several ICPMS isotope dilution applications were reported (C70,C72, C73, C75, C91, C92). 2. Microwave Induced Plasma (MIP) Mass Spectometry Sources. Microwave induced plasmas have been investigated over the last 2 decades as possible low-cost, li htweight, equivalent-excitation alternatives to the popular 18P. Although never practically successful as emission sources (other than for chromatographic detectors), these devices remain an analytical curiosity. The original cou li of a MIP provided source with a uadrupole mass spectrometer the interface %esignthat was successfully incorporated into the ICPMS instruments of today. Two recent reports concerning the use of MIP ionization sources were published during the review period; both were investigated as alternative sources for ICPMS instruments. A helium MIP was utilized as a MS ion source to determine halogens (C107);characteristics of the MIPMS interface and the effect of operational parameters on ion intensity were discussed. Use of a microwave induced nitrogen discharge a t atmospheric pressure (MINDAP) was also described and assessed (C108) by using a noncommercial ICPMS system (C42). Further investigation of the MIPMS approach is necessary before any judgment of utility or promise can be made. 3. Glow-Discharge and Hollow Cathode Sources for Mass Spectrometry. Harrison has pointed out that gas discharges resembling the glow discharge (GD) were used as the “natural” ion sources during the formative years of mass spectrometry (C110). Thus, glow discharge sources are not particularly new or innovative so much as revisited and refined for current solids mass spectrometry problems. Harrison and co-workers provide excellent reviews of recent developments with glow discharge and hollow cathode ion sources for mass spectrometry (C110, C111);another review of the glow discharge as an emission and ionization source is also available (C112). A glow discharge mass spectrometer employing a magnetic sector MS is commercialIy available and has been described (C113). The GDMS technique is ideally suited for the analysis of solid samples and is therefore competing with SSMS; the GDMS advantages of a more stable ion current and a lower power, less complex ion source promise to hasten the demise of the SSMS technique (an estimated 20 GDMS instruments are now installed in user laboratories). Recent developments in glow discharge and hollow cathode mass spectrometry include the use of h e r resonance ionization in combination with glow discharge sputtering (C114),the demonstration of a hollow cathode plume as an ionization source (C115),and the description of two new GD ion sources interfaced with a quadrupole mass spectrometer (C116). Quadrupole based GDMS instruments are, as yet, available only in a few research laboratories around the world, in spite of their considerably lower apparent cost. The potential use of pulsed plasmas as MS ion sources has also been proposed (C117). Applications of the GDMS technique are beginning to appear more frequently in the literature. Applications have been described for metals and electronic materials (Cl16, C118), steel (C119, C120),and nuclear materials (C121). The use of GDMS in the steel industry to determine ppm levels of C, 0, and N as well as the traditional trace and minor steel con-
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(&$
comitants is of special interest. Use of the hollow cathode plume as a possible ionization source for geological sample analysis was also implied (C122) in a study of its emission characteristics. Hess and Marcus have recently reviewed the analytical applications of GD sources (C123). The future growth of glow discharge mass spectrometry appears to be predicated upon a combination of factors: high cost of the available instrumentation, future availability of quadrupole-based GDMS instruments, and relative demand for bulk (i.e., GDMS) versus surface or depth-specific chemical information (SIMS, etc.). From all current indications however, it appears that GDMS will become (is becoming?) the heir apparent to SSMS.
