Geological and Inorganic Materials - Analytical ... - ACS Publications

Anal. Chem. 1995, 67,71R-85R

Geological and Inorganic Materials L. L. Jackson,**t P. A. Baedecker,* T. L. Fries$ and P. J. Lamothet

U.S. Geological Survey, P.O. Box 25046, DFC, MS 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, Califomia 94025 Review Contents

Geostandards Sample Preparation and Dissolution Atomic Absorption Spectrometry Plasmas: ICP-AES and ICPMS Mass Spectrometry X-ray Spectrometry Electron Microbeam Techniques Particle-Induced X-ray and y-ray Emission Nuclear Activation Methods Miscellaneous Spectroscopic Methods Chromatography Methods Miscellaneous Methods

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This review surveys the literature on the analysis of geological and inorganic materials for the two-year period October 1992 through October 1994. The review was conducted in the same fashion as our previous reviews and with the same focus. A computerized search of Chemical Abstracts and a manual search of numerous primary journals were performed. We have continued in our attempt to highlight current trends and identify geoanalysis methods primarily for the determination of inorganic species that offer significant changes to research and routine work. Generally, we have not included applications or techniques for qualitative identification or elucidation of structural properties of minerals. Despite the many incremental changes and important but relatively minor improvements that we cover in this review, there is not much that is really new in the analysis of geological materials. Many geoanalytical laboratories are being asked to determine more elements in more samples each year and as one of our colleagues aptly put it, ‘They (ke., geologists and geochemists) are trying to solve the problem with smaller and smaller samples”. This is readily apparent with the increasing interest in microbeam techniques and in particular with techniques such as laser ablation inductively coupled plasma mass spectroscopy (ICPMS). The development and application of ICPMS in the geosciences is probably the most signifkant advance in geoanalysis in the last decade. Although in other areas of geoanalysis nothing so striking has occurred, trends that have been ongoing such as the prolieration and characterization of geostandards, development of speciation schemes, and a greater push for standardization, at least by the European community, and an increasing use of chromatographic techniques have continued during this review period. Several noteworthy books appeared during the review period. Riddle edited a general text, Analysis of Geological Materials, that covers many aspects of geoanalysis ( A l ) including chapters on an overview of geochemical research, sampling, sample prepara+

U S . Geological Survey, Denver, CO.

’ U S . Geological Survey, Reston, VA.

4 US.Geological Survey, Menlo Park, CA.

This article not subject to U.S.Copyright. Published 1995 Am. Chem. SOC

tion, analytical techniques, geostandards, quality assurance/ control programs, and interpretation of geochemical results. Unfortunately, the examples do not include much emphasis on the environmental aspects of geochemical analysis. Vander Voet and Riddle have also prepared a practical guide for analysis of geological materials (A2) to accompany the Ontario Geological Survey’s manual of methods (A3). Another Canadian effort covers the sampling and analysis of soils (A4). Manuals on sampling of contaminated soils (As) and soil testing have been updated (As). A symposium volume on characterization of soil minerals contains numerous chapters on analytical techniques (An. Lastly, for those analysts who are tired of decomposing their samples, there is a new text-Remote Geochemical Analysis: Elemental and Mineralogical Composition (AS). 0 EOSTANDARDS Geostandards Newsletter plays the critical role in the international distribution of information on geostandards. This is no more apparent than in the latest special issue on the compilation of working values for 383 geostandards (Bl). In addition to the large increase in the number of standards included (the previous 1989 edition included 272 standards), there has been a slight increase in the amount of trace element data reported for individual standards. One minor problem regarding the ambiguity of the definition of the nature or form of carbon reported has still not been clarified. Geostandards information is also available in computerized databases (B2,B3). Another massive compilation (B4)has also been published which includes data on about 500 geostandards and has a greater emphasis on ores and refractories than the Geostandards Newsletter special issue. A compilation of data on National Institute of Standards and Technology (NISQ reference materials has been updated (Bs). Data for rare earth elements (REE) Sc, Y, Zr, and Hf have also been compiled for 26 Geological Survey of Japan reference materials (B6). Abbey has revised his “five-mode method” for determining “usable values” and evaluated it on new and old data for four Canadian ironformation samples (B7). The results of several international collaborative studies on the first GIT-IWG rock reference materials (B8-Bll) and Belgium sedimentary rocks (BIZ) have been published. Data are beginning to appear on new Russian lake silt and brown coal ash samples (B13), three new Japanese rocks rich in rare metals (B14), and 14 South African silicates (B15). S i new Canadian standards that were intended for certification for Au and platinum group elements (PGE) (B10)and a NIST refractory gold ore (SRh4 886) have been developed. Independent laboratory data are also beginning to be published for the three new NIST soils: S M s 2709-2711 (B16,B27). In a very useful departure from the past, NIST has begun to report leachable element concentrations by US. EPA method 3050 in addition to the certitied values and noncertified information values for total element concentrations Analytical Chemistty, Vol. 67, No. 12, June 15, 1995 71R

for the three soil materials (B18).An insider’s perspective on the use of analytical techniques in the certifying of reference materials was recently presented (B19).Several routine and nonroutine analysis procedures used to characterize the US. Geological Survey organic-rich and sulfur-richshale material, SDO1, have been described (BZO). A new microprobe anorthoclase standard (B21)and new materials for nitrogen isotope ratio measurements (BZZ) have been reported. As has been recently noted (B23), international standards and information on these latter two types of materials has generally been lacking. Geostandards Newsletter has continued its annual bibliography on literature related to geostandards (B24, BZ5). SAMPLE PREPARATION AND DISSOLUTION In the analysis of geologic and inorganic materials “the problem is often the solution”. Whether the solution is the fused glass disk often used for X-ray fluorescence 0, the collection button from fire assay, or the acid digest used for atomic absorption spectrometry (AAS) , inductively coupled plasma OCP) spectrometry, and various separation techniques, the problems are similar. Resistant mineral phases, inhomogeneous samples, volatile species, precipitates, and excessive time requirements continue to generate interest in sample preparation and dissolution. One of the more recent innovations in this area is the microwave decomposition technique. Descriptions of microwave methods with the potential advantages of rapid, controlled heating are common, and documenting the use of microwave in place of conventional methods is an important application area. Leaching of sediments, soils, and sludges, by using closed-vesselmicrowave methods, has been compared with conventional methods (C1). Results for the two methods were in good agreement. Total dissolution methods and automation of microwave procedures were also discussed. In another area, microwave heating was compared with conventional procedures in Tessier’s extractions of sediments (CZ). Results for extractions prepared using microwave heating were similar to those obtained using conventional methods, but the extractions were completed more rapidly using the microwave approach. Microwave heating has also been applied to the on-line digestion of various materials, including sewage sludge, for direct, automated, determination of Cu, Mn, Pb, and Zn by flame AAS (C3). Digestion was carried out on slurried samples flowing through a Teflon coil in a microwave oven. Although Zn results were generally low, data for other elements were acceptable. At the other end of the spectrum is fire assay, with origins dating to the very beginnings of geochemical analysis. This is still an area of active application developmentwith the recent focus on nickel sulfide fire assay techniques for PGE. Improved recovery of PGE was reported using a modified nickel sulfde method and ICPMS (0).The method simplifies sample handling, reduces waste generation, and eliminates potential loss of PGE during the recovery phase. Difficulties encountered during the determination of PGE in kimberlites by using nickel sulfide fire assay combined with neutron activation analysis (NAA) resulted in a radiotracer study that detailed problems with incomplete melting and losses during dissolution (C5). Lower detection limits than previous NAA analyses were reported, and results from previous studies of PGE in kimberlites were questioned. Nickel sulfide fire assay, lead fire assay, and leaching by aqua regia have been compared for the determination of PGE 72R

