Electron probe microanalysis

Electron probe microanalysis (EPMA), also known as electron probe X-ray microanalysis, electron microprobe analysis (EMPA) or electron probe analysis (EPA) is a microanalytical and imaging technique used to non-destructively determine the chemical element composition of small volumes of solid materials. The device used for this technique is known as an electron probe microanalyzer (also abbreviated EPMA), often shortened to electron microprobe (EMP) or electron probe (EP).

In EPMA, the instrument bombards the sample with a high-intensity electron beam, which then emits X-rays. The X-ray wavelengths emitted are characteristic of particular chemical elements and are analyzed using X-ray spectroscopy. The same principle is also employed in wavelength- or

energy-dispersive X-ray spectroscopy (WDX, EDX) commonly used in scanning electron microscopes (SEM), but EPMA is characterized by a fixed electron beam rather than a scanning one and primarily used for elemental analysis rather than imaging.

Principles

An electron gun produces an electron beam focused on the sample through a series of magnetic lenses, much like a SEM. 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. The electron beam has a significantly higher beam current than is typical of a SEM and 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 reflected X-ray's first-order diffraction peak is detected. 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.