Principles of electron microprobe analysis

During electron microprobe analysis, a sample is bombarded with a beam of electrons. The interaction of the electron beam with the sample material results in formation of X-rays, which can be analysed by the microprobe. The wavelengths and energies of these X-rays provide informations about the chemical elements present in the sample (qualitative analysis). When compared with reference materials, the measured X-ray intensities can be used to determine element concentrations (quantitative analysis).
In the following, the principle structure of an electron microprobe, the underlying physical processes and the analytical posibilities are briefly described.

Schematic sketch showing a cross-section through an electron microprobe 'Cameca SXFiveFE' (source: Cameca).

Electron-specimen interaction

Under the influence of electron bombardment, two major interaction mechanisms with the sample are of importance. One type of inelastic interactions is removal of an orbital electron of the sample material by a primary electron. Subsequent filling of this "electron hole" with outer orbital electrons, leads to formation of X-rays with discrete wavelengths. As the electron configuration is characteristic for different elements, also the X-rays produced are 'characteristic X-rays'. A primary electron that does not collide with an orbital electron decelerates by the Coulombic field associated with the sample atoms. Result of this inelastic scattering process is a continuous X-ray spectrum (Bremsstrahlung). This radiation of continuous energy forms the background for the characteristic X-ray lines.
Further interaction processes are release of secondary electrons and of backscattered electrons as well as the occurrence of cathodoluminescence.
Alltogether, the effects of electron-specimen interaction allow for imaging and for qualitative and quantitative chemical analysis of the sample material.