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using ICPMS (C6). The aqua regia leach generally produced incomplete recoveries, and nickel sulfde lire assay gave the best general results. However, it is important to note that lead lire assay provided better recoveries for Au, Pd, and Pt. An improved lead fire assay method for the determination of Au, Pt, and Pd for a wide range of sample types has been described (C7). ICPMS was used and detection limits lower than those obtained with traditional methods were reported. Nugget effects were documented, and results were reported for geological reference materials. Although many dissolution methods employ extremely vigorous schemes of attack, there are always exceptions. Acceptable results for the determination of Mn in rocks, ores, and minerals were reported by using room-temperature decomposition with hydrofluoric acid and aqua regia (C8). Samples were allowed to react for 24 h, and Mn was determined by spectrophotometry. In another application, an ultrasonic bath was used for the digestion of sediment, sludge, and soil (C9). Aqua regia or an aqua regia/ hydrofluoric acid mixture was used, and Cd, Cr, Cu, Mn, Ni, Pb, and Zn were determined by AAS. Acceptable results were obtained for all elements in the sludge sample, for Cd, Cr, and Cu in the sediment, and for Cd, Cu, and Zn in the soil. In the field of coal analysis, a very low-temperature ashing method has been reported (C1O). To decrease the reaction rate, thus preserving the mineral matter to be extracted, the oxygen used in ashing was mixed with helium. Conditions were shown to vary with coal rank. Precipitates are often cited as potential problems in dissolution procedures. These precipitates are seldom identified, and the exact nature of the problem they cause is not often defined. An exception to this is a study of fluoride precipitates and their effect on Sm and Nd determination (C11). Three precipitate phases that fractionate Sm and Nd were identified, and solutions to the precipitate problem were presented. Sample preparation and decomposition techniques for specific methods are found in the appropriate section of this review. ATOMIC ABSORPTION SPECTROMETRY The application of atomic absorption spectrometry to the analysis of geological and inorganic materials continues to generate a tremendous number of publications. A comprehensive inventory of the literature can be found in the biennial listings in Atomic Spectroscopy (01-04, and exhaustive reviews of AAS applications in this area are included in the EnvironmentalAnalysis Updates section of the Journal of Analytical Atomic Spectrometry (05).In addition to these encyclopedic efforts, a mostly historical review of the application of AAS to the analysis of geological materials at the Geological Survey of Canada has been published

(06). In spite of the substantial number of publications devoted to geological and inorganic applications of AAS, new applicationsare relatively rare. This can probably be attributed to the vastness of the existing literature and to the maturity of the theoretical and applied aspects of AAS. Most of the current application work in this area is focused in two general directions. The first of these is devoted to increasing the throughput of the method to take advantage of the relative simplicity, low cost, and high sensitivity of AAS and to make AAS competitive with multielement techniques such as inductively coupled plasma atomic emission spectroscopy (ICP-AES) and ICPMS. The approaches taken include renewed interest in simultaneous,

multielement AAS, applications of solid sampling techniques, standardless analysis of real samples, and modified pretreatment programs in electrothermal atomization graphite furnace atomic absorption spectrometry (ETA-AAS), The second area is represented by some interesting a g proaches to isolating various analytes. These include automated sample preparation steps for separation, concentration, and introduction. In most cases, these methods could be utilized for a number of different instrumentaltechniques and are not unique to AAS. Methods for which the strengths of AAS make it the obvious detector of choice are included in this section. The commercial availability of simultaneous multielement atomic absorption spectrometersmay be a key factor in extending the useful lifetime of AAS. Such an instrument has been used for the determination of Ru, Rh, Pt, Ir, Pd, Ag, and Au in rocks, ores, and other materials by electrothermalatomization (07).The elements Ru, Rh, Pt, and Ir were determined in one firing and Ag, Au, and Pd in a second firing. Although higher atomization temperatures were required to achieve sensitivities comparable to conventional ETA-AAS, results for reference materials compared well with accepted values. Simultaneous multielement electrothermal AAS has also been applied to the determination of Sc, Eu, Dy, Ho, Er, Tm, Yb, Y, Nd, and Sm in rocks (08). After dissolution and concentrationit was possible to determine Y, Nd, and Sm in the low microgram per gram range and Sc, Eu, Dy, Ho, Er, Tm, and Yb in the low nanogram per gram range. Multiple determinationswere made on single firings, and results were in general agreement with ICP-AES and ICPMS data. Direct analysis of solid materials continues to be an area of active application development. This approach is being used in both flame and electrothermalatomization AAS with an emphasis on introducing the sample as a slurry. Slurry sampling coupled with flow injection has been applied to the flame AAS determination of Fe, Ca, and Mg in silica-based materials (09). A variablevolume chamber allowing on-line dilution of the slurry has been described, and precision in the 1-5% range was reported for diatomaceous earth samples. A fast temperature program has been used in conjunction with slurry sample introduction for ETAAAS determination of Cd, Zn, and Mn in silica-based materials (010). Aqueous standards and conditions optimized for diatomaceous earth were shown to produce acceptable results for reference materials. Slurry sampling for ETA-AAS has also been described for the determination of lead in marine sediments (011). The effects of various suspension agents and particle sizes were investigated, and a precision of 5% was reported. Results for samples prepared by using an acid digestion were not significantly different from those obtained by using the slurry method. A method utilizing ~ WHOin Ar demonstrated significant background reduction in the analysis of soil and sewage sludge by using ETAAAS and slurry sampling (012). This approach resulted in correctablebackground signals and results for certified reference materials that were generally acceptable. Standardless analysis is another approach to making AAS a universal method. Because improvements in fundamental data are critical for real-life applications in this area, developments tend to come in bursts and are often based on the analysis of synthetic samples. Several investigations of the effects of atomization temperature on characteristic mass and atomic absorption coefficients, with results for real samples, have been published. These include studies on the determination of Ag and Cd (013), In

(014,and Cr (015)in sediments and other geological samples.

Results were generally in agreement with published data. The excellent sensitivity of ETA-AAS for many elements has made it the method of choice for many types of geological and inorganic analyses. One weak point of this approach has been analysis times of 2-3 min per determination. To overcome this shortcoming, various fast furnace protocols continue to be investigated. The application of fast furnace methods to the determination of geologically important elements including As, Pb, Se, Tl, Cd, Cu, Cr, and V indicated that procedures can be completed in less than 1min per determination with acceptable results (016). Although fast furnace programs may seem to be an easy path to increased throughput, they do not address the real-world problems of multiple matrices and matrix modifiers. Fast furnace programs have been compared with traditional ETAAAS methods to demonstrate the application of this approach to a range of matrices (017). This study included multiple instruments and considered the role of matrix modifiers in fast furnace programs. The determination of vapor- or hydride-forming elements has probably generated more publications than any other subject in the field of AAS. For the most part, these experiments are applicable to several different detection methods, but the determination of Se and Hg seem to remain the domain of AAS. A comparison of fluorometry, hydride generation AAS, hydride generation ICP-AES, hydride generation ICPMS, and radiochemical neutron activation analysis for the determination of Se indicates that AAS remains the routine method of choice for all but the lowest Se levels (018). Cold-vapor AAS also seems to remain the method of choice for the determination of Hg in a wide variety of sample types, in spite of interferences in complex matrices. In one recent example, this method coupled with flow injection has been applied to the determination of trace Hg in zinc ore (019). A detection limit of 0.08 pg/g was reported, and results for Hg in CZN-1 agreed with certified values. In addition to these traditional gas-phase AAS determinations, a method for the determination of Cd employing a continuous-flow vapor generation system and atomic fluorescence spectrometry has been described (020).This method was successfully applied to the determination of Cd in sewage sludge and water with a detection limit of 20 ng/L. Whereas the last application is not an AAS application in the strictest sense, it is an example of the kind of incremental development that will allow AAS to retain a role as an important, modem analytical tool in the field of geological and inorganic analysis. PLASMAS ICP=AESAND ICPMS

Because of their high excitation temperatures, plasma sources (such as inductively coupled, direct current, microwave, and glow discharge) are popular atomization and excitation devices for the analysis of geological and inorganic samples. In this section we will limit our discussions to the two most popular versions of plasma spectroscopy in use in the field of geoanalysis, namely, ICP-AES and ICPMS. Space limitations prevent us from including an exhaustive review of all work published in this field. Therefore, the ZCP Information Newsletter (R. M. Barnes, Editor) is a particularly useful periodical for plasma spectroscopists-it abstracts papers presented at national and international meetings, presents special reports, and provides an annual bibliography in the January issue. Analytical Chemistry, Vol. 67, No. 12,June 15, 1995

