Anal. Chem. 2005, 77, 3717-3736
Geochemical and Cosmochemical Materials Michael E. Lipschutz,*,† Stephen F. Wolf,‡ F. Bartow Culp,† and John M. Hanchar§
Department of Chemistry, Wetherill Laboratory, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2038, Department of Chemistry, Indiana State University, Terre Haute, Indiana 47809-5901, and Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X7, Canada Review Contents GEOSTANDARDS SAMPLE PREPARATION FOR BULK ANALYTICAL TECHNIQUES MULTITECHNIQUE SYMPOSIA AND REVIEWS QUADRUPOLE-BASED ICPMS Solution Laser Ablation SECTOR-BASED HR- AND MC-ICPMS Solution Laser Ablation MASS SPECTROMETRY ELECTRON MICROBEAM TECHNIQUES NUCLEAR TECHNIQUES MISCELLANEOUS LITERATURE CITED
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This review surveys the literature on the chemical analysis of terrestrial and extraterrestrial solids for the two-year period October 2002 through September 2004. We continue to include extraterrestrial materials as we did for the previous periods (A1) because analyses of these samples on Earth continue to present new challenges to the analytical geochemist. 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 online searches of the pertinent chemistry and geoscience literature to support the reviewers’ individual reading and subject expertise. We searched the following databases: GEOREF, established by the American Geological Institute in 1966, is the online complement of the printed Bibliography and Index of Geology. It is the most comprehensive database in the geosciences, with references dating back to 1785, containing 1.9 million references to journal articles, books, conference proceedings, reports, and theses. The particular format used for these searches was the online version of the database covering 20022004. CAPLUS, produced by the Chemical Abstracts Service (CAS) of the American Chemical Society, is the most current version of Chemical Abstracts Online; it includes the entire Chemical Abstracts file from 1907 with additional new and unindexed entries. It covers the world’s chemical literature comprehensively, with * To whom correspondence should be addressed. E-mail: rnaapum1@ purdue.edu. † Purdue University. ‡ Indiana State University. § Memorial University of Newfoundland. 10.1021/ac050566b CCC: $30.25 Published on Web 04/28/2005
© 2005 American Chemical Society
specific sections on geochemistry and cosmochemistry. We used SciFinder Scholar, the search engine developed by CAS, to search this database. In searching each database, we followed two strategies. First, we very broadly searched over the period (2002-2004) combining the concept terms for “geochemistry” and “chemical analysis”. These searches resulted in large numbers of recordssover 450 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 The Analyst, Analytica Chimica Acta, Analytical Chemistry, Chemical Geology, Critical Reviews in Analytical Chemistry, Earth and Planetary Science Letters, the newly renamed Fresenius Journal of Analytical Chemistry, Geochimica et Cosmochimica Acta, Geostandards Newsletter: the Journal of Geostandards and Geoanalysis, Journal of Chromatography, Journal of Geophysical Research E, Meteoritics and Planetary Science, Nature, Science, and Talanta for bulk elemental and isotopic analysis of geochemical and cosmochemical materials during the period of interest. During this reporting period, several publications appeared that are worth mentioning for their applicability to topics covered in this review. The Handbook of Rock Analysis (A2) covers wet chemistry and instrumental methods, including a thorough treatment of various applications of mass spectrometry. The 10-volume Treatise on Geochemistry (A3) devotes its first volume to meteorites, comets, and planets, with information on specific analytical techniques scattered throughout the work. As with many other fields of endeavor, geochemistry and cosmochemistry are well represented on the Internet. The following selective list of Web sites contains numerous links to further resources: The International Association of Geoanalysts (IAG) maintains a straightforward and reasonably current Web site , with links to meetings, journals, and other organizational sites. It also contains a useful online guide to many instrumental methods used in geochemical analysis. Elsevier maintains a Web site , covering 70 scientific journals, online articles, books, etc., in the Earth and Planetary sciences. The Geology section of the U.S. Geological Survey Web site is particularly useful for its numerous publications in geochemistry, many of which are now available in full text online. Of particular note is the open file report Analytical Methods for Chemical Analysis of Geologic and Other Materials (A4). Analytical Chemistry, Vol. 77, No. 12, June 15, 2005 3717
GeoScienceWorld (GSW) is a new Internet endeavor that contains links to many geoscience publications. The is a fee-based service (that had an initial free trial period) . The Chemical Institute of Canada’s Hamilton Section has compiled a useful site containing links to research groups, societies, and several software programs of geochemical interest. A number of educational institutions have also assembled useful Web sites for cosmochemistry, geochemistry, or both: Cornell University’s Web site is quite extensive; however, it has not been updated since 1999 and contains many dead links. Likewise, the International Association of Geochemistry and Cosmochemistry’s site at Michigan State University is sadly out of date. The University of Hawaii maintains a Web site for Planetary Science Research Discoveries which usually has a strong chemical component. A Web site is maintained at Washington University for information on