Anal. Chem. 1993, 65, 12R-28R
Geological and Inorganic Materials L. L. Jackson,**+ P. A. Baedecker,t T. L. Fries,$ and P. J. Lamothel
US.Geological Survey, P.O. Box 25046, DFC, M S 973, Denver, Colorado 80225, U S . Geological Survey, National Center, MS 923, Reston, Virginia 22092, and U.S. Geological Survey, 345 Middlefield Road, MS 938, Menlo Park, California 94025 Review Contents Introduction Geostandards Sample Preparation and Dissolution Atomic Absorption Spectrometry Plasmas: ICP-AES, ICP-MS, and DCP-AES Mass Spectrometry X-ray Spectrometry Electron Microbeam Techniques Particle-Induced X-ray and y-ray Emission Nuclear Activation Methods Instrumental Neutron Activation Analysis Epithermal Neutron Activation Analysis Fast Neutron Activation Analysis Delayed Neutron Activation Analysis Prompt y-ray Activation Analysis Chemical Separation Neutron Activation Analysis Radiochemical Neutron Activation Analysis Charged Particle Activation Analysis Photon Activation Analysis Miscellaneous Spectroscopic Methods Chromatography Methods Miscellaneous Methods
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INTRODUCTION This review surveys the literature for the two-year period since our previous review (AI), November 1990 through October 1992. In order to prepare this review we performed a computerized keyword search of Chemical Abstracts and manually searched several of the primary journals. Our literature search identified more than 2900 references. A similar number were obtained for each of our two previous reviews and we have cited about 500 of those references. In general, obscure references have not been cited. In many instances, foreign language articles have been reviewed solely from the abstract. We have followed a format similar to our previous reviews, although our contributing author list has changed. We have continued to focus on and highlight current trends in geochemical analysis. Our primary emphasis has been to cite applications of the determination of inorganic species in geological materials that offer significant changes to research and routine work. For many of the topics covered, the references are simply too numerous to cite and only representative examples are included. We have not included applications or techniques for qualitative identification or elucidation of structural properties of minerals. The Review Contents lists the areas and techniques that are discussed in this review. During this review period many applications to the analysis of geologic materials have been published, but changes and improvements have been minor, for the most part. Applications of inductively coupled plasma mass spectrometry, particularly with laser ablation of the solid sample, are still maturing and coming into more widespread use. Although
* Author to whom correspondence should be sent. + f
U.S. Geological Survey, Denver, CO. U S . Geological Survey, Reston, VA.
5 U S . Geological 12R
Survey, Menlo Park, CA. 0003-2700/93/0365-0012R$12.00/0
techniques such as atomic absorption spectrometry have widespread use, there are few, if any, major advances in their geochemical application. Techniques for determin’ element speciation in geologic materials, regardless of thzefinition of speciation, continue to be problematic and controversial. Precious metals and rare earth elements have continued to be of general interest. A new cookbook-style text focused on methods of analysis of geological materials for the precious metals (A2). Another text published during this period was an updated review of instrumental techniques for soil analysis (A3). The many facets of the analysis and characterization of marine particles have been compiled in one volume (A4). The volume is an excellent overall review that includes specific aspects of different analyses and recommendations for successful use of the techniques for both organic and inorganic analysis of suspended particles in the oceans. Reviews on aspects of data quality and the use of geologic reference materials, decomposition techniques, and the most prominent instrumental techniques for applied geochemical analysis have been brought together in one “Geoanalysis” special issue of the Journal of Geochemical Exploration (A5).