D. THERMAL IONIZATION MASS SPECTROMETRY Thermal ionization (also known as surface ionization or hot-anode) sources, along with gas discharge sources, were among the first ionization devices used when mass spectrometry was developed in the 1920s and 1930s. Thermal ionization mass spectrometry (TIMS), as it is now popularly known, found its original use in the early mass spectrometric studies of the elements’ isotopic compositions. Thermal ionization still remains as the method of choice when measuring precise isotopic ratios, as necessary in various geologic, environmental, nuclear, or life science investigations involving isotope fractionation or decay processes. The high precision of the isotope ratio measurements obtained obviously make this ionization source ideal for isotope dilution analysis using mass spectrometry, as Heumann (01-04) and others ( 0 5 )have pointed out in recent reviews. Herzog ( 0 6 ) has commented that the fundamental elements of the TIMS technique are basically unchanged from earlier instruments; the primary changes being user features such as automated sample introduction, computerized instrument control and data processing, and the availability of adjustable multiple collectors. The literature essentially corroboratesthis assessment; very few publications involve advancement or innovation in instrumentation or techniques. Several studies reported the progress or results obtained upon computer automation control of older TIMS instruments ( 0 7 - 0 9 ) . Most of TIM instruments employ a ma netic sector mass spectrometer, but several reports descrited results obtained by using a TI-quadrupole instrument (010, 011). This TIMS configuration was found to yield comparable concentrationvalues for several elements by using separation/ preconcentration and isotope dilution techniques and has the advantages of lower cost and compactness relative to the traditional TIMS instruments. In other technique developments, a surface ionization-diffusion (SID) source (incorporatinga Pt-plated Re filament) was developed for Np determination by isotope dilution analysis mass spectrometry (IDAMS) ( 0 1 2 ) and application of a microwave dissolution technique for IDAMS was described ( 0 1 3 ) . A nonconventional TIMS source, using a high efficiency, thermally heated cavity, was described for the determination of transuranic elements in very small (lo-” g) samples ( 0 1 4 ) . The thermal cavity described was equipped with an aperture for extracting the ionized sample; no chemical separation of analytes was required. Most of the TIMS studies published during the review period involved technique applications. Almost all published TIMS reports involved either isotope ratio or isotope dilution analyses; approximately equivalent applications were found in the geological, environmental, nuclear, biological/clinical, and metallurgical fields, as indicated by the tabulation provided in Table 111. TIMS determinations of Pb and U in the geological, environmental, and nuclear studies appeared especially numerous, probably due in part to the lack of other widely available, sensitive methods for these elements. Although most elements or ratios are measured by using atomic ions, several studies reported the use of the molecular ions, measuring Li2B02+(D15),BOz- (DIS),CsB02+ ( 0 1 7 , GIB), and Lao+ (037) for Li, B, B, and La concentration or isotope ratio determinations,respectively. Negative ion measurements were made for BO2- (D16) and C1-, Br-, and/or I- ( 0 3 6 , 0 3 7 ) using TIMS. Due to the high ratio precisions typically required in TIMS, analyte separation/preconcentration techniques are prevalent nndjor required; studies using ion-exhange ( 0 1 0 , D l l , 0 2 0 , 0 2 2 , 0 2 3 , 0 4 0 , 0 5 5 ) , precipitation (021), electrodeposition ( 0 1 0 , D l l ) , and thermal evoluANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15, 1988
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tion/evaporation (038, 0 4 6 , 0 4 7 ) methods were reported. Several reports compared TIMS results with those of other GFAAS (028033), NAA techniques, including ICPMS (OX), (033,D50),and y-spectrometry (058). Finally, it should be pointed out (to correct the misconce tion that TIMS is a technique applicable to only a few e ements) that over 40 different elements are represented in the application studies reported in Table 111.
P
E. STABLE ISOTOPE RATIO MASS SPECTROMETRY Natural variations in the abundances of C, H, N, 0, and
