Electron microprobe
An electron microprobe is an instrument that fires a focused electron beam at a polished sample and measures the X-rays produced to find the exact chemical composition of tiny spots.
The electron probe microanalyser (EPMA) was developed by the French physicist Raimond Castaing, whose 1951 doctoral work laid out the principles of quantitative analysis. A beam of electrons, focused to about a micrometre, strikes a polished, carbon-coated sample. Each element emits X-rays of characteristic energies, which are measured mainly with wavelength-dispersive spectrometers. Comparison with standards and matrix corrections gives accurate concentrations of major and minor elements, typically down to roughly a hundred parts per million. Back-scattered electron imaging and element maps show how compositions vary across a sample.
The microprobe is the workhorse of meteorite classification. It provides olivine fayalite and pyroxene ferrosilite values for ordinary chondrites, reveals compositional variation in unequilibrated chondrites, identifies rare minerals and analyses metal phases in irons. It is largely non-destructive, so the same section can be used again. Light elements such as hydrogen and lithium cannot be measured, and trace elements need other methods.
A famous application is the carbonate globules in martian meteorite ALH 84001, whose chemical zoning in calcium, iron and magnesium has been mapped in detail. Samples must be flat, polished and conductive, usually polished thin sections or mounted blocks.
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