GEOSTANDARDS Geological reference materials are a necessity for calibration of sophisticated multielement instrumental analysis techniques and for quality assurance-quality control materials to meet dataquality objectives ofresearch and regulatory aspects of the geosciences. Currently there is an ever increasin production of reference materials that are being distribute! internationally ( B I ) . Recently produced reference materials include the following: an extensive suite of Russian materials, 7 magmatic rock samples (B2),4 sedimentary rock samples (B3),19sediment samples (B4),and coal ash and silt samples (B5);Polish fly ash and apatite concentrate samples (B6); 2 Scottish chromite samples (B7); 5 Nevada precious metal samples (B8);and 14 synthetic rare earth element orthophosphates (B9)and 13 mineral standards for oxygen (BIO) for electron microprobe analysis. The National Institute of Standards and Technology (NIST) has also issued several standard reference materials (SRM) that are oriented to environmental studies: three Tennessee River sediment reference materials, RM 8406-8408,for mercury determinations and three soil samples, SRM 2709-2711, for determination of a variety of inorganic elements from background to contaminated levels ( B I I ) . Geochemical reference material programs have been reviewed (B12, BI3) and a number of authors have addressed the general use of SRMs and specifically their use in quality assurance-quality control programs (B14-BI9). Geochemical reference materials have frequently been misused. The use of U S . Geological Survey (USGS) geochemical exploration reference materials GXR-1 through GXR-6 for other than their intended purpose is a classic example. The samples were originallydeveloped for exploration programs-programs where field and laboratory semiquantitative techniques are commonly used. When high-precision analysis techniques are used, the bottle to bottle heterogeneity may cause significant problems (B20). In the development and subsequent use of geochemical reference materials, the determination of the “certified”, “best”, “consensus”, “recommended”, “working”, or other descriptor of the concentration of an element in reference material has generated considerable debate. The use of definitive analysis techniques to quantify element concentrations and thus “certify” the value for a reference material is an obvious issue (B21).The use of isotope dilution mass spectrometry as a definitive method has been illustrated in 0 1993 American Chemical Society
GEOLOGICAL AND INORGANIC MATERIALS
Larry L. Jackson is a Research Chemist with the U.S. Geological Survey, Branch of Geochemistry. He received a B.S. degree in chemistry from Kansas State University in 1973 and a Ph.D. in analytical chemistry from Colorado State University in 1978. His research interests include the application of electroanalytical techniques to the analysis of geological materials, environmental geochemistry of sulfur, element cycling in wetlands, and the use of biogeochemistry studies for air-quality monitoring.
Paul J. Lamothe is a Research Chemist at the U.S. Geological Survey, Branch of Geochemistry. He received his B.S. in chemistry from the University of San Francisco in 1968andhis Ph.D. in analytical chemistry from Marquette University in 1973. Prior to joining the USGS in 1976, he was a research chemist with the Environmental Protection Agency, Research Triangle Park, NC. His research interests are in analytical spectroscopy and trace element analyses.
PhWlpA. Baedecker is a ResearchChemist with the U.S. Geological Survey, Branch of Geochemistry. He received his B.S. degree in Chemistry from Ohio University in 1961 and his Ph.D. in inorganic and radiochemistry from the University of Kentucky in 1967. Prior to joining the USGS in 1974 he held postdoctoral positions at the Massachusetts Instituteof Technology and the University of California at Los Angeles. At the USGS he has served as Chief of the Branch of Analytical Chemistry and as Chairman of the Materials Effects task group of the National Acid Precipitation Assessment Program. His research interests include the application of nuclear analytical methods to problems in geochemistry, computer methods in y-ray spectroscopy and activation analysis, and the effects of acidic deposition on carbonate stone.
