Anal. Chem. 1999, 71, 1R-20R
Geochemical and Cosmochemical Materials Michael E. Lipschutz,*,† Stephen F. Wolf,‡ John M. Hanchar,§ and F. Bartow Culp†
Department of Chemistry, BRWN/WTHR Building, Purdue University, West Lafayette, Indiana 47907-1393, CMT/205, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439-4837, and Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick Hall, University of Notre Dame, South Bend, Indiana 46556 Review Contents Geostandards Sample Preparation and Dissolution Atomic Absorption Spectrometry Inductively Coupled Plasma-Atomic Emission Spectrometry Inductively Coupled Plasma Mass Spectrometry Mass Spectrometry X-ray Spectrometry Electron Microbeam Techniques Particle-Induced X-ray and γ-Ray Emission Nuclear Methods Miscellaneous Spectroscopic Methods Chromatography Miscellaneous Methods Literature Cited
1R 2R 4R 4R 5R 8R 10R 10R 13R 13R 14R 15R 16R 17R
This review surveys the literature on the chemical analysis of terrestrial and extraterrestrial solids for the two-year period October 1996 to October 1998. We have chosen to expand coverage of geochemistry to include extraterrestrial materials because analyses of these samples on Earth present new challenges to the analytical chemist. We focused upon rocks and minerals and did not include gaseous species. We included aqueous solutions in our survey only where these bore upon geologic processes, but not environmental problems. We carried out comprehensive on-line searches of the pertinent chemistry and geoscience literature to support the reviewers’ individual reading and subject expertise. We searched the following databases: (1) GEOREF, established by the American Geological Institute in 1966, is the on-line complement of the printed Bibliography and Index of Geology. It is the most comprehensive database in the geosciences, containing over 2 million references to journal articles, books, conference proceedings, reports, and theses. The particular format used for these searches was the CD-ROM version of the database covering 1996-1998. (2) CAPLUS, produced by the American Chemical Society, is the most current version of Chemical Abstracts Online, including the entire CA Online file from 1967 with additional new and unindexed entries. It covers the world’s chemical literature comprehensively, with specific sections on geochemistry and cosmochemistry. In searching each database, we followed two strategies. First, we very broadly searched over the period (1996-1998) combining †
Purdue University. Argonne National Laboratory. § University of Notre Dame. ‡
10.1021/a1990003n CCC: $18.00 Published on Web 03/09/1999
© 1999 American Chemical Society
the concept terms for “geochemistry” and “chemical analysis”. These searches resulted in large numbers of recordssover 400 from each database. These records were scanned by the reviewers to check for their relevance to the scope of the review. Accordingly, we compiled a list of key terms for specific analytical procedures and carried out follow-up searches using these terms in the context of geochemistry. In addition, we manually searched American Mineralogist, The Canadian Mineralogist, Chemical Geology, Contributions to Mineralogy and Petrology, Geochimica et Cosmochimica Acta, Earth and Planetary Science Letters, Journal of Geophysical Research E, Meteoritics and Planetary Science, Nature, and Science for the period of interest. We chose to highlight state of the art geochemical research methods for quantifying inorganic species, i.e., those that are not routine applications but, rather, are novel either from a technical standpoint or from the problem being addressed. One continuing important theme in geo- and cosmochemistry involves more sensitive studies of and ability to analyze ever-smaller samples. We omitted qualitative studies and mineral structure research. GEOSTANDARDS With the first issue of 1997, the Geostandards Newsletter changed its name to the Journal of Geostandards and Geoanalysis and increased its publishing scope to include developments in geoanalytical techniques (A1). Under either name, the Newsletter/ Journal has continued to be the primary worldwide source for geostandards information. Recent issues have featured special topics on laser ablation inductively coupled mass spectrometry (LA-ICPMS) (A2) and new needs and requirements for certified reference materials (CRMs) (A3), and serve to illustrate the expanded nature of the journal. It also has an Internet presence (http://www.geostandards.lanl.gov/), maintained at present by Ernest Gladney at Los Alamos National Laboratories (A4). In addition to archives of previous issues and editorial guidelines, the site has a number of useful links to related web pages. Sidney Abbey has contributed his reminiscences to the Newsletter as well as his experiences with reference samples (A5). He has applied the “five-mode method”, first developed by him in 1992, to evaluate interlaboratory analytical data on three GITIWG rock reference samples (A6). He also introduced the graphical moving mode, a new procedure used to provide a clearer picture of the distribution of raw data for each constituent. Gladney and Roelandts have also updated their 1987 study of the distribution of Canadian Certified Reference Materials Project (CCRMP), NIST, and USGS reference material data in the literature (A7). A compilation of concentration data was presented for up to 36 Analytical Chemistry, Vol. 71, No. 12, June 15, 1999 1R
