Electron Microprobe (EPMA)
JGB5.106a; telephone 471-4949
Electron microprobes, cousins, to SEMs, use a beam of accelerated electrons to “probe” specimens. Although outfitted with some of the same imaging capabilities as SEMs, the primary aim of the microprobe is quantitative elemental analysis of minerals. The microprobe can generate elemental analyses of 1 to 10 µm spots on a wide variety of minerals, by allowing comparisons of generated X-ray intensities between samples and standards. The integration of quantitative mineral analysis with qualitative information on crystal compositional heterogeneities is the great strength of microprobe analysis. The microprobe is applied to a wide range of problems in many areas of geology, whenever it is important to know the details of mineral compositions. Installed in 2002-2003, the JEOL 8200 has 5 wavelength dispersive spectrometers (two are “high-intensity”), an energy dispersive detector (EDS), and imaging in secondary and back-scattered electron modes.
In addition to the JEOL software, a third party software, Probe for EPMA, developed by John Donovan et al., was also installed on this instrument. Probe for EPMA and related programs are very user-friendly and they store data in Microsoft Access database files. Information on Probe for EPMA software can be found at the Probe Software web site.
Probe samples must be polished (and good polish is one important key to good results) and coated with a standard thickness of evaporated carbon. Typical samples are standard thin sections of slightly greater than ordinary thickness. One-inch round sections (useful for grain mounts) and very thin (<3 mm) polished slabs can also be used.