extraction of metals from sediments, sludges, soils, and solid wastes. Microwave decomposition techniques may speed up digestions, but they do not necessarilyalleviate problems that were encountered in the more conventional acid digestion, as has been discussed for the extraction of Sb from soils by the EPA SW846 method 3050 (C2, C3). Many of the two dozen or so articles on microwave digestions that we noted during this review period were based on comparisons and optimization of an individual laboratory's conventionalacid digestion recipe using microwave heating. One group has applied a chemometric approach to the optimization of microwave digestions (C4, C5). Other groups have used microwave heating to assist in vapor-phase acid digestions (CS)and in automated systems using robotics (C7) and flow injection analysis with on-line slurry digestion (C8). Numerous papers have addressed specific digestion problems. Fire assay procedures using nickel sulfide have become more commonplace. Several analysis techniques have been used to examine the completenessof chromite digestion using a modified nickel sulfide fusion process (C9). Platinum group elements (PGE) and Au were determined using neutron activation analysis after fusing a chromite-rich sample with lithium tetraborate, sodium carbonate, nickel, sulfur, silica, and sodium hydroxide a t 1200 "C. After 1 h of heating a second batch of flux was added. This flux recipe produced a less viscous melt which was not saturated with chromium oxide and there were no detectable undissolved chromite grains. Biases in the determination of gold using an aqua regia digestion have been revisited (ClO,C l l ) . Problems encountered in incomplete digestion due to the nature of the extracting solutions and the sample to extracting solution ratio have been discussed briefly. Unfortunately, this work has used the USGS GXR series of standards, which are probably not the best samples to use for comparison of methods (B19). Another potential bias in Au determinations was the apparent loss of Au in ground samples due to electrostatic adhesion to the walls of plastic storage containers
Terry L. Frles is a Research Chemist at the U.S. Geological Survey, Branch of Geochemistry. He received his B.S. in chemistry from California State University, Fresno in 1976 and his M.S. degree in analytical chemistry from San Jose State University in 1982. His research interests are in the application of atomic spectroscopy to trace element analysis.
the correction of certified sulfur concentrations in NIST bauxite SRMs (B22). Various objective and subjective methods of determining the true element concentration in the referencematerial from the available analytical data have been the subject of numerous articles (B23-BZ8). Standards for terminology to be used in describing the array of descriptors of element concentration in reference materials have been proposed (B29). However, one problem that has occurred frequently is not just how the mean concentration is derived and what it is called, but an inadequate descriptor of what is measured or inappropriate compilation of data from different methods (B30). Publishers of data on reference materials must include more details on their methods of analysis than has been common practice in the past if these data are to be useful to others. Geostandards Newsletter has continued to publish annual bibliographieson geochemical reference materials (B31,B32) and compilationsof data on various materials. Several USGS reference materialswere the subject of two recent compilations (B33,B34). Rare earth elements in ferromanganese nodule reference materials have also been examined (B35).
SAMPLE PREPARATION AND DISSOLUTION Decomposition techniques of geological materials by acid digestion and fusion and applications to the determination of specific elements or groups of elements have been reviewed (CI). The review includes a discussion of microwave decomposition techniques. Microwave decomposition techniques are quite common now. The U.S. Environmental Protection Agency (EPA) has even approved their use for the
(C12).
One problem that was not addressed directly in the work on Au was the linear range of analyte concentration that the digestion procedure would extract. However, the linear range of several acid digestion procedures for trace metals of environmental interest has been examined (C13). There was considerable variability in the linear range for different elements in different matrices using four digestionprocedures which included EPA SW846 method 3050. Lastly, one novel sample preparation problem that has been examined is the preparation of purified monominerallic quartz samples from rocks and soils for the determination of in situproduced cosmogenic nuclides (C14). A series of relatively simple acid leaches in a heated ultrasonic bath have been found to produce large quantities of purified quartz suitable for the measurement of loBe and 26Al. Other decomposition techniques are discussed throughout this review.