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A comprehensive review (El) of the application of atomic spectroscopy to the field of environmental analysis contains 997 references and has major sections devoted to the analysis of soils, plants, and geologic materials using various plasma spectrometric techniques. It is well-known that the one of the major factors influencing accuracy in the analysis of geologic materials is the efficiency of solid sample dissolution. Incomplete digestion of acid-resistant mineral phases and the loss of volatile species during sample preparation are two perennial problems that face geoanalysts. One way of circumventing sample dissolution problems is to volatilize the elements of interest directly from a solid sample by using a high-temperature reaction chamber interfaced to an ICP-AES instrument. Such a device was used, in combination with Freon12 as a halogenation reagent, to determine Cu, Mn, Zn, Al, and Fe in marine sediment and coal fly ash samples (E2).Another way of avoiding sample dissolution problems is to introduce the sample in the form of a slurry. However, it has been found that slurries (in the absence of dispersing agents) do not behave like aqueous solutions even when particle sizes are reduced to ~ 1 . 5 pm (E3).Quantitative analyses are possible if correction factors are determined daily by using matrix-matched reference materials. Improving the sensitivity of ICP-AES analyses via the generation of volatile species of the element of interest was the subject of two reports. The first report ( E 4 describes a method for efficiently generating plumbane (FbH4) using a simultaneous combination of dichromate and lactic acid to achieve a Pb detection limit of 1pg/g in sediment samples. The second report (E5) is an extension of Nakahara’s earlier work on the determination of low concentrations of iodine in brine samples. The method is based upon the generation of volatile iodine by the oxidation of I- by using NaNOz and sulfuric acid and then sweeping the vapors into the argon stream of an ICP-AES. The detection limit of the method was reported to be 0.39 ng of I/mL, which is 5 times lower than previous achievements utilizing microwave plasma spectroscopy. Two new separation and preconcentration methods for ICPAES determinationshave appeared since our last review. A new chelating resin, containing pyrocatechol violet as a functional group, has been used for the quantitative separation and determination of Ti, Zr, and Ga in geochemical samples (E@. The separation of Au from copper concentrates by solvent extraction permitted the subsequent ICP-AES determination of Au down to 0.1 pg/g (ET). The field of ICPMS has continued to mature to the point where it is now in the final stages of receiving EPA approval as SW846 method 6020 for the determination of Ag,Al, As, Ba, Be, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, TI,and Zn in liquid and solid wastes. A review (ES)on the application of ICPMS to geoanalysis contains a section on geological applications which includes isotope ratio measurements, matrix elimination, and speciation studies. Another popular approach to circumventing problems associated with the dissolution of solid samples is to use laser ablation as a sampling device for ICPMS measurements. The problem then becomes one of preparing representative and homogeneous sample surfaces that will couple with a pulsed laser beam operating at a wavelength of -lo00 nm. Fusion of a powdered sample with a lithium tetraborate flux was the subject of a recent article (E9)which concluded that there is no apparent matrix effect when this technique is used to prepare samples for laser 74R

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ablation ICPMS determinations of trace elements in silicate matrices. However, in our experience, matrix independence usiig fused samples is evident only when laser ablation is performed with the laser in a Q-switched (high power density) mode. A popular sample preparation method for laser ablation work is to mix the powdered sample with a binder and then press a pellet. This pressed pellet technique was the subject of two reports (El0,Ell) which investigated the use of both external calibration and internal standardization for the determination of REE in soils and silicate rocks. Internal standardization using 43Ca (E12) also has been shown to be an effective way of determining Sr, Y, Ba, and REE in carbonates using non-matrixmatched standards for calibration. The accurate determination of light REE (La, Pr) is hampered by the presence of oxychloride ions, specifically CaC104+,according to Longerich (El3),who found enhancements in several calcium and magnesium oxychloride ions in HC104 solutions compared to equimolar solutions of HCl. Furthermore, these interferencescannot be corrected using blank subtraction strategies because blanks typically do not contain either Ca or Mg. The attenuation of polyatomic oxide interferences can also be accomplished by the addition of hydrogen to the nebulizer gas in ICPMS (E14)-a feat that was achieved by earlier workers using nitrogen gas addition or spray chamber cooling procedures. In the early 1800s, dry chlorination was used to prepare samples for the determination of PGE. This technique, which involves subjecting heated samples to a stream of chlorine gas, has been modified over the last several years by Perry et al. (El$, who developed a method for handling large samples (250 g) for the ICPMS determination of PGE in rocks, clays, and sands. Currently, a widely used method for the determination of PGE is nickel sulfide fire assay followed by ICPMS. A slightly modified version of this technique has been reported (El6) to yield improved recoveries of PGE from geological samples. Of the PGE, Os is the most d ~ c u l to t determine because of its low natural abundance, because of its relatively poor recovery by NiS (-65%), and because Os04 is highly volatile. A slurry nebulization method (El 7) has been developed for PGE determinations by ICPMS which exhibits excellent accuracy for Os. The only shortcoming of this slurry method is its poor recovery of Pt,which could be caused by the presence of intermetallic Pt species that are both refractory and acid resistant. The traditional Pb f i e assay procedure has been combined with ICPMS (El8)in an attempt to achieve subnanogramper gram detection limits for Au, Pt, and Pd. The authors’ goals were achieved for Pt and Pd (0.1 and 0.5 ng/g detection limits, respectively), but they were only able to obtain a detection limit of 2 ng/g for Au because of problems with flux contamination. The advantages of using flow injection as a sample introduction system for ICPMS was first demonstrated in 1988for the analysis of biological fluids. A pair of recent reports (El9,E20) demonstrate that flow injection can be used to minimize matrix effects in samples containing high total dissolved solids such as brines and fusion preparations. Detection limits using flow injection are 3-9 times better than those for continuous-flowsample aspiration of the same matrix. The topic of chromium speciation analysis is always of great interest because of C r O toxicity. Chromium speciation analysis by ICPMS using hydraulic high-pressure nebulization and ion-

pair chromatography to separate and determine both CrgII) and C r O was reported (E21). The determination of As and Se by ICPMS is complicated because both 75Asand 77Sesuffer from isobaric interferences due to ArCI. One way of overcoming this limitation used selenium hydride generation in combination with a tubular membrane gas/ liquid separator to minimize entrainment of chloride aerosols (E22). Another approach that has been used to eliminate chlorine and sulfur interferences in ICPMS is anion-exchange chromatography (E23). Using this technique, it is possible to simultaneously determine V, Cr, Cu, Zn, As, and Se in soils and other geological materials. Group separation of Nb, Ta, W, and Mo from silicate matrices by using N-benzoyl-N-phenylhyroxlamine in CHCl3 has made possible the accurate determinations of submicrogram per gram concentrations of Nb, Ta, and W in HF digests of rock samples (E24). The solvent extraction using cyclohexane and ICPMS quantitation of 99Tc is the subject of two reports (E25, E26) that focus on the minimization of the isobaric interference from 99Ru. MASS SPECTROMETRY

Over the past several decades, the technique of thermal ionization mass spectrometry has been used by geoscientists for the precise measurement of both radiogenic and stable isotope abundances in their quest to characterize terrestrial and extraterrestrial samples. Recently, a novel approach to the in situ determination of oxygen isotopes was published by Sharp (FI), who described a laser-assisted fluorination technique that was used to analyze silicate minerals. During the past two years, several papers have been published (F2-F5) that describe various modifications to Sharp’s technique and extend it to the analysis of sulfur isotopes in sulflde minerals. One problem with conventional ion-exchange procedures for the separation of Sr and Rb in rock samples is that if the content of Rb is high, it is difficult to obtain a Rbfree Sr fraction for isotopic analysis. An improved sample preparation scheme ( F a for Rb and Sr uses HPLC in combination with ion exchange to achieve favorable results for Rb/Sr ratios as high as 3001. A new generation of extraction chromatographic resins, comprised of various crown ethers, has been developed by Horwitz et al. These materials have been applied to the isolation of Pb (F7) and Sr (FS)from geological samples for subsequent mass spectrometric isotope determinations. Other workers have used these crown ether resins to perform efficient Ba/Ra separations for measuring Ra isotopes in volcanic rocks (F9). Negative thermal ionization mass spectrometry (NTIMS) is renowned for its enhanced sensitivity relative to positive thermal ionization mass spectrometry. Isotopic analysis by NTIMS was developed by Heumann and co-workers, who recently published an article (FlO) on the determination of Re isotope ratios using a V20j-coated Ni filament to reduce the Re blank. With this technique, they were able to measure 1a5Re/1s7Reratios with a relative precision of 0.04%. NTIMS was also used for the simple and precise determination of B isotopes in carbonates and natural waters (F11) with an uncertainty of 0.07%, which is a 5fold improvement in analytical precision compared to other negative ion techniques. A new Zn reagent method for the quantitative conversion of HzO to Hz for the hydrogen isotopic analysis of hydrous minerals and whole rocks has been developed (F12). Analyses by this

