Electron probe microanalysisWhat’s the point?PowerPoint PresentationSlide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Electron probe microanalysisAccuracy and Precision in EPMA:The Role of StandardsRevised 11/12/09What’s the point?EPMA’s claim to fame as a microanalytical tool rests upon (1) faith in a correct matrix correction and (2) use of “good”, “correct”, “true” standards.How do you know to trust a standard?StandardsIn practice, we hope we can start out using the “best” standard we have.* There have been 2 schools of thought as to what is the “best” standard is:• a pure element, or oxide, or simple compound, that is pure and whose composition is well defined. Examples would be Si or MgO or ThF4. The emphasis is upon accuracy of the reference composition.• a material that is very close in composition to the unknown specimen being analyzed, e.g. silicate mineral or glass; it should be homogenous and characterized chemically, by some suitable chemical technique (could be by epma using other trusted standards). The emphasis here is upon having a matrix that is similar to the unknown, so that (1) any potential problem with the matrix correction will be minimized, and (2) any specimen specific issues (i.e. element diffusion, volatilization, sub-surface charging) will be similar in both standard and unknown, and largely cancel out.* This is based upon experience, be it from prior probe usage, from a more experienced user, from a book or article, or trial and error (experience comes from making mistakes!) It is commonly a multiple iteration, hopefully not more than 2-3 efforts.Standards - Optimally• Ideally the standard would be stable under the beam and not be able to be altered (e.g., oxidizable or hygroscopic) by exposure to the atmosphere.• It should be large enough to be easily mounted, and able to be easily polished. • If it is to be distributed widely, there must be a sufficient quantity and it must be homogeneous to some acceptable level. However, in the real world, these conditions don’t always hold. “Round Robins”On occasion, probe labs will cooperate in “round robin” exchanges of probe standards, where one physical block of materials will be examined by several labs independently, using their own standards (usually there will be some common set of operating conditions specified). The goal is to see if there is agreement as to the compositions of the materials.Sources for standards :• Purchased as ready-to-go mounts from microscopy supply houses as well as some probe labs ($1200-2000) • Alternately, most probe labs develop their own suite of standards based upon their needs, acquiring standards from:• Minerals and glasses from Smithsonian (Dept of Mineral Sciences: Ed Vicenzi, free)• Alloys and glasses from NIST (~$100 ea)• Metals and compounds from chemical supply houses (~$20-60 ea)• Specialized materials from researchers (synthesized for experiments, or starting material for experiments) – both at home institution as well as globally (some $, most free)• Swap with other probe labs • Materials from your Department’s collections, local researchers/ experimentalists , local rock/mineral shop (e.g., Burnie’s) or national suppliers (e.g., Wards)USNM Standards• 1980: Gene Jarosewich, Joe Nelen and Julie Norberg at the Smithsonian Dept of Mineral Science (US National Museum) published results of an effort to develop epma standards for minerals and glasses. They had crushed, separated, then examined for homogeneity; once a subset found, it was analyzed by classical methods (wet chemistry), and then made available for distribution. This list included 26 minerals and 5 glasses. In 1983, Jarosewich and MacIntrye published data on 3 carbonate standards (calcite, dolomite and siderite), and in 1987, Jarosewich and White published data on a strontianite (SrSO4) standard. These all are available at no cost to probe labs.• These are excellent standards. Users must be aware of course that the “official value” represents a bulk analysis and individual splits may be slightly different. One problem is the small size of many grains (~100-500 m).• Another problem recently discussed (Albuquerque M&M 2008) is the presence of small inclusions in a not insignificant fraction of the grains. This requires the prober be very careful.Other Mineral Standards• In the 1960s, Bernard Evans developed a suite of silicate and oxide mineral standards (at UC Berkeley) that were available for EPMA work. Some of these are still around. • 1992, McGuire, Francis and Dyar published report on evaluation of 13 silicate and oxide minerals as oxygen standards. They included data for all elements. Available from Harvard Mineralogical Museum for small cost (~$100-150).• Here in Madison, I have evaluated several minerals from the Mineralogy collection for standards and found some very good: casserite (SnO2), wollastonite (CaSiO3), Mg-rich olivine and enstatite. Other minerals from Wards have been found to be useful (biotite and F-topaz). On the other hand, other efforts have been unsuccessful (e.g., ilmenite from Wards -- zoned/exsolution lamallae)Synthesized Standards• 1971, Art Chodos and Arden Albee of Caltech contracted Corning Glass to produce 3 Ca-Mg-Al borosilicate glasses (95IRV, W and X) containing a number of (normally) trace elements, at 0.8 wt% level, to be used as EPMA trace element standards. They are available now from the Smithsonian.• 1971, Gerry Czamanske (USGS) synthesized 73 sulfides and 3 selenides/tellurides (for phase equilibria studies). Some of these were made available to EPMA labs. We have them here.• 1972, Drake and Weill (U. Oregon) synthesized 4 Ca-Al silicate glasses each with 3-4 REE elements.• 1991: Jarosewich and Boatner published data on a set of 14 rare-earth (plus Sc and Y) orthophosphates (synthesized by Boatner). These are also available at no charge from the Smithsonian. (A recent study by Donovan et al. showed that many have some unreported Pb impurities.)• John Hanchar (Memorial University , NFLD) has been working on synthesizing zircon, hafnon, thorite and huttonite; some are now available for standards.• There are other synthetic standards available, usually in limited quantities; one discovers these sources by “asking around”.• Have skilled users (who have