ATOMIC ABSORPTION SPECTROMETRY Atomic absorption spectroscopy (AAS) continues to see broad application to the analysis of geologicalmaterials. This is in spite of the advent of sensitive, multielement methods such as inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). The continued interest in AAS is ANALYTICAL CHEMISTRY, VOL. 65, NO. 12, JUNE 15, 1993
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due to the robust nature of the instrumentation, the huge body of literature on the use of AAS in solving geochemical problems, the ease of interfacing AAS equipment to other analytical devices, and the universal availability of AAS equipment. Worldwide interest in the application of AAS to the analysis of geologicalmaterials is reflected in the fact that nearly 50 % of the papers considered for this review appeared in a language other than En lish. Comprehensive reviews of this literature in the Journafof Analytical Atomic Spectrometry (01,0 2 ) and biennial listings in Atomic Spectrometry (03-06) are valuable presentations of the AAS literature. A broad review of the use of AAS in geochemical exploration outlines AAS theory, instrumentation, sample preparation, and analytical requirements for the ap lication of AAS in this important field (07).More methocforiented reviews of AAS in applied geochemistry ( 0 8 ) and in the analysis of marine materials (09) are useful additions to the literature. Refinements to the AAS method have been the key to continued development and research. The introduction of solid samples is an active area of interest as is sample introduction using flow injection techniques. Modifications of the graphite tube atomizer and extensions of the use of matrix modifiers remain active fields of research. Innovative separation schemes provide pathways to still lower detection limits, to the elimination of interferences, and to quantitative data for speciation studies. The analysis of solid samples by electrothermal atomization atomic absorption spectrometry (ETA-AAS),after introducing them in the form of slurries, has been successful for a number of elements and matrices. In the determination of Pb in soils, it was found that magnetic stirring of the soilwater suspension produced a bias due to lead-containing magnetic articles adhering to the stir bar. Vortex mixing eliminatef this problem, and calibration with aqueous standards produced excellent results for lead in two certified reference soils (010). A fast-temperature program coupled with slurry ETA-AAS provided a more rapid approach to the determination of lead in soils and demonstrated that matrix modifiers were not required for this type of analysis (011). A study of homogenization techniques, suspension media, atomization conditions, and particle size distribution for the determination of lead and cadmium in sediment slurries by ETA-AAS demonstrated that excellent results could be obtained using a PdCl2 matrix modifier and either aqueous or slurry calibration standards. The critical roles of mixing and particle size distribution were studied,and vortex mixing was found to be better than other mechanical methods, although ultrasonic homogenization produced the best results of all mixing methods investigated (012). The role of matrix modifiers in slurry sample introduction for ETA-AAS has been even less clear than in the case of aqueous samples. Scanning electron micrographs and pyrolysis data for the use of alladium as a matrix modifier for the determination of leaf in sediments using slurry ETAAAS provide new data on this question. Physical mechanisms were described and slurry particles were shown to be the stabilizing factor rather than the addition of palladium matrix modifier (013). The problem of loadin a slurry prepared from a solid sam le for analysis using #TA-AAS has been approached by comginin slurry samplingwith probe atomization. A custommade prote system produced detection limits in the lo-" g range for Bi, Cd, Cu, Sn, and Sb in geological materials. Calibration was carried out using aqueous standards, and good agreement with published results for reference materials was claimed (014-016). Direct analysis of solid materials by AAS is an analytical area that generates interesting analytical methods. The direct determination of indium by ETA-AAS usin an automatic probe system was investigated. This methocfwas compared with ion-exchange separation followed by ETA-AAS, interferences and atomization mechanism were described, and results for geologicalreference materials were reported (017). By placing a mixture of sample and graphite powder in a raphite cup for introduction into the furnace, Li, Be, Co, Ni, u, Rb, Cs, Pb, and Bi were determined in nine standard reference rock samples. Results were found to be strongly dependent on article size, sample size, sample to graphite mixture, ande!t use of ionization suppressants for Rb and
