Electron microprobe

An electron microprobe (EMP), also known as an electron probe microanalyzer (EPMA) or electron microprobe analyzer (EMPA), is an analytical tool used to non-destructively determine the chemical element composition of small volumes of solid materials. The instrument has some similarity to a scanning electron microscope (SEM): the sample is bombarded with an electron beam, emitting X-rays at wavelengths characteristic to the elements being analyzed. However, a key difference from a SEM is that the electron beam is fixed rather than raster scanning, which makes it incapable of producing scanning electron micrograph images. Instead, the electron beam has a significantly higher beam current than is typical of a SEM, and also is highly stabilized and focused using a special beam stabilization system. This allows the electrons to more deeply penetrate the sample, producing characteristic X-rays at a high signal-to-noise ratio.

The characteristic X-ray signal is typically analyzed by one or more wavelength-dispersive X-ray spectrometers (WDS), which use a pivoting-crystal goniometer to discern the angle relative to the crystal's surface at which the intensity of the reflected X-ray's first-order diffraction maximum is greatest. Using this angle and the known distance between lattice planes of the reflecting crystal, Bragg's law can then be applied to derive the wavelength of the characteristic X-ray emitted from the sample, which is unique to a particular chemical element. An EMP may also have a number of other detectors, such as an energy-dispersive X-ray spectrometer, detectors for secondary and backscattered electrons, or for cathodoluminescence.

This enables the abundances of elements present within small sample volumes (typically 10-30 cubic micrometers or less) to be determined,^[2] when a conventional accelerating voltage of 15-20 kV is used.^[3] The concentrations of elements from lithium to plutonium may be measured at levels as low as 100 parts per million (ppm), material dependent, although with care, levels below 10 ppm are possible.^[4] The ability to quantify lithium by EPMA became a reality in 2008.^[5]