S isotopes occur due to the relatively large mass differences between the isotopes of these elements and their consequent isotopic fractionation in biochemical and geochemical processes. The determination of an element’s stable isotope ratio (heavy/main isotope ratio typically used) by mass spectrometry is commonly known as stable isotope ratio mass spectrometry (SIRMS). Gaseous samples (CO,, H2, N,, 02,SO2) of the above elements are obtained by using vafious nonfractionating conversion treatments on original samples; the gas sample is then ionized using electron impact methods and isotope ratios are derived from mass spectrometric measurements of the appropriate molecular ions. Stable isotope ratio (SIR) analysis is used extensively and routinely in biological, geological, and clinical studies; its use is so common that SIRMS data are included in the literature of these fields without abstract or keyword reference, therefore appearing transparent in literature searches of the type conducted for this review. Major developments,highlights, and applications of the SIRMS technique published during the review period are summarized, however. For example, Schmidt reviewed the determination of stable isotope ratios in food ( E l ) ,and Bulman reviewed the development and application of SIRMS mass spectrometry in biochemical tracer, archeological, and geological studies (E2). Hatchey and co-workers also provided an extensive review of SIRMS techniques in the nutrition and biomedical fields (E3). The subject of sample preparation for SIRMS measurements dominated the published literature in this area in the last 2 years. A low-blank, stepped heating preparation procedure was described for the measurement of carbon isotopes using nanomole samples of CO, (E4). Florkowski examined the use of the controversial Zn-reduction technique for hyE6) and found fractionation effects drogen generation (E5, that were dependent on the mass, producer, and even batch of Zn used in the reduction of HzO to Hz. Alternativereducing agents, including Mo-Pt (E7)and a porous resin Pt catalyst (E8)were also investigated for Hz gas sample preparation. Wong and Klein extensively reviewed Hz sample preparation methods for the analysis of biological materials; techniques reviewed included reduction, electrolysis, and combustion (E9). Sofer described (E10)and applied ( E l l ) a direct high temperature pyrolysis technique for the conversion of organic H in hydrocarbons to H2 for D / H isotope ratio analysis. Sample preparation for 0 isotope ratio measurements were also covered in a review by Wong and Klein (E9)and included discussion of electrolysis, chemical conversion, fluorination, and thermal decomposition approaches. The 0 isotope composition of natural and heavy waters was also determined after direct chemical oxidation using either Na2S20sor ClF,; no fractionation effects were reported ( E l 2 ) . A method for the determination of 0 isoto e fractionation and H202concentrations in rainwater was &scribed (E13);the method involved use of evacuation, ultrasonic agitation, oxidation, and sample sparging. A simple syringe technique for preparation of C02 for 0 isotope analysis was reported (E14),offerin advantages of simplicity and expediency. A new procedure, Eased on the classical Schuetze-Unterzaucher method and usin a commercial CHN combustion analyzer, was describe% for the determination of l80in organic materials (El5). A new value for the COZ-HzO oxygen fractionation factor, normalized to conventional isoto ic standards, was reported by Dugan and Borthwick (El6). &fide and sulfate minerals were converted to SO2for S isotope determinations; the conversion procedure involved evolution of SO3 with subsequent reduction with Cu to produce SOz (E17).Small samples,including single mineral grains, were prepared and analyzed using this procedure. The problem of memory effects in S isotope ratio mass spectrometry was reported to be minimized by using C12introduction
into the mass spectrometer prior to sample (SO,) introduction (E18).A high-voltage arcing technique was applied for the determination of 15Nin N2 and NzO in air (E19). Evaporative systems for concentrating 15N samples were evaluated by Lober and co-workers (E20);atmospheric contamination and cross-contamination among samples were found to present problems. Finally, deviations among N isotope values for ambient Nz reported by different laboratories were attributed to deviations and effects of Ar in the air samples; no additional clarification was available in the abstract reviewed (E21). Instrumentation developments in SIRMS included the adaptation of a cycloidal mass spectrometer design for H isotope determinations (E22). A sensitivity improvement of lOOX over traditional magnetic sector spectrometers was reported. Interfacing of SIRMS instruments with a gas chromatograph(ED)and a commercial N analyzer (E24)were reported for determination of C and N isotope ratios, respectively. Computers were used to obtain a m / z 3 null point in H isotope ratio determinations (E25) and to control an improved,high-precision isotope ratio detection system (E26). Comparisons of SIRMS isotope determinations with other potential isotope analysis techniques, including infrared spectrophotometry (E27),nondispersive infrared heterodyne ratiometry (E%), and molecular emission spectrometry (E29), were reported during the review period. Reported SIRMS applications included analysis of fruits (E30),Allende meteorite samples (E31),carbonates (E32),and heavy water (E33). The availability of five new water standards for H and 0 isotopes was also reported (E34). As mentioned in the introduction of this section, many other applications of SIRMS were undoubtedly reported but were not retrievable with the literature search conducted.