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Cs. Good agreement with recommended values was achieved using aqueous standards for calibration (018). In a short note, the direct analysis of microgram-size samples for Cu, Nb, Zr, and Hg by AAS has been reported. A commercial sputtering device was used to atomize samples that had been mounted with collodion in a presputtered crater in a pure Fe target. Examples of the analysis of dust, soil, and oxide materials are presented, but no data is reported for reference materials (019). Hydride generation (HG) has long been a useful sample introduction method for enhancing the sensitivity of AAS for the determination of hydride-forming elements. More recently this approach has been coupled with flow injection (FI) to provide rapid, automated, low-level analysis. Conditions for the determination of Sn in steel using HG-FI-AAS with a uartz cell have been outlined, and both the source and m J e of occurrenceof interferences have been established. An Ar+O2 mixture for the carrier gas stream was found to produce better sensitivity and more symmetrical peaks, but for this determination, as well as others where the matrix affects peak shape, the use of integrated absorbance was recommended over peak absorbance (020). Trapping of the hydrides generated by HG-FI on a Pd-treated graphite platform and subsequent analysis by ETA-AAS has been presented as a promising approach to lower limits of determination of hydride-forming elements and the elimination of interferences related to hydride release rate and transport. The trapping process was effective for As, Bi, Ge, Sb, Se, Sn, and Te, but data are presented only for the determination of Sn (021). A more conventional approach using HG-FI-AAS and a heated quartz tube atomizer for the determination of Bi in international geologic reference samples demonstrates another application of this method. Usin a Varian AA6 instrument and a basic FI module, a rapit method for Bi determination at levels as low as 10 ng/g was developed by integrating FI into an established technique using masking agents to control interferences from Cu and Ni. Results for reference materials are clearly presented, and for materials with recommended values, agreement is good (022). In the past, the addition of tungsten or tantalum liners or coatings to the graphite tube atomizer used in ETA-AAS has allowed the determination of low levels of refractory materials, the elimination of some memory effects, extended lifetimes for the graphite tube in certain applications, and the possibility of standardless analytical methods. Recent work using a tantalum foil held in place in the graphite tube by a tungsten spiral continues investigations in this area. No interferences were found in the determination of Yb in a number of geological materials, and results for reference materials were in agreement with accepted values. Peak area measurement was required for satisfactory results, and although pyrolytic coated graphite was deemed totally unsuitable for standardless measurements, the tantalum-lined tube was touted as a potential atomizer for such work (023). This approach was also used for the determination of Er with results similar to those found for Yb (024). A study using a tantalum foil platform in apyrolytic gra hite tube compares the theoretical characteristic mass for Ed with experimental values for atomization temperatures in the range from 1000 to 2200 K. Palladium, NaH2P04, and ascorbic acid were evaluated as matrix modifiers, and results for the standardless analysis of a variet of materials including geological materials are includeJ(D25). A tungsten tube atomizer has been used for the determination of titanium in geological materials. This atomizer allowed the use of high charring temperatures to eliminate interferences, and a characteristic mass of 7.5 pg was achieved for Ti (026). The development of sample introduction techniques, improved atomizers, and matrix modifiers for AAS analysis of geochemical materials has been driven by the need for lower limits of detection and the elimination of interferences. In many cases separation methods have also been able to solve these problems. A calcium nitrate matrix modifier was used in conjunction with anion exchan e to eliminate interferences in the determination of Sn. Tfis permitted sample dissolution by fusion using a flux mixture of LiZCO3and H,B03 to ensure good analyte recovery (027). A similar approach was also used for the determination of In in silicates, and ood agreement with workin values was achieved (028). iolvent extraction was appliefto the determination of Ga in
QEOLOGICAL AND INORGANIC MATERIALS