elements in 93 geochemical reference samples (GRS), using instrumental neutron activation analysis (A8). The International Organization for Standardization (ISO) has developed a number of technical standards and guides concerning the standardization of various commercial and scientific activities. The article summarizes those guides most relevant to the production of geochemical reference (A9). The 16th and 17th additions to the annotated GRS Bibliography, covering 1995 and 1996, respectively, continue to provide references from the major analytical chemistry and geochemistry journals (A10, A11). Among the added features of these compilations are tables showing the distribution of GRS references in the literature and the analytical methods most used to report GRS results. A variety of standards work has been reported from international sources. Analytical data have been compiled on nine Geological Survey of Japan (GSJ) sedimentary rock series reference samples (A12). The reproducibility of elemental concentrations for JB-1, a GSJ rock sample, has been studied (A13). The GSJ has also set up an Internet site (http://gsj.go.jp) to facilitate the retrieval of information on 31 GSJ rock reference samples (A14). Another helpful feature of this site is a listing of over 400 geoscience organizations around the world. Also reported is an improvement on the acid digestion method of rare earth element (REE) samples (A15) and elemental analyses of 15 Japanese igneous reference rocks (A16). Certified values for 11 silicates and 9 limestones have been derived by the Chinese Institute of Geophysical and Geochemical Exploration (A17). A lead isotope standard has been prepared by the Chinese Academy of Geological Sciences (A18); and three Chinese CRMssplastic clay, potassium feldspar, and soda-lime-silica glassshave been prepared (A19). New reference samples of the magmatic rocks quartz diorite SKD-1 and aviatonossite SSV-1 have been prepared (A20). The Geochemical Survey of Finland provided rock samples for the ICPMS measurements of 34 geochemically significant trace elements (A21). The IUPAC has published its biennial revision of the atomic weights of the elements (A22). Among the items relevant to this review are the following: (1) An improvement to the Can ˜on Diablo Troilite (CDT) standard used for reporting the relative sulfur isotope abundance has been recommended by the IUPAC. It proposes the establishment of a Vienna Can ˜on Diablo Troilite (VCDT) scale defined by the internationally distributed silver sulfide reference material IAEA-S-1. (2) New guidelines are recommended for reporting stable hydrogen, carbon, and oxygen isotope ratio data. (3) Many elements exhibit a different isotopic composition in various nonterrestrial materials. Further illustrating the importance of the LA-ICPMS technique to the field, a new technique for GRS synthesis of zircons has been described (A23). REE have been determined in 16 silicate reference samples by ICPMS after Tm addition and ion-exchange separation (A24). A comparison of LA-ICPMS and SIMS results from NIST standard 614 and 616 glasses has been reported (A25). The USGS has issued a report on its evaluation program for standard reference samples distributed in September 1996. Included in these samples are metals and trace constituents (A26). 2R
Analytical Chemistry, Vol. 71, No. 12, June 15, 1999
SAMPLE PREPARATION AND DISSOLUTION One of the principal rate-determining steps in geochemical analysis involves the efficient conversion of samples into solutions suitable for quantification by some specific technique. During the period covered by this review, advances in sample preparation and dissolution have generally been associated with solution ICP methods, the most widely used geochemical analysis technique. The direct analysis of solutions with total dissolved solids (TDS) greater than ∼0.2% can be problematic for ICPMS. Potential interferences from polyatomic species and signal degradation resulting from solid deposit buildup on the nebulizer and sampling cones have led to the development and application of alternative strategies for sample pretreatment, sample introduction, and instrumental calibration for analysis of