method are of an accuracy comparable to those using the conventional U reagent extraction method, and they are simpler, faster, and less expensive. The method is also applicable to the quantitative determination of water in minerals. A mass spectrometric method for the determination of cosmic ray-produced 21Nein terrestrial rocks (F13) has implications for studies involving surface exposure ages, erosion rates, and extent of glacial cover. To a great degree, advances in isotope geochemistry have paralleled advances in the capability of analyzing smaller and smaller samples. Therefore, techniques suitable for high-resolution microanalysis such as secondary ion mass spectrometry (SIMS) and accelerator mass spectrometry have become increasingly important in the earth sciences. Because of the lack of adequate theoretical models, quantitation in SIMS is mainly empirical, involving comparisons between elemental intensity ratios in the sample and a standard of similar composition. A move toward standardless quantitative analysis requires modeling of ion yield behavior and the ability to predict yields when no standard is available. Recent advances in the determination of SIMS relative sensitivity factors for Ru, Rh, Pr, Eu, Tm, Lu, Re, Os, and Ir have been made (F14) through the successful implantation of these ions into Si and GaAs. Particle accelerators interfaced to mass analyzers are capable of measuring isotopes at very low abundances in small samples. This technique, commonly labeled accelerator mass spectrometry (AMs),has been used to study long-lived isotopes such as ‘OBe, 14C,26Al, 36Cl,41Ca,and Iz9I. It has recently been used for the determination of 59Niin meteorites (F15)and lunar materials (F16). AMs is the subject of several recent reviews (F17-FZO) which point out that further developments may allow the measurement of 32Si,53Mn,and 6oTh in geological samples. The technique of resonance ionization mass spectrometry (RIMS) uses tuned laser beams to selectively photoionize specitic species of interest (even isotopes) in a vapor cloud. Recent progress in the application of RIMS to geochronological studies is discussed in a review (F21).Laser-induced isotopic effects in the measurement of Ti isotopes were reported (F22), and similar biases were observed in the measurement of Th isotopes (F23). X=RAYSPECTROMETRY

The application of X-ray spectrometry to the analysis of geological and inorganic materials encompasses a wide range of methods and techniques. Applications include various XRF techniques for quantitative elemental studies and studies of oxidation states in a wide range of materials using X-ray absorption spectroscopy (XAS) or X-ray photoelectron spectroscopy (XI’S). X-ray diffraction techniques are also briefly discussed in this section, although it is not technically a form of X-ray spectrometry. A wide range of analytical applications was covered in a recent text on analytical X-ray emission spectrometry (GI). This comprehensive handbook includes sections on X-ray techniques, instrumentation, applications for numerous sample types, quantitication procedures, and sample preparation methods. Sample preparation for energy-dispersiveX-ray fluorescence (EDXRF) in particular has also been reviewed (G2). Samplingand preparation of heterogeneous materials such as soils and geologic materials are emphasized. Methods for preconcentration and separation of analytes for determination by X-ray spectrometry were also discussed. A review of microscopic X-ray fluorescence has also Analytical Chemistry, Vol. 67,No. 12, June 15, 1995

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been completed (G3). Both tube and synchrotron radiation (SR) X-ray sources are included in the review, with an emphasis on the analysis of geologic materials. In a comparison of EDXRF and wavelength-dispersive X-ray fluorescence spectrometry 0 for the determination of Rb and Sr in silicate rocks, similar precision was found for the two methods below 20 pg/g, but WDXRF was found to be more precise between 50 and 100 pg/g (0.The precision of the EDXRF determinations could not be improved by longer counting times because of the low-power source used in this study, and no benefit was found in the simultaneous determination of Rb and Sr by using EDXRF. The method of standard additions for the determination of lead in sediments by XRF has been shown to overcome matrix and background problems (GS). Aqueous spikes of a sediment matrix were used for calibration, and results compared favorably with data obtained by using EPA methods. Although encouraging results are also reported for the determination of Zn, Br, and Cd with this technique, shielding problems make this approach impractical for most elemental determinations. A simple approach to the elimination of matrix effects, the use of a high flux-to-sample ratio (1:10), has been shown to be effective in the analysis of sulfide and Fe-Mn ores by XRF (G6). Acceptable results were obtained for reference materials. XRF has been used to determine trace levels of REE in geological samples after anion-exchange separation of the REE and precipitation of the REE as thin films on a membrane filter (G7). Results were compared to certified values, and data for inductively coupled plasma optical emission spectrometry was also presented. The determination of REE by EDXRF using a fundamental parameters method has also been reported (G8). Data for 11REE and Th were calculated and precision in the range of 2-2% was reported. Detection limits that were in the 300 pg/g range for the heavy REE were 1 order of magnitude higher for the light REE. A fundamental parameters method using both EDXRF and WDXRF information has also been described (G9). Geologic reference materials were used to test the method, and improved results for low atomic number elements were reported. A theoretical approach to background measurement has been applied to the determination of Ce, La, Ba, and Cs in geologic samples (G10). A formula defining the continuum as a function of wavelength was fitted to background measurements on either side of the peak of interest, and the background under the peak was calculated. A semiempirical calibration method for XRF analysis has been applied to various sample types, including sediment and soil standards (G11).Results were compared to multiple regression methods for the determination of all elements in the sample and for the determination of selected elements. A semiempirical procedure has been used to correct for transparency effects in the determination of Sn by WDXRF (G12). A detection limit of 4 pg/g was obtained by using a rhodium tube. A method for determining sample mass thickness and photoelectron cross section, by using incoherent and coherent radiation, has been applied to the multielement analysis of coal fly ash (G13). Unweighed samples were prepared as relatively low density pressed material, and 16 elements were determined from a 1OO@s count. In the area of non-SR X-ray microfluorescence 0 , elemental mapping continues to be the focus. The application of 76R

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this method to large-area mapping of a rock thin section and a multiphase alloy sample has been described (G14). Twodimensional X-ray maps as large as 50 mm by 50 mm were possible, and principal component analysis was used for develop ing multielement maps. A capillary-basedXRMF system with 3@ pm resolution was used for both microanalysis and mapping of geologic materials, particles, and glass disks (G15). Total reflection X-ray fluorescence continues to develop as a powerful analytical tool. It has been applied to the multielement analysis of fractions separated from sediments (G16). Application of this method to various sample types and details of the method, theory, and instrumentation have also been discussed (G17). As new synchrotron radiation facilities come on line, X-ray spectrometry employing this focused, high-intensity source is applied to more and more analytical problems. Applications of SR-XRF and X-ray absorption near-edge spectrometry (XANES) to elemental imaging and oxidation-state studies of meteorites, art objects, and other sample types have been reviewed (G18). A variety of SR applications for geologic and other materials have also been described including the determination of REE by SRWDXRF and the study of Cr oxidation state in soils and minerals by using XANES (G19). SR-XRFhas been used for the determination of REE in mineral grains and synthetic glass samples (G20). The study demonstrates the superiority of the wiggler source for this type of analysis, and favorable results were reported for comparisons with data from other analytical techniques. Similar techniques have been applied to the determination of TI,W, Ta, Nb, and Au in lepidolites (G21). Excitation conditions and data processing for the application were also described. SR-XRFanalysis of individual fluid inclusions in quartz has also been reported (G22). For synthetic fluid inclusions that were of known composition, results were normally within 30% of the nominal ratio. X-ray photoelectron spectroscopy continues to be developed as an analytical tool for the surface analysis of geologic materials. The application of XPS to the study of a variety of problems related to soil surfaces has been reviewed ((223). Analyses related to speciation, oxidation states, and exchange sites on various surfaces were discussed. XPS applications for the study of oxide minerals and aluminosilicateshave also been reviewed (G24). This review included a discussion of instrumentation and theory as well as applications to clay mineralogy. X-ray diffraction (XRD) is also an important technique for analysis of geologic and inorganic materials. New applications continue to be developed in the areas of mineral determination, quantification, and automated procedures. Quantitative phase analysis and standardless XRD are among the topics covered in a recent NIST publication (G25, G26). A variety of XRD applications are presented in a special issue of Analytica Chimica Acta devoted to the measurement of crystalline silica (G27). An XRD method for the determination of thickness, composition, and mass absorption of a sample deposited on membrane filters has been described (G28). Accuracies of 3%, 1%,and 4% were reported for the determination of mass thickness, mass absorption, and quantitative mineralogy, respectively. A number of artificial intelligence or “expert” systems for XRD analysis have also been described (G29-G31).