iron-rich meteorites. Gallium was extracted into methyl isobutyl ketone as a chloro com lex after reduction of iron using potassium iodide. ETA-&S results compare satisfactorily with neutron activation results for eight meteorites (029). An aqua regia digestion followed b separation using proprietary “molecular reco ition ligan&” was used as an determination of Pd, Pt, and alternative to fire assay for Rh by ETA-AAS in a standard stream sediment sam le. The molecular recognition ligands are macrocycles, suc! as 18crown-6, bonded to silica el and they achieved complete recovery of Pd, Pt, and Rh rom a synthetic solution without interference form Co, Cu, Cr, Ni, Pb, or Zn. Good agreement with lead fire assay was found for Pd and Pt in a sediment ’ ,was attributed sample, and the low recovery of Rh, about 80% to incompletesample attack by aqua regia ( 0 3 0 ) . Low levels of silver in copper tailings and copper-zinc ores were determined using flame AAS by first condensing volatilized Ag in a quartz tube and then recovering the Ag with HNOB and HC1rinses. By usin a MgSiO3-CaOadditivewithheatin in air at 1200 OC for 2 satisfactory results were obtainef for samples with Ag concentrations in the lO-3% range (031). A combination of approaches was also used to obtain good results for thallium in a number of materials including river and marine bottom sediments. Thallium was first complexed with potassium xanthogenate and the complex absorbed onto activated carbon. An aqueous slurry of the activated carbon was then loaded into the furnace for analysis (032). One last area of AAS that continues to see innovative applications is in the use of AAS as a detector for studies of analyte speciation in soil and sediment. Flame AAS, coupled with a custom-made slotted-tube atom trap, was applied to the determination of seven organic and inorganic forms of arsenic in soil. The AAS system was used on-line with a high-performance li uid chromatography (HPLC) system, and it was found t h a t h S sensitivityfor As was not dependent on its molecular form. Although ICP-MS was shown to be a much more powerful detector, AAS provided sufficient sensitivity for this study ( 0 3 3 ) . Alkylation of mercury compounds with subsequent separation using as chromatography (GC) and determination using AAb has been proposed for the study of mercury speciation. In this study atomicfluorescence spectrometryis favorably compared with AAS for the determination of elemental Hg (034). Methods have been developed for the determination of selenium in volatile compounds generated from soils. The volatile Se compounds were separated using GC or preferentiall aband sorbed onto a palladium-coated graphite furnace analyzed using AAS. Conditions are described for an on-line application of this method usin ETA-AAS (035, 036). Liquid nitrogen cold traps, contro!l ed heating, and a number of separation-digestion procedures have also been combined with HG-AAS to determinethe concentration d five Se species in sediment water extracts (037). Though AAS is a mature technique, there seems to be no limit to the analytical problems it can solve. The apparently unlimited versatility of AAS seems to assure a long future for the method.
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PLASMAS: ICP-AES, ICP-MS, AND DCP-AES Plasma spectrometry is a popular and versatile class of methodologies being used by eoscientists for the analysis of geological and environmen samples. In this section we will limit our discussions to the three versions of plasma spectrometry which have been widely utilized in commercial, academic, and government laboratories, namely, inductively coupled plasma atomic emission spectrometry (ICP-AES), direct current plasma atomic emission spectrometry (DCPAES), and inductively coupled plasma .mass spectrometry (ICP-MS). All three ofthese methods exlubit a linear response e of more than 5 orders of magnitude, and all are amenable i x e determination of major, minor, and trace elements in geological and inorganic materials. A useful periodical for plasma spectroscopists is the ZCP Information Newsletter (R. M. Barnes,Editor), which reports on recent developments in the field by listing abstracts of national and international meetings, issuing special reports, and providing an annual bibliography in the January issue. During 1992,two journal special issues were published that deserve special mention. The first, “Plasma Spectrometry
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in the Earth Sciences”, edited by Jarvis and Jarvis (El), presents 15 papers which provide an excellent overview of a wide range of research by earth scientists who routinely use plasma s ectrometry in their work. This volume includes articles &at review developments in the fields of sample dissolution, sample introduction, instrument calibration, and trace element determinations as well as articles that discuss the effect of analytical bias on geological interpretations. The second s ecial issue, ”Geoanalysis”, edited by Hall (EZ), is designecfto give the reader an in-depth review of the state of the art of several of the prominent eoanalytical techniques. Two of the 10 articles in this s eciafi issue feature the topics of ICP-AES and ICP-MS. Al!? scientists involved with the practice of geoanalysis would benefit from readin both of these special issues from cover to cover. Selectef articles from these two volumes will be highlighted throughout the following section. Two comprehensive reviews focus on plasma s ectroscopy in general and ICP-AES in particular. The prasma spectrosco