solutions with high TDS. For these reasons, the use of microwave dissolution techniques continues to receive considerable attention in the literature. Several studies have focused on the comparison of microwave procedures with other digestion methods (B1), with a variety of trace and ultratrace elements (B2), or with different acids as digesting agents (B3). Lamble and Hill (B4) critically reviewed microwave digestion procedures for environmental matrixes. This paper discusses and compares open and closed digestion systems, closed microwave digestion techniques, and on-line digestion techniques, for use on biological and geological samples. A volume dealing with microwave analytical applications has been published in the ACS Professional Reference Series (B5). Other applications integrate microwave dissolution into improved sample preparation procedures. Chakraborty and coworkers reported a generalized method for determination of Ni by electrothermal atomization atomic absorption spectrometry (ETAAS) that includes rapid microwave digestion of sealed samples at power levels between 330 and 550 W (B6). A microwave digestion method for in situ trapping of Hg on Au, Pd-Au alloy and Pt-Rh alloy in a graphite furnace has been described (B7). A method previously used for analyzing fly ash was developed and optimized to study trace element balances in coal-fired power plants (B8). It incorporated microwave digestion of both coal and coal fly ash in a mixture of HNO3 and HF in closed vessels. A flow injection (FI) system based on on-line microwave-assisted digestion was applied in the analysis of silicate rocks (B9). The procedure developed enables up to 10 samples/h to be analyzed with a detection limit of 60 ng/mL. Chemical separation techniques continue to be developed and applied to the preparation of geological samples prior to ICPMS analysis. A classical acid digestion method was developed to decompose sulfide ores of platinum group elements (PGEs), Au, and Ag without the usual preroasting step (B10). Detection limits were in the range 0.01-0.04 ng/mL of the digested solutions. Three methods have been used for the separation and preconcentration of PGEs. Typically, PGE determination has involved preconcentration into a Pb button followed by cupellation, a step that can result in PGE loss. A minicolumn with tetraethylenepentamine resin was used to separate PGE from solutions generated by dissolution of ores, silicates, and Fe-formation rocks (B11). The methodology relative standard deviation (RSD) was calculated to be 3.5%, with limits of quantitation in the range of 0.5-4.0 ng/g for determination of several PGEs by ICPMS.
Jarvis et al. (B12) described new procedures for separation (on Dowex 1 × 8 resin) and determination of PGE and Au by ICPMS. Acceptable results were obtained by ICPMS determination of most elements containing high concentrations (>1 µg/g) of PGEs (B12). Another procedure (B13) combined microwave digestion and alkali fusion, followed by separation of PGEs by cation-exchange chromatography and analysis by ICPMS. Results indicated that the method was applicable to larger (5 g) sample sizes and offered an alternative to fire assay (B13). Rehka¨mper and Halliday (B14) described a simple anion-exchange separation scheme for the isolation of PGEs and other trace elements with small (∼1.25 mL) columns from relatively large samples. Samples are loaded onto the column and Cd, Zn, Ag, Ru, Pd, Re, Ir, and Pt are sequentially eluted with dilute HCl. Reversed-phase chromatography has been combined with an on-line matrix-elimination procedure in the detection of Th and U in phosphate rocks automatically using a programmable HPLC pump (B15). After phosphate and other anions were removed by cation exchange using dilute HNO3, Th(IV) and UO22+ were trapped on a C18 column while transition and REE metals were eluted with phenylhydroxyacetic acid. Finally, Th(IV) and UO22+ were transferred onto a reversed-phase C18 column, eluted, and determined spectrophotometrically using R-hydroxyisobutyric acid. An oxine cellulose column was used to separate and enrich REE prior to detection with ICP-AES (B16). A comparison study was made on the performance of a heated spray chamber desolvation chamber for sample introduction into an ICP-AES (B17). Improvement of the detection limits over a normal spray chamber was observed, down to levels of microgram to nanogram per liter. Two papers described new methodologies for separation and preconcentration of REE. An ion-exchange chelating fiber with aminophosphonic and dithiocarbamate groups based on polyacrylonitrile was used to preconcentrate REE and separate them from seawater prior to determination by ICPMS (B18). All REE were found to be retained and eluted quantitatively. The separation with subsequent ICPMS analysis was found to yield RSDs of