ELECTRON MICROBEAM TECHNIQUES Electron probe microanalysis (EPMA) has wide usage in the geosciences. One area that is experiencing a lot of attention is the characterization of individual particles for various environmental studies. Applications of EPMA and several other microbeam techniques to the analysis of aerosols have been reviewed (HI, H2).In order to identify particles, numerous multivariate techniques are used, such as a nonlinear mapping technique that has been applied to the interpretation of EPMA spectra of mine dust and lacustrine aerosol samples (H3, H4). EPMA and automated image analysis have been used to quantitatively determine iron sulfides in coal ( H a . There is an increasing use of EPMA to characterize the source of elements and their bioavailability in soils at residential, industrial, and mine sites. Because the application of this technique to the evaluation of Pb in soils is particularly important in health risk assessments, but standardized methodology does not exist, an interlaboratory comparison was performed using two mine wastecontaining soils ( H a . Although EPMA results were generally comparable between the four laboratories that participated, more interlaboratory variability was observed for samples with complex multiplephase associates. It was also suggested that standard nomenclature for soil particles and phase associations be adopted. Numerous problems exist in EPMA analysis, such as movement of elements because of the electron beam. Because this mobility significantly effects EPMA results, the anisotropic diffusion of F and C1 in apatite has been quantified (H7). The mounting of EPMA specimens does not always permit subsequent analyses such as for 40Ar/39Arlaser microprobe dating. Thus, an alumina ceramic mounting technique was developed for sequential analysis and dating of minerals (H8). A new volume in the Reviews in Mineralogy series focuses on the application of transmission electron microscopy to mineral studies (H9). PARTICLE4NDUCED X-RAY AND PRAY EMISSION Particleinduced X-ray emission (PIXE) is a method capable of providing data on major, minor, and trace elements in geological materials. It is used for bulk analysis in either a thin- or a thicktarget mode and, more recently, as a microanalytical technique 01-PIXE). Particle-induced y-ray emission (PIGE) is similar in practice to PIXE except that photons arising from nuclear radiations are measured, rather than X-rays. PIGE is used primarily for the determination of light elements. A laboratory that does one type of analysis frequently does the other, and therefore, we have grouped them together in this section. In most cases, samples are bombarded by protons in an accelerator, causing ionization and subsequent emission of characteristic X-rays (PIXE) or nuclear transformation and the emission of characteristic prays (PIGE). pPIXE, also know as the proton microprobe, is similar to EPMA except that protons are used instead of electrons. Although the application of PIXE and PIGE to geological samples has been growing, the techniques are not as widely applied to geochemical studies as the other analytical techniques described here because of the need for an accelerator facility. PEE. Two papers have reviewed the application of p-PIXE to problems in geology and mineralogy (11,12). A third review treats the application of high-energy (40-60 MeV) proton beams to the analysis of geological samples by PIXE, PIGE, and charged

particle activation analysis (13). The proceedings of two 1992 conferences on PRE/PIGE have been published: Third International Conference on Nuclear Microprobe Technology and Ap plications, Sweden (14) and S i International Conference on P E E and Its Applications, Japan (15). Both proceedings contain a number of papers on geochemical applications, particularly applications of the proton microprobe,but are not reviewed here. ,u-PIXE has been applied to the determination of major elements in mineral specimens by using 75GkeV protons (16)and major and trace elements in magmatic inclusions (17). The latter paper describes modifications to the GUPM software for the analysis of PIXE data. p-PIXE has been applied to the analysis of silicate reference standards to demonstrate the feasibility of using the technique to study trace-element zoning and diffusion in petrogenetic studies of intermediate to silicic igneous rocks (Is). PME has also been applied to the determination of solid/liquid partition coefficients for Ni, Cu, Zn, Rb, Sr, Y, Zr, and Nb between plagioclase and pyroxene phenocrysts and the associated ground mass in calc-alkaline dacites (19). p-PIXE has been used to study the partitioning of several trace elements between experimentally produced amphibole and silicate melts having variable F content (120). Ionoluminescence has been combined with p-PIXE and applied to the microcharacterization of geological materials (Ill). p-PIXE and p-Rutherford backscattering have been used to study the distribution of trace elements in PGErich mineral grains (122). Modifications of a proton microprobe facility to aid the analysis of geological samples have been described (123). A comparative study of instrumental neutron activation analysis and PIXE for the determination of heavy metals in estuarine sediments has been reported (114). PIGE. PIGE and PIXE have been applied to the analysis of Nigerian tar sands, PIGE was applied to the determination of Si, S, N, C, and 0,whereas PIXE was used to determine Si, S, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, and Pb (115). B, B isotopes, and Li have been determined in tourmaline samples by using a proton microprobe (116).A nuclear microprobe using 1.8MeV deuterons has been applied to the determination of nitrogen in minerals (117).The detection limit was between 10 and lOOpg/g. LiCO3 has been used as a nonanalyte spike in order to correct for the relative stopping power of 5MeV protons between samples and standardsfor the determination of Si in chromite ore samples (118) and F in phosphate rocks (119). The potential application of pPIGE for the determination of isotope ratios of Li,B, C, N, 0, Mg, Si, and S in various mineral phases has been evaluated (120). NUCLEAR ACTIVATION METHODS Nuclear activation analytical methods are all based on the transmutation of stable nuclei into radioactive states, which then decay emitting characteristic radiation. The goal of most nuclear activation analysis as applied to geological materials is the measurement of elemental concentrations, although it is sometimes used for determining isotopic ratios. The scope of this section includes studies of most geologic materials but excludes studies relating to the measurement of natural and cosmogenic radionuclides, or environmental contamination from nuclear explo sions or by the accidental release of byproducts from the nuclear power industry. In addition to the computerized abstract search, the geochemical journals, Geochimica et Cosmochimica Acta,

Chemical Geology, Earth and Planetary Science Letters, Contributions to Mineralogy and Petrology, and Geostandards Newsletter; the Analytical Chemistry, Vol. 67, No. 12,June 15, 1995