y review (E3) encompasses ICP-AES, ICP-MS, and DCP-IES, from instrument design and o eration to interferences and analyticalperformance. In ad ition to discussing the relative merits and limitations of the three plasma techniques, they attempt to put plasma spectrometry into perspective by comparing it with other routine instrumental methods such as atomic absorption, X-ray fluorescence, and neutron activation analysis. Their review on ICP-AES in ex loration geochemistry (E4) contains some of the same inkrmation from their earlier review, but it also contains specifics about dissolution, separation, and preconcentration procedures that are particularly valuable for exploration work. It is generally recognized that the dissolution of solid samples is one of the critical factors controlling accuracy in the analysis of geological materials. Incomplete digestion of refractory mineral phases and the lws of volatile speciesduring sample preparation are two perennial problems that face eoanalysts. A comparison of three of the more popular issolution techniques has appeared (E5)which evaluates LiBOz fusions, open-vessel HF-HC104 digestions, and microwave-heated sealed-vessel acid digestions. Surprisingly, the authors concluded that acid attack with HF, either at atmospheric or elevated pressure, yielded accurate results for rare earth elements (REE) in most cases. This is inconsistent with our experience and the reports by others (E6,E7) who find that digestion of samples solely via an acid attack often leads to inaccurate chondrite plots of the REE due to incompletedissolution of the heavier REE. The paper also presented a method whereby by Watkins and Nolan (E7) Hf can be determined simultaneouslywith the REE, Y, and Sc using a cation-exchangeseparationste that includes oxalic acid in the final eluant, followed by ICP-XES. An automated sample preparation lasma spectrometric measurement system has been descriged (E8)which uses a sodium peroxide sinter for sample decomposition. Although the sodium peroxide sinter method has been used extensively in France for more than 10 years, it has not gained wider popularity primarily because of the difficulty of homogenizing the flux with the sample prior to sintering. One way of circumventin the problems associated with the dissolution of solid sampfes is to introduce the sample in the form of a slurry. The role of slur nebulization in the analysis of geological samples is the s g j e c t of a review (E91 which examines the progress made in this area of sample introduction over the past 5 years. A recent report on the suspension nebulization of clays ( E l 0 ) confirms the work of earlier researchers who found that the particle size needed to be reduced to 30pm) fluid inclusions ( E l 3 )by ICP-AES. More than 10 individual inclusions need to be sampled, and the results averaged, if accurate quantitation is desired. Severe memory effects in laser ablation ICP-MS using either a fused glass bead (E14)or a pressed pellet (El5)sample preparation procedure were experienced by van Heuzen. The memory
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effects were probably due to particle deposits in the transfer tubing between the ablation cell and the ICP torch. The determination of trace elements in carbonates using laser ablation ICP-MS was the sub’ect of two papers (E16, E l 7) which focused on the problems of preparing adequate standards for the technique. The authors concluded that homogeneous synthetic standards can be produced by doping carbonate powders with small aliquots of aqueous standard solutions. Mineral microanalysis by laser ablation ICP-MS was made possible (E18)through a slight modification to a Nd:YAG laser which yielded ablation craters of 20-30 pm in diameter. The major obstacle to mineral analysis by this method is the limited availability of single mineral reference materials for standardization. A nuance to the technique of laser ablation sampling is laser-produced plasma emission spectroscopy (E19),which has also been labeled laser microvaporization atomic emission spectroscopy (E20). Both of these reports discuss the analytical information which can be extracted from the millimeter-sized plasma that is created above the surface of solids that are subjected to laser pulses. The various strategies for the multielement calibration of ICP-AES spectrometers were reviewed by Ramsey and Coles (E21), who concluded that the application of the parameterrelated internal standard method to the intensity measurements of the calibration solutions improves precision and accuracy and obviates the need for matrix matching. The use of multiple internal standards for the analysis of geological samples by ICP-AES was reported (E22)which suggests that three internal standard elements (Ga, Cd, Li) are needed for the precise determination of major elements in silicate rocks. The use of these matched internal standards improves the precision obtainable under routine analytical conditions to about 0.5% RSD. Significant matrix effects have been