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How an electron microprobe works

If electrons of sufficient energy strike a material, characteristic X-rays are emitted from the sample. Each element present in the sample emits characteristic X-rays of wavelengths unique to that element. If the intensities of X-ray production at these wavelengths are detected and measured, compared to a standard, and matrix corrections applied, a quantitative analysis may be obtained of a volume with a resolution of a few microns.

Information derived in such a manner can be a single quantitative spot analysis, or spatial elemental information may be obtained in the form of an image. The electron microprobe is one of the most versatile of laboratory instruments.

This detailed information can be applied to fields as diverse as geology, archeology, materials science, metallurgy, chemistry, physics, gemology, electronics, biology, medicine, dentistry, environmental science and engineering, and forensics, to name a few.

The electron microprobe is essentially a scanning electron microscope (SEM), with an electron beam being accelerated to typically 15 to 20 kV and focused onto the sample with magnetic lenses. The primary differences being that a microprobe has wavelength dispersive spectrometers (WDS) and very stable electronics.

For a WDS spectrometer, X-rays impinge upon a diffracting crystal, which is set to an angular position that is unique to diffract only the characteristic X-ray of interest (Bragg’s Law). The diffracted X-rays enter a gas-filled proportional counter and are counted. The position of the diffracting crystal and associated counter can normally be adjusted to accept a range of characteristic wavelengths. Thus, several elements can be measured sequentially on a single spectrometer. A microprobe usually has from three to five WDS spectrometers, to accommodate a wide variety of wavelengths as well as shorten acquisition time when several elements are being analyzed.

Most electron microprobes also have an energy dispersive spectrometer (EDS). This solid-state detector accepts all wavelengths continuously, and multiple electron-hole pairs are produced in proportion to X-ray energy.

The two detection techniques are largely complimentary.  The primary advantages of WDS are higher spectral resolution (which is better in the case of wavelength overlaps) and higher peak/background. The disadvantages are that they are sequential (i.e. they only detect one wavelength at a time), expensive, and their precise moving parts are prone to getting out of adjustment.

The primary advantages of EDS are that the entire X-ray spectrum is acquired simultaneously, there are no moving parts, and they are somewhat less expensive.

Typically, EDS provides a rapid qualitative overview of the composition of a sample, and can also be used for quantitative analysis of some major elements, while WDS is used for quantitative major, minor and trace element analysis.

Most microprobes, including ours, also have a back scatter electron detector (BSE), which is a valuable imaging technique. Some electrons impinging the sample do not create X-rays but rather are elastically scattered. The back scatter efficiency is a function of the average atomic number of the sample. Thus BSE image contrast is due to differences in the average atomic number of the various phases in the sample.

Sample preparation:
For accurate quantitative elemental analysis, the sample must be flat, highly polished, with the surface precisely normal to the electron beam. Typically a thin section is made of the material, and mounted on a glass slide 26 x 46 mm. In our other type of sample holder, we can mount pieces of material up to 5 mm in thickness, and as large as 25.5. We will be happy to discuss your sample preparation requirements.

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