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analytical chemistry journals AnaEytical Chemistry, Talanta, and Analytica Chimica Acta; and the nuclear analyticaljournalsJournal of Radioanalytical and Nuclear Chemisty, Nuclear Instruments and Methods in Physics Research, and Nuclear Geophysicswere searched manually in preparing this section. The most common method of nuclear activation involves the irradiation of the sample by neutrons followed by y- (and sometimes B-) ray spectrometry to measure the induced activity (generally called neutron activation analysis, NAA). Most often the samples are irradiated by thermal neutrons in a reactor followed by direct counting of the induced y activity (referred to as instrumental neutron activation analysis, INAA). In some applications, the samples are irradiated with high-energy neutrons in order to preferentially activate nuclei with high cross sections for epithermal neutrons (ENAA) , thereby improving sensitivity for certain elements, or to take advantage of reactions with fast neutrons (FNAA) when convenient thermal neutron activation products do not exist. Detection limits can often be improved with a chemical purification of the element of interest either before irradiation (chemical separation, CSNAA) or after irradiation (radiochemical, RNAA) . Other nuclear activation methods are also applied in geochemical analysis, although their use is more limited because of the need for access to an accelerator or an on-site reactor. Th and U can be determined by counting delayed neutrons (DNAA) after a short reactor irradiation. Nuclear activation, again followed by measurement of induced activity, may be accomplished by irradiating samples with energetic photons (PAA) or charged particles (CPAA). In some applications, elemental concentrations are determined by measuring prompt y-rays (photons emitted in the nuclear reactions) during neutron irradiation (PGNAA) or during particle bombardment (PIGE). INAA and ENAA. Instrumental neutron activation analysis remains an important tool for the routine analysis of geological samples as indicated by the large number of geochemical papers in the literature that report INAA trace element data. We estimate that -40% of the papers that report trace element data in Geochimica et Cosmochimica Acta, Earth and Planetay Science Letters, Chemical Geology, and Contributions to Mineralogy and Petrology report data obtained by neutron activation. It remains the most commonly used method for trace element analysis, followed by ICPMS. Elements that are of particular interest in petrological studies, for which INAA is a preferred analytical method, include Cs, most of the REE, Hf,Ta, Th, and U. Routine applications in the recent literature include many studies of igneous rocks, ores, mantle and crustal xenoliths, meteorites, lunar rocks, sediments, soils, and coal. The papers referenced in this review are those papers that gave more than the minimum detail regarding analytical methodology or that represent an application to an unusual sample matrix. INAA has been evaluated for the analysis of soils, and the quality of the data for various elements was placed in four categoriesranging from “very good” to “unsatisfactory”VI). The relative advantages of thermal, epithermal, and Compton suppression counting for the INAA analysis of soils have also been evaluated VZ). The relative advantages and disadvantages of INAA and ICPMS for the determination of trace elements in crude oils, oil fractions, and source rock bitumens have been reported v3). An INAA method has been developed for the analysis of large samples of heavy metal concentrates and stream sediment 78R

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samples of interest for mineral resource assessment by using a “slow-poke 2” reactor (14). Fission product interferences on the determination of Zr, Mo, Ba, La, Ce, and Nd have been reevaluated US,J6) for thermal and epithermal activation. Techniques for the handling of small samples (40- 100 yg) have been applied to the analysis of green-glassparticles from lunar soils and breccias

(27). Recent developments of the k, method have been reviewed (18), and the application of the method to the measurement of

elements having short-lived indicator radionuclides has been described v9, 110). The method has been applied to the determination of 32 elements in sediment reference standards (111).

A rapid INAA method for the determination of F using the 11.0s activity 20F has been developed (112). Activation with epithermal neutrons and coincidence spectrometry have been applied to the determination Ir and Se in sediments at the K-T boundary v13). A method using separate thermal and epithermal irradiations and measurement of the 2.25min “Al activity has been applied to the determination of Al and Si in desert aerosols (114). The use of 252Cfas an irradiation source for borehole cores, followed by scanning the cores with a Ge(Li) detector, has been evaluated for the determination of Au (115). An automated system for the analysis of large (160-300 g) samples of gold ore has been described with a sensitivity for Au of 0.01 yg/g The underestimation of the gold content of ores due to self-shielding during epithermal neutron irradiation has been measured 7). A 33% reduction was observed in a sample with particle size in the range of 150-250 ym. A method has been developed for the determination of In and Sn in cassiterite which may be of archaeological interest for potential identification of ore sources for tin/bronze artifacts V I S ) . Radiography following neutron activation has been used to study the distribution of Au, Ag, and other elements in mineral and ore samples (/IS),as well as for Al, Mn, and Na in rocks (1.0) .

FNAA. Fast neutron reactions initiated with isotopic neutron sources (Am-Be or Pu-Be) have been used to measure F concentrations in drill core samples by measuring the induced I6N activity (J21) and for the determination of F, Si, and Ba in fluorites and barites (f22). Cyclic activation analysis with 14MeV neutrons has been applied to the determination of eight major and minor elements in four rock standards (123). DNAA. DNAA is a well-established technique for the nondestructive determination of U and Th that involves counting “delayed neutrons” from short-lived (0.2-60-s half-life) fission products. An automated system (250 samples/day capacity) for the analysis of large (up to 30 mL) environmental samples has been described c124). PGNAA. A pair of recent papers describe the potential application of the NIST cold neutron source for PGNAA, including count rate data for 14 elements in the Allende chondrite standard (/25) and the determination of H at the microgram level in Si and quartz (126). Another recent paper reviewed elemental analysis through the measurement of prompt y radiation produced by inelastic scattering of accelerator-produced neutrons ‘$3‘). PGNAA and the related method of monitoring y-rays following neutron inelastic scattering are finding increased application for in situ analysis of rocks and coal in bore holes, and in process control in the coal and minerals industries using neutron sources,

principally using z5zCfas the neutron source. The in situ analysis of minerals and coal using nuclear methods including PGNAA with isotopic neutron sources has been reviewed with 86 references cited (128). Several papers that deal with the evaluation of coal quality through bore-hole logging and the use of on-line PGNAA methods during coal processing appeared during the review period (129-133). Additional applications of PGNAA using isotopic neutron sources include the determination of Fe, Al, Si, and Ti in rock samples (134),the determination of 23 elements in hematite and 11 elements in phosphate ore (133, and the determination of lime, silica, alumina, and FeO in cement raw materials (136). PGNAA well-logging data have been compared with laboratory analysis of the core material for nine elements (137). The impact of statistical uncertainties of elemental concentration on the interpretation of geochemical well logs has been evaluated (138). Monte Carlo methods have been described for modeling neutron and photon transport during well logging (139) and the analysis of neutron capture pray spectra from a spectrum library (140). CSNAA. Fire assay preconcentration of the PGE and Au by using NiS as the collector remains an important tool for the geochemical determination for those elements (141,142). The method has been applied to the analysis of kimberlites; alternative flux compositions were recommended for the analysis of peridotites and high-carbonate samples (142). A separation procedure that involved the collection of Pd, Ir, Pt, and Au on polyaniline at low acidities, followed by elution of Pt, Pd, and Au by using thiourea and Ir with ascorbic acid followed by 10 M HCl, has been described (143). For the determination of U, a preirradiation separation procedure into 4-(5nonyl)pyridine/benzene has been reported (144). Activated charcoal has been used as a collector for the preirradiation separation of I that is released by heating from soil samples (145) and as an absorbent for trace elements released in volcanic emissions (146). RNAA. As noted in the previous review period, much of the work on RNAA methods as applied to geochemical studies is directed toward developing improved methods for the determination of the PGE. An RNAA method has been developed for the determination of Pd, Pt, Ir, Au, Ag, Se, As, and Sb by coprecipitation in metallic Te by reduction with SnClz (147). RNAA methods for the determination of Pt in environmental and geological samples via I*Au (148) and Au, Ag, Pt, and Pd (149) have been described. RNAA methods have also been presented for the determination of Ir (150) and Ir and Os (151) in K-T boundary sediments. A method for the determination of Re, Os, and Ir in chondritic meteorites involved the separation of Os by distillation and Re and Ir by anion-exchange chromatography (152). A group separation scheme for the determination of 25 elements by using ion-exchange techniques and tetracycline as a complexing agent for removing interferences (153) has been described. Iridium was determined by removing the major activities with AG 50 W x 8 cation resin. Pd, Ir, and Pt have been determined in ultramafic rocks by extraction with DDTC and DBDTO (154). In the same work, Cs and Na were determined by recovery with tetraphenyl borate, REE by cation exchange, and U and Th by TOP0 extraction. An RNAA method was developed for the determination of the REE in monazites that involved their separation into three groups by selective elution from AG 1-X8 resin (155). Microfire assay methods that used lead, tin, or iron as collectors for the