observed in ICP-AES due to the presence of HC1, HN03, or HC104at concentrations below 1% (E23)which could not be explained by changes in nebulizer or transport efficiency or by variations in droplet size distributions. However, a correction scheme based on the Myers-Tracy signal compensation procedure (E24) was shown to minimize the suppressive effects of mineral acids on analyte emission of Cu and Mn for acid concentrations as high as 20% (viv). Several reports have appeared since our last review that deal with improvementsto the plasma spectrometric analysis of selected elements. The topic of ReiOs isotope ratio determinations by ICP-MS was reviewed (E25),and a new method for the chemical separation of Re, Os, and Mo (E26) has been presented which prevents polymerization of Mo species and is a plicable to isotope geochemical studies of ore deposits anzmeteorites. An ICP-MS method has been designed which is capable of differentiating between exotic and indigenous Au in humus samples at concentrations less than 1ng/g (E27). This method is particularly useful as an aid to gold exploration in areas of glacially transported overburden. A method for the determination of Au in soil and sludge using ICP-AES has been described (E281 which separates and preconcentrates the Au via a thiol-cotton fiber column. A comparison of various methods for the extraction of soluble boron from soils (E29) prior to ICP-AES determination has appeared. Also, a rapid technique for the estimation of precious metals in geological samples (E301 is based upon an aqua re ia leach followed by ICP-MS determination of Au, Pt, P$ Rh, Ru, Os, and Ir. For ophiolitic samples, only Au and Pd are quantitatively extracted because many of the platinum group element (PGE) mineral species are.actually intermetallic alloys which are insoluble in aqua regia. The field of ICP-MS is exhibiting the signs of an analytical technique that is beginning to mature. The literature has shown a shift from fundamental studies toward application studies, and the number of review papers, relative to the number of truly innovative application papers, has increased dramatically over the past two years. A review article by the late Alan Date (E311discusses instrumentation, performance characteristics, and application of ICP-MS to trace element and isotope ratio determinations in geological materials. A comprehensive review of ICP-MS in geoanalysis (E32) contains over 115 references and includes segments on instrumentation, optimization,interferences, and calibration, as well as an extensive applications section. An interesting, 16R
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but somewhat disappointing, article was published which presented a statistical analysis of data comparin ICP-MS, ICP-AES, and GFAAS data obtained from a multiyaboratory study (E33)of water and “digested”geolo ical samples. The authors concluded that ICP-MS results difnot generally agree well with GFAAS or ICP-AESresults for many of the elements studied. Unfortunately, the authors’ conclusion that the “variation among laboratories is significant” leaves the reader to wonder if the variance among techniques (and indeed even between *types” of ICP-MS instruments) is statistically independent of the variance among laboratories. The utility of using a flow injection sample introduction system with ICP-MS was demonstrated in a recent report (E34)which presented results obtained for the determination of PGE in the standard PCC-1. Although the results for some elements were less than impressive,most of the problems could be obviated through the use of a more optimal sample digestion protocol. A novel two-stage solvent extraction and back-extraction system for the separation and concentration of REE has been developed 0335) for ICP-MS. Its novelty stems from the fact that it is a continuous on-line system that results in a dilution factor of only 5 for solid materials prepared using either alkali fusion or acid decomposition procedures. Chlorination is an ancient but seldom used technique for the decomposition of platinum metal samples. Its use dates back to the early 1800sand involves subjectinga heated sample to a stream of chlorine gas. A recent report discusses the development of a dry-chlorination ICP-MS procedure for the determination of PGE and Au in rock pulps (E36). Rea ent contamination was low, recoveries of most PGE (Os an Re were not included) generally exceeded 90 % , and the results on three standard reference samples compared favorably with NiS fire assay ICP-MS results obtained from commercial laboratories. Even though the recovery for Au was on1 approximately 60 5% for the three reference materials studied: the Au values produced by this technique were superior to commercial laboratory results obtained using either Pb or NiS fire assay techniques. In addition to its capabilities in the area of trace and ultratrace element determinations, ICP-MS can be a suitable method for the determination of P b isotope ratios. Biases for ratios to *04Pbcan be controlled to
