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Central Michigan University - Department of Geology ELECTRON MICROPROBE FACILITY Dr.
David Matty, Director What is a microprobe? A microprobe is nothing other than a electron microscope that has been optimized to produce a very stable electron beam that remains focused on one spot rather than scanning over a sample as the more common SEMs (Scanning Electron Microscopes) do. The microprobe is used to determine the chemical composition of solid materials - from minerals and glasses in rocks and meteorites, to metals, and even composite materials. It is able to determine the chemical composition 1) because it produces a very stable, highly focused electron beam; 2) because most high-energy electrons that penetrate materials interact with the atoms in the materials to generate small amounts of X-rays; and 3) because every element has a known, characteristic set of X-rays. By counting the number of characteristic X-rays produced from electrons impacting suitable standard materials as well as unknowns, it is therefore possible to both qualitatively and quantitatively determine the chemical makeup of a sample. The probe does this through Wavelength-dispersive and Energy-dispersive analyses.... Wavlength-Dispersive Quantitative Analyses The main function of the electron microprobe is to produce highly accurate and precise quantitative chemical analyses of areas as small as 1-2 microns (1-2 thousandths of a millimeter) in diameter using wavelength dispersive spectrometry (WDS). Our microprobe is optimized to analyse elements in the range of Fluorine (Z=9) through Uranium (Z=92), however, we can arrange to analyse elements as light as Boron (Z=5). Some 'probes can analyse elements as light as Beryllium (Z=4). Detection limits for common elements using WDS are in the range of 100-500 ppm, although the instrument can be set up to detect and analyse elements with concentrations in the single ppm range (given very long count times). Our new Advanced Microbeam automation system now allows us to obtain about 25-30 nine-element analyses per hour once standardization is completed. Analytical totals of anhydrous minerals commonly fall within acceptable limits (98.5-101%) and almost all analyses produce stoichiometrically correct formulae. Energy-Dispersive Qualitative Analyses We can also produce qualitative analyses using our energy-dispersive (EDS) detector. EDS rapidly (~5 sec) provides information about the chemical characteristics of a sample (what's there & an idea of how much). EDS does not provide highly accurate concentration data and sometimes has problems analysing samples containing elements with overlapping elemental peaks. An example of an X-ray spectrum determined by EDS is shown here:
Backscattered Electron and X-ray Imaging Since the microprobe also functions as a Scanning Electron Microscope (SEM), we can generate highly magnified images of our samples. Using the backscattered electron detector, we can distinquish between different minerals in a sample based on differences in their bulk atomic number. The WDS spectrometers can also be used to produce "maps" of the distribution of elements within a sample. An example of such an X-ray map is shown here:
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