radiochemical separation of various metals, including the PGE, have been reviewed (156'). An RNM procedure for the REE by using multiple precipitations as the fluoride has been developed and applied to the analysis of the GOG1 gabbro (157). RNAA methods for the determination of REE (158) and C1, Br, and I (159) have been applied to the analysis of 15 igneous rock reference samples prepared by the Geological Survey of Japan. As, Sb, Se, and Mo have been determined by coprecipitation with Bi& (160). A method for the determination of 196Hg/202Hg ratios in meteorites has been reported in which Hg is released by stepwise heating (161). A rapid method has been devised for the determination of In, Zn, and Cu in chondritic meteorites and standard rocks (162). CPAA. This technique finds almost no application for routine geochemical analysis. A method for the determination of Pt in chromite sulfide ores by a-particle bombardment (163)was noted during this review period. A microactivation technique for the determination of 0 in diamonds has been developed using a beam of 3He2+from a tandem accelerator (164). e+ annihilation radiation was measured by NaI counting. I*F activity was separated from other e+ emitters by decay curve analysis. PAA. PAA also finds few routine applications in geochemical research. A method has been reported for the determination of N in silicates (165). A rapid radiochemical separation of the 9.97min N13 activity used prior to NaI y counting of the annihilation radiation. PAA has also been applied to the determination of up to 24 elements in Cu-Mo ore samples by using microtron bremsstrahlung radiation (166). Miscellaneous Methods. Plastic fission particle track detectors that are packaged along with the analytical sample for neutron irradiation have been used to measure the U content in bulk samples of coal and rock following irradiation using both reactor (167) and isotopic (168) neutron sources. MISCELLANEOUS SPECTROSCOPIC METHODS Classical spectrophotometricmethods of analysis continue to be used routinely in many laboratories and at field sites around the world. During a two-year period, the reports on spectrophotometric methods for geoanalysis typically number several hundred. However, we have not reviewed them for this article because they generally focus on applications of new indicators and/or minor mod~cationsof analysis conditions for specific sample types. Many other spectroscopictechniques that are not covered elsewhere in this review are primarily used for mineral identification and structural studies. Therefore, we only briefly review recent applications of such techniques as Mossbauer, Raman, and nuclear magnetic resonance spectroscopy (NMR) below. A new book on Mossbauer spectroscopy discusses both theory and applications of the technique for geochemists (K1).It also touches on environmental and archeology applications. Although more costly, the advantages of high-field Mossbauer analysis of iron-containing phases have been discussed (K2). For garnets with mixed-valenceiron states, recoil-free fraction ratios have been obtained leading to a better procedure for the estimation of the Fe III/II ratio (K3). Ig7AuMossbauer spectroscopy was used to study Au adsorption on As and Sb sulfides and indicated that reduced, elemental Au was not formed on the metal sulfide surfaces (K4). Analytical Chemistry, Vol. 67, No. 12, June 15, 1995

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Laser Raman microprobe has proved a valuable tool for studying 5uid inclusions. In an exchange of comments, numerous points regarding the determination of 5uid compositions have been discussed and guidelines for documenting Raman analyses have been proposed (K5, K6). To aid the analysis of 5uid inclusions, Raman spectral parameters for binary mixtures of CHI and Nz have been reported for pressures up to 700 bars (K7). h a n techniques have also been used for mineral identiiication with Fourier transform spectra of a wide variety of minerals being reported (K8). A laser Raman microprobe study of 20 sulfide minerals demonstrated the power of the technique for mineral identiiication in hand specimens and thin sections with a spatial resolution of -1 pm (K9). Initial results from a study of the laserexcited 5uorescence of REE in fluorite crystals has shown that quantitation of individual REE with a spatial resolution of -3 pm by using a Raman microprobe is possible (KlO).However, the applicability of the technique to other minerals may be very problematic. Laser Raman microprobe has been used to characterize carbonaceous material (K11)and the influence of laserinduced sample temperature rise on the Raman characterization of graphite has been investigated (K12). Solid-state NMR with magic angle spinning has been used in numerous mineral studies. The application of 27Al NMR to investigations of aluminosilicates has been reviewed (K13).zgSi NMR has been applied to the determination of crystalline silica in natural iron oxide pigments (K14).Utilizing high fields and high spinning speeds, the f i s t 25MgNMR spectra for a variety of mineral classes have been reported (K15). Infrared methods along with a variety of other sophisticated spectroscopic techniquesfor use in clay mineral studies have been reviewed (K16,K17). Applications of near-infrared re5ectance spectroscopy to a wide variety of materials including geologic and inorganic materials has also been reviewed (K18). CHROMATOGRAPHY METHODS Liquid chromatography (LC) continues to be used in geoanalysis for separation or preconcentration steps. Numerous applications are cited throughout this review. Liquid chromatographic techniques are also coupled, somewhat more frequently than in the past, directly to a variety of spectroscopic techniques. The coupling of chromatography with ICP-AES and ICPMS has been reviewed, and a few geologic applications were noted (L1). Generally, these coupled techniques are applied to very limited types of geoanalysis problems such as the speciation of organ@ metallic compounds in soils and sediments. For example, lead speciation has been performed by using liquid chromatography coupled with isotope dilution ICPMS (La.Ion-exchange chromatography has also been used with ICPMS to determine tributyltin (L3).Gas chromatography (GC) coupled with atomic spectroscopicdetection methods provides another avenue for the determination of organometallic compounds. The determination of organolead compounds by this method has been reviewed (LA), and the advantages of in situ ethylation of butyltin species prior to GC-AAS determination have once again been touted (Ls). Methods for the determination of organotin (L6)and Se species (L7)have been reviewed. Interferences in a hydride generation GC-AAS procedure for organotin have been evaluated (Ls). Also, supercritical fluid extractions have been developed for organotin species in sediments (L9, L1O). 80R

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In the last decade, there has been an increase in the use of liquid chromatography techniques for the determination of metals, commonly with either pre- or postcolumn derivatization. The most active area has been the determination of specific suites of REE where the chromatographicconditions have been varied to avoid coelution of elements. For example, La, Ce, Pr, Nd, Er, Tm, Yb, Lu, and Y have been determined in several Japanese rock standardsby using ion-interactionchromatography (~511).Yttrlum was also determined in rare earth ores by a somewhat similar method (L12).Another group has determined 11REE in various Japanese, US., and Canadian geologic reference materials (L13, L14). Based on their observed trends in chondrite-normalized REE plots compared to previously published data, they suggest that their chromatography method provides more accurate results for several of the REE than spectroscopic and nuclear activation techniques. Other metals determined by chromatographic techniques include Th and U in ilmenite, rutile, and zircon mineral sands by using ion chromatography (L15);Nb over the range of 0.3-100 pg/g in rocks by using reversed-phase high-performance liquid chromatography (HPLC) (L16);and metal-cyanide complexes including Au and Ag in ore processing solutions by using capillary zone electrophoresis (L17)and ion-interaction chromatography (L18). Although anions in geologic materials are commonly determined by chromatographicmethods, in many laboratories iodide is not determined routinely by any method. In order to improve recovery of iodine from soils, an alkaline ashmg procedure was developed which was followed by a solvent extraction step and GC analysis (L19).Iodine concentrationsfrom about 1to 5 pg/g were determined in several NIST soil and sediment SRMs with precision on the order of 1-4%. MISCELLANEOUS METHODS As noted in our last review, anodic stripping voltammetry has been used for field testing in Au exploration. Development of this technique has continued (MI). Based on an aqua regia decomposition of 10 g of sample followed by an ethyl acetate extraction, reasonable results were obtained for a variety of geologic materials by using a portable voltammetry instrument. The detection limit for the procedure was 10 ng/g. Less successful results were obtained when cyanide leach solutions were analyzed. Various stripping techniques have also been used for field screening of Cd, Cr, Cu, Pb, and Zn in sediments (M2). Chemically m o d ~ e delectrodes have been used for the voltammetric determination of Au (M3, M4), but their application to geoanalysis has not been well tested and it is doubtful that they will see much use in the near hture. This is generally true for most electrochemical methods other than potentiometry. However, coulometry is used somewhat for the determination of C and HzO in geologic materials. A coulometric method for total S in rocks has been recently reported (M5). The determination of forms of sulfur in geologic materials has always been problematic. Numerous extraction schemes have been employed, and frequently the same extraction procedure is used for materialswith varying age and concentrationsof organic matter. Ancient sedimentary rocks present particular problems where it has been found that although mild acid extractions (e.g., 6 N HCl) recover the majority of acid volatile sulfides (AVS) from modem sediments,hot 6 N HCl plus stannous chloride is required