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F. LASER MICROPROBE MASS SPECTROMETRY Laser microprobe mass spectrometry (LMMS) has become popular in recent years for the sensitive (down to g), multielement and isotopic microanalysis of solids. Both bulk and surface analyses can be performed to a spatial resolution of 0.5-1 pm. The availability of commercial instruments has popularized the acronyms LAMMA (laser microprobe mass analysis) and LIMA (laser ionization mass analysis); at least three such instruments are available that operate in the transmission, reflection, or combined transmission/reflection irradiation-detection modes. Both positive and negative ion detection capabilities exist with the technique, although very few applications of the latter capability have been reported. Quantitation and limited accuracy of the technique have been constraints, but the impressive sensitivity and resolution characteristics outweigh these limitations in many applications. Instrumentally, both TOF and sector mass analyzers have been used, but the former is most popular, because it is ideal for the pulse-shot nature of the laser analysis experiments and because it provides high ion transmission. Lasers used are typically of the Nd:YAG variety and are $-switched to allow reduced pulse lengths ( w 10-20 ns) at the doubled, tripled, or quadrupled frequencies. A comprehensive review of laser microanalysis by Moenke-Blankenburg includes a treatment of laser microprobe mass spectrometry ( F l ) . The University of Antwerp (Belgium) group has included an extensive and thorough chapter on laser microprobe mass analysis in their new book (F2),as well as in other reviews of the technique in life science (F3) and medical (F4) applications. Kaufmann also reviewed the state-of-the-art of LAMMA as applied to biomedical samples (F5). Keller and Snyder briefly reviewed laser ionization detection techniques, including laser mass spectrometry ( 6 ) . Houk presented a concise discussion of laser ionization techniques, includingthose applied to elemental solids analysis, in a recent chapter (F7). Also available is the latest in a series of updated laser mass spectrometry reviews/bibliographies from Conzemius and co-workers (F8); over 1400 citations were assimilated and categorized in this endeavor, which indicates the popularity and applicability of this technique. Any of these reviews are recommended for indepth treatments of the status and capabilities of laser microprobe mass spectrometry techniques. The quantitative capabilities of LMMS are less than optimal, and most results at this point are semiquantitative at best. Problems in quantitation stem from inability to accu-
d
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ATOMIC MASS SPECTROMETRY
rately model or describe laser-solid interactions, evaporation volumes, ion yields, ion transmission, or detector efficiencies. Consequently, empirical approachesto quantitation in LMMS are employed, using relative sensitivity factors (RSF) and standard reference materials or synthetically prepared standards. Ramendik and co-workers showed that RSFs can be derived from atomization energy and ionization potential considerations(B4). Selected mineral samples were examined in order to set reproducible analysis conditions and to allow calculation of RSFs in another study (F9).Investigators at the Ames Laboratory (U.S.A.) examined RSFs for impurity elements in metallic samples (steels, brasses) and found near-unity RSFs for metallic impurities but lower RSFs for nonmetallic impurities; laser power density of >5 X 1@W cm-2 was recommended (F10). Chinese investigators identified five vital factors affecting the possibilities for standard-free quantitative analysis of solids: laser power density, volume of the laser-induced plasma, angular expansion of the plasma, laser focusing characteristics, and number of laser pulses shot at a sin le site (F11). Standard-free quantitative analysis was claimecf if appropriate conditions and parameters relative to these five critical aspects were utilized. Work at the National Bureau of Standards in developing single particle standards for LMMS (F12) included measuring isotopic ratios and assessing the feasibility of Ni speciation by LMMS. General aspects of LMMS quantitation were discussed by Southon and co-workers,with illustration by specific sample analysis (F13).Other workers noted that whereas calibration curve methods are applicable with LMMS if the total ion current or the sampling volume can be determined, the use of RSF is nonetheless effective, easy, and quantitative within an error range of 10-30% (F14). Variability in the results of binary metallic systems analyses was attributed to lasemurface interactions and to instrumental effects (F15).Theoretical studies of ion transmission through a LMMS TOF spectrometer were performed (F16);results indicated a strong transmission dependence on ion formation conditions and an apparent important contribution by high-energy,angularly emitted particles. Peak shapes in LMMS were investigated and calculated by Vertes and co-workers, who observed that peak broadening, shifting, and asymmetry could be explained by ion energy distribution considerations (F17). Developmental work in LMMS included that of Ishimori and co-workers, who used LMMS to analyze samples under atmospheric conditions (F18). An LMMS employing a magnetic TOF mass analyzer was described by Eloy, with claims of optimized ion transmission and mass resolution (F19). This work included comparison of analytical results on WS SRM glasses, obtained with the described instrument and the commercially available LAMMA and LIMA instruments. Depth-profilingcapabilities of LMMS were discussed by Odom and co-workers, with demonstration using loB implanted in Si (F20). Low laser power densities and defocused conditions have been utilized in LMMS for application to