Lunar meteorites . A Web site at the NASA Johnson Space Center is maintained dealing with Martian meteorites . We continued to focus on a limited number of exciting areas, and to highlight trends in trace analysis, particularly of smaller samples. In fact, this continues to follow analytical trends in geochemical and cosmochemical research that involve getting ever more compositional information from ever smaller samples. As in our previous review (A1), inductively coupled plasma mass spectrometry (ICPMS) with all of its hyphenated permutations continues to be the analytical method used most widely. Other trace analysis techniques are of lesser import, particularly for noteworthy studies. We tried not to be myopic in our search, so that we could identify important studies outside of our focus areas. A second trend in geochemical research that continued to broaden during the past two years is the application of two or more advanced techniques to characterize a terrestrial sample or suite of samples. This “consortium approach” has long been a staple in cosmochemistry, where samples are rare and usually small and where bulk host material is not generally plentiful. In geochemical studies where bulk material is usually plentiful, the major appeal of the multitechnique approach is that data sets generally prove synergistic. Even in such materials, it may be only rare grains that are of interest (for example, a single zircon that is to be dated). Also, as techniques improve so that more and more information can be obtained from lesser quantities of sample, compositional heterogeneity becomes a potentially more serious problem. Whether this problem is serious can be determined by gathering several sorts of data, to establish whether sample heterogeneity may be affecting the conclusions of the primary study. GEOSTANDARDS Beginning in 2002, Valladon continued the annual retrospective of the geostandards literature compiled for many years by Roelandts. In his 2002 review, Valladon (A5) expanded coverage 3718
Analytical Chemistry, Vol. 77, No. 12, June 15, 2005
to include new developments in geoanalysis methods. He notes the general increase in the volume of geostandards publications, partially attributing it to the increasing necessity for including quality control in experimental results. The primary publication in the field of geostandards changed its name in 2004 from Geostandards Newsletter to Geostandards and Geoanalytical Research. By any name, it continues to be the most frequent publisher of new geochemical Certified Reference Materials (CRMs) from laboratories around the world. It also has continued the useful and timely “Back to Basics” reviews; the latest surveys analytical techniques applied to the Re-Os isotopic system (A6). Three new CRMs have been reported for the Re-Os system: two molybdenites and a serpentinized peridotite. Concentrations and isotopic ratios of the molybdenites, HLP and JDC, were determined by three laboratories using negative thermal ionization mass spectrometry (NTIMS) and ICPMS (A7). The UB-N peridotite was also characterized in a three-laboratory effort, which, despite recognized sample inhomogeneity, yielded welldefined Re and Os concentrations (A7). Because of the increasing sensitivity and precision of analytical techniques, uncertainties caused by sample inhomogeneitysboth elemental and isotopicshave become an increasingly significant part of the total analytical uncertainty. The specific issue of isotopic homogeneity for Mg, Cd, Ca, Li, and Si RMs was addressed by Carignan et al. (A8), who also discussed the varying notation systems used to describe isotopic ratios. Other authors made specific proposals for standardization of notations for Mg (A9), Cd (A10), and Ca (A11), along with suggested RMs for isotopic ratio measurements. Eggins and Shelly (A12) report extensive compositional heterogeneity in the widely used NIST SRM 610617 glasses and state that their reliability as RMs extends to only half to two-thirds of the elements with which they were doped. A microheterogeneity study of NIST, MPI-DING, and USGS glass RMs using synchrotron microscopic X-ray fluorescence (µ-XRF) was carried out to establish minimum representative masses for calculation of homogeneous measurements (A13). Continuing the drive for greater compliance with ISO Guide 35 recommendations, the IAG developed a protocol for certifying geological and environmental RMs (A14). The goals of this protocol follow, in general, those established for the GeoPT Proficiency Test programs but also include the need for traceability and estimation of uncertainties for reference values. The IAG certified a slate sample, OU-6, for 12 major and minor elements through a 30-laboratory program (A15). This certification closely complies with ISO guidelines and includes the traceability of the sample. To provide a widely available reference for zircon, the 91500 zircon crystal sample was reexamined by a variety of techniques in an interlaboratory comparison (A16). While Wiedenbeck et al. (A16) do not recommend the use of 91500 as a primary standard for trace analysis because of previously undetected zoning in samples, it is well suited for calibrating oxygen isotopic ratios in situ. An outcome of that study was the observation that for many techniques used there was excellent agreement for analyses done at a given laboratory (EPMA, SIMS, LA-ICPMS) but poor agreement between different laboratories using the same technique. These results clearly indicate the need not only for homogeneous standards but also for standardized analytical protocols to improve accuracy.
Zircon dating by the U-Pb method is important in geochronology with the 238U-206Pb scheme being particularly valuable for ages