for complete recovery of AVS from older materials ( M a . It has also been found that various commonly used acid and Cr(II) extractions apparently solubilize more organic S species than previously believed (M7). Because there is considerable concern about the association of metals with sedimentary sulfides and their potential toxicity, the determination of simultaneously extracted metals along with AVS and disulfides is of renewed interest. A simplified acid extraction with a colorimetric determination of evolved H2S has been used for AVS determination in conjunction with determination of the associated metals (M8). A diffusion procedure followed by potentiometric determination of the H2S has also been developed, but 10%more AVS was recovered than when the more common purge and trap procedures were used (M9). The trace metals associated with AVS and pyrite can be overestimated owing to contributions from metals released by organic matter during the extractions. Heavy liquid separation of pyrite prior to analysis and several extraction schemes for the bulk sediment that were designed to remove organic matter but not pyrite were recently tested (MlO). It was concluded that the concentration of trace metals associated with pyrite was most accurately estimated from density-separated pyrite whereas the total amount of pyrite was best determined by extraction methods. The determination of reactive iron for interpreting paleoenvironments or iron-limitedsulfide formation has been revisited. One of the techniques used in the past for iron extraction was a 1 N HCl leach. This leach was tested on estuarine sediments and based on Mossbauer results it was found that the Fe oxidation state was maintained during the extraction ( M l l ) . By using this leach in conjunction with dc polarography, the extracted Fe(l1I) and Feu0 were measured and AVS was determined from the evolved H2S. This same leach was also compared with dithionite and concentrated HC1 leach procedures to evaluate their ability to extract iron from pure mineral phases and sediments (Ml2). It was observed that dithionite and boiling concentrated HCl extracted all iron oxides that were studied but that cold, dilute HCl did not. Thus, as a result of this pbservation and other considerations, it was recommended that these former two extracts be used to define the degree of pyritization and ironlimited pyrite formation. Techniques for the determination of reactive silica in sediments have also been evaluated (M13). Many different speciation or element/phase association schemes have been developed in the past. Reports on this subject frequently have several common themes such as examination of model compounds (Ml4), comparison of sequential and simultaneous techniques ( M I $ , removal of organic matter (Ml6), and extractions of specific toxic elements such as Cr (M17).Many of these themes are discussed in a special issue of International Journal of Environmental Analytical Chemistry, which covers the proceedings of a European workshop on the sequential extraction of trace metals from soils and sediments (Ml8). One important aspect of the work described is the concerted effort of the European Community Bureau of Reference (BCR) to standardize extraction procedures (M19). This has resulted in work on testing the feasibility of developing standards that are certified for extractable element contents (MZO) and evaluating a BCR proposed three-stage sequential extraction scheme for sediments (M21-M23). The results of standardized extraction schemes may vary unpredictably from one sample type to another, such as in the case of a former US.EPA method for the alkaline extraction of

C r O where spike recoveries were unreliable. However, the method has been found to be satisfactory when restricted to soils with oxidizing conditions (M24). Measurement of additional soil parameters may be required to verify whether the sample is suitable for the method or whether the soil would reduce any added C r O . Two different estimators, developed by Gy and by Ingamells, for the error associated with sampling particulate materials have been reexamined with a particular focus on Au assay (M25). It was concluded that the Gy method has more validity in practice. The Gy estimator was also looked at in regard to the sampling of cyanide-contaminated soils (M26). A method for estimating the segregation error was developed. ACKNOWLEDGMENT We thank the USGS Library staff in each of the regional centers, Denver, CO, Menlo Park, CA, and Reston, VA, for their assistance in the literature search. The use of trade names is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey. We acknowledge Chemical Abstracts Service for providing access to STN International to aid in the literature search used in the preparation of this work. Larry L. Jackson is a Research Chemist with the U S . Geological

Surve Branch of Geochemisty. He received a B.S. degree in chemisty ffrom om i Colorado gansas StateStateUniversity in 1973 and a Ph.D. in analytical chemist University in 1978. His research interests inch$

the application of electroanalytical techniques to the analysis ofgeological materials, environmental geochemisty of sul r, element cycling in wetlands, and the use of biogeochemisty studieskairquality monitoring.

Philip A. Baedecker is a Research Chemist with the U S . Geological Survey,Branch of Geochemisty. He received his B.S. degree in chemisty from Ohio University in 1961 and his Ph.D. in inorganac chemisty and radzochemisty from the Um'verstty of Kentucky an 1967. He has served as Chzef of the Branch of Anal tzcal Chemistry and as. Ch~azmzanof the Matenals E ects task group of tie Natzonal Acid Prect itatzon Assessment Pro ram. %is research interests include the appication of nuclear anafytical methods to problems in eochemisty, computer methods in y-ray spectroscopy and actzvation anahis, and the effectsof acidic deposition on carbonate stone. Terry L. Fries is a. Research Chemist at the US.Geological Survey, Branch of Geochemzsty. He received hzs B.S. in chemisty from California State University, Fresno in 1976 and his M.S. degree in analytical chemzstyfiom San Jose State University in 1982. His research znterests are zn the applzcatzon of atomzc spectroscopy to trace element analysis. Paul J. Lamothe is a Research Chemist at the U S . Geological Surve Branch of Geochemisty. He received his B.S. in chemisty from t& University of San Franczsco in 1968 and his Ph.D. in anal ical chemist from Marquette University in 1973. Prior to joinin the $SGS in 1 9 7 r he was a research chemzst wzth the Envzronmentaf Protection T n c y , Research Trian le Park, NC. His research interests are in ana ytzcal spectroscopy a n i trace element analyses.

LITERATURE CITED (Al) Analysis of Geological Materials; Riddle, C., Ed.; Marcel Dekker: New York, 1993. (A2) Vander Voet, A. H. M.; Riddle, .C. The Anal is of.Geologica1 Matemals, Vol. I: A Practrcal Guzde;Ontano ~ o l o g c aSurvey l Miscellaneous Paper 149, 1993. (A3) Ontario Geolo 'calSurve The Analysis ofGeolo ical Materials, VoL II: A &nual of Gethods; Ontano Geofogical Survey Miscellaneous Paper 149, 1990. (A4) Soil Samplin and Methods ofAnalysis; Carter, M. R, Ed.; Lewis Pub.: Boca %aton, FL, 1993. (A5) Bouldmg, J. R Description and Sampling of Contaminated Soils, A Field Guide, 2nd ed.; Lewis Pub.: Boca Raton, FL, 1994. (A6) Handbook on Reference Methods for Soil Analysis, 2nd ed.; Soil and Plant Analysis Council, St. Lucie Press: Delray Beach, FL, 1992. (A7) Quantitative Methods in Soil Mineralogy; wonette, J. E., Zelazny, L. W., Eds.; Soil Sci. SOC. Am.: Madison, WI, 1994. Analytical Chemistry, Vol. 67, No. 12, June 15, 1995

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QEOSTANDARDS

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&,

SAMPLE PREPARATION AND DISSOLUTION

(Cl) Kingston, H. M.; Walter, P. J. Spectroscopy 1992, 7, 20, 22, 24-7.

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(C7) (C8) (C9) (C10) (C11)

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ATOMIC ABSORPTION SPECTROMETRY

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Analytical Chemistry, Vol. 67, No. 72, June 75, 7995

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4

I 1

l+(/-ou.

(D20) Ebdon, L.; Goodall, P.; Hill, S.J.; Stockwell, P. B.; Thompson, K. C. J. Anal. At. Spectrom. 1993,8, 723-9. PLASMA ICP-AES AND ICPMS

P.; Cave, M Anal (E2) Ren, J. M.; S d n , E. W 7 - 7 GI " .

c

""I

(E3) Halicz, L.; Brenner I. B.; Yoffe, 0.J. Anal. At. Spectrom. 1993, 8(3), 475-80. (E4) Valdes-Hevia y Tem rano, M. C.; Femandez de la Campa, M. R; Sanz-Medel, A. -?Anal. At. Spectrom. 1 9 9 3 , 8(6), 821-5. (E5) Nakahara, T.; Mori, T.J. Anal. At. Spectrom. 1994,9(3), 1596.5

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---

1167-71 . . -.

(E24) Goguel, R. Fresenius'J. Anal. Chem. 1992, 344(7-8), 32633. (E25) Tagami, K.; Uchida, S.Radiochim. Acta 1993,63, 69-72. (E26) Morita, S.;Tobita, IC;Kurabayashi, M. Radiochim. Acta 1993, 63, 63-7. MASS SPECTROMETRY

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~

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iini-6.

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h. %.;

1aA-G Id1 Y.

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