surface and film analysis, with comparable results to dynamic SIMS (F21). A new ion generation mechanism, employing a high electric field and a short-duration laser pulse applied to the tip of a field ion microscope, was described for ionization of surface atoms and analysis by TOF MS (F22). An improvement of sensitivity in LMMS was noted with the addition of a tunable dye laser, which allows selective irradiation for certain elements (F23). Application of a tunable dye laser LMMS to steel alloy analysis was also demonstrated by these investigators (F24) [note that resonance ionization mass spectrometry, which these last two investigations utilized, will be covered in the following section]. Nonresonant multiphoton ionization of laser microprobe ablated neutrals (i.e., postablation ionization) was shown to yield generally larger ion yields than the conventional laser-ablation-only mode of LMMS (F25). A signal height discriminating timer, designed to correct for pulse pile-up and counting loss effects in LMMS, was described and evaluated by measurement of Ni and W isotopes (F26). Collimation efficiency (i.e., the effectiveness of offcenter radial positions on the sampled surface in contributing to the observed total ion signal) was measured and evaluated by Ohashi and co-workers (F27). LMMS was utilized for the determination of elemental concentrations and isotopic compositions of C, H, N, and 0 in various geologic and synthetic samples (FZS).In this study, isotopic fractionation associated with the formation of both 120R
ANALYTICAL CHEMISTRY, VOL. 60, NO. 12, JUNE 15. 1988
Table IV. Selected Applications of Laser Microprobe Mass Spectrometry sample matrix
sample type biological
kidney section tissues parathyroid glands neuron cytoplasm rat kidneys
elements
Ca
Fe, A1 Al, Ca Cd
kidneys aortic wall
Pb Pb
plant latex
K,Mg, Ca
semiconduc- Si, GaAs, GaP
tor, electronic
materials
G A S , InAs GaAs, GaP
GaP
sc
asbestos, minerals
miscellaneous
Fe
circuit plating amosite fibers asbestiform fibers chrysotile fibers montmorillonite, sepiolite, crysotile fibers Cr-Fe-Ni alloy, glasses BN, graphite Ni particulates aerosols coal
ref
F33 F34 F35 F36 F37 F39 F39 F40 F41 F42 F43 F44 F45 F46 F47 F48 F49 F50 F51
F52 F53 F54 Li, Ti, Ba, Sr, F, C1 F55 Ni species
CO and COz species was observed, although the degree of fractionation appeared reproducible enough to allow for correction of the obtained data. Determination of the light elements H, Li, Be, B, and C by LMMS was described by Kohler and co-workers (F29). Elemental ion mapping, providing ion maps of high lateral resolution, was described by Hercules and co-workers; application to integrated circuit bonding pads (Au, Al) and coal maceral inclusions (Fe, S) was demonstrated (F30). Inorganic cluster ions were studied by Linton and co-workers in attempts to elucidate relationships between sample composition and characteristic cluster ion signals (F31). Carbon cluster ions containing 12Cand 13Cfrom separated layers of these isotopes were studied by Van Grieken and co-workers in an analysis of geometric effects and recombination reactions in LMMS (F32). Laser microprobe mass spectrometry was originally developed for sensitive elemental analysis of thin biological sections, but its use has also become widespread in the analysis of semiconductor, metallurgical, and other solid samples. Practical analytical applications of LMMS are given in Table IV as illustrations of the versatility of the technique. Additional, less recent LMMS application studies can be also found in earlier reviews and bibliographies ( F I , F8).
G. RESONANCE IONIZATION MASS SPECTROMETRY Resonance ionization mass spectrometry (RIMS) is a powerful new MS technique that was developed to overcome the nonselective and nonuniform ionization characteristics of most atomic MS ion sources. The RIMS technique uses resonant laser photoionization to selectively ionize elements (or isotopes) of analytical interest (after atomization by other means), combined with conventional mass spectrometric separation and detection. The primary advantage of resonance ionization for MS is the ability to avoid isobaric interferences in the ionization process; this advantage is necessary for the determination of ultratrace concentrations or extremely small (C10-6) isotope ratios. A secondary advantage is the separation of the atomization and ionization processes, through which matrix-induced effects on atomization and ionization can be segregated and mitigated. The state-of-the-art in RIMS is best reflected in the proceedings of the Resonance Ionization Spectroscopy (RIS) symposia, the latest of which (at time of writing) was held in 1986 (GI). By the time this fundamental review is published, however, the 4th RIS symposium will have been held (April 1988); when available, the proceedings of this symposium should be consulted for the latest advances. Several generic
ATOMIC MASS SPECTROMETRY
RIMS reviews offer technique familiarization ((3247); Fassett et al. (G2) and Nogar et al. (G4)provide very readable general reviews while Travis et al. (G3)provides a more detailed review of recent RIMS progress. The RIMS technique is fairly young as an analytical method, consequently much development work was reported during the review period. Many types of atomization, ionization, and mass spectrometer systems have been used in RIMS investigations. Thermal atomizationusing conventional TIMS instruments and apparatus has been the most common (G3),but ion beam sputtering and resonance ionization (or sputter-initiated RIMS, SIRIMS) for the direct analysis of solids has received much recent attention. For example, argon was used to sputter-atomize U metal, U02, and U308samples (G8). A signifcant increase in sensitivity (compared to SIMS) was observed for the SIRIMS analysis of the metal. Analysis of an Fe-implanted Si sample resulted in an observed sensitivity of
