12.2.2: 12.2.2 Electron Microprobes

For the most accurate chemical analyses, we use wavelength dispersive X-ray spectroscopy (WDS) instead of EDS. Special detectors called spectrometers measure the intensity of X-rays having specific wavelengths, characteristic of specific elements. Because most scanning electron microscopes do not have spectrometers, they cannot produce WDS analyses.

Figure 13.38: An electron microprobe

Electron microprobes, more properly called electron probe micro analyzers (EPMAs), are instruments similar to SEMs but specifically designed for obtaining WDS analyses (Figure 12.38). Microprobes have a more intense electron beam than SEMs, and may have four or more spectrometers besides imaging and EDS analytical capabilities. However, we cannot obtain WDS analyses as quickly as EDS analyses because a spectrometer only measures one element at a time. For the most accurate results, we analyze standards to compare with unknowns, adding additional time to the procedure. The WDS approach is much more accurate than EDS and can detect elements present in smaller amounts, with precision about 1 to 2% of the amount present (for major elements). The analytical accuracy depends primarily on how well we have standardized the microprobe. Today’s microprobes can do all the same imaging that an SEM can do. The four monitors in Figure 12.38 allow viewing of different kinds of images and analytical data simultaneously.

Scanning electron microscopy and electron microprobe analysis have several major advantages over most other analytical techniques and are therefore widely used. First, they allow us to examine rocks, mineral grains, or thin sections with little sample preparation. Second, microprobe analysis is a nondestructive analytical technique. So, we analyze samples without destroying them. Perhaps most important, microprobe analyses focus on very small spots. We can analyze single mineral grains, or many spots in the same grain, to look at very small-scale changes in chemistry. No matter the exact technique used, because the electron beam typically excites a sample volume of only 30 cubic microns, it can resolve very fine compositional features. Additionally, individual X-ray peaks may be isolated to construct elemental maps, showing the distribution of different elements in different parts of the sample.

One shortcoming of both EDS and WDS analysis is that elements with low atomic number are difficult to analyze. This problem arises because low-numbered elements emit X-rays of low energy. For several reasons, such X-rays may not make it to the detector or spectrometer, or may not be measurable once they get there. Technology varies, but even the best EDS detectors cannot analyze elements lighter than boron. WDS analysis can, in principle, analyze elements from Be to U but technical complications occur at both extremes.

An additional complication arises because most geological materials do not conduct electricity. Unless we coat them with a conducting material, static charge builds up, distorting images and analyses. So, we vaporize gold, carbon, or other conducting material in a vacuum and let it precipitate on our specimen. Gold coatings work very well for some applications, producing excellent images. However, we cannot easily remove the gold, and it interferes with analyses. So mineralogists and petrologists generally use carbon coating instead. Unlike gold, we can remove the carbon coating from a thin section by light polishing. Instead of coating samples, we can reduce or eliminate charging problems by using an environmental scanning electron microscope (ESEM) instead of a standard SEM. ESEM is a technology that, unlike a conventional SEM, does not require that the sample be under a high vacuum. A small amount of gas (up to 20 torr/2.7 kPa) provides a medium to conduct electrons away from the sample. ESEMs require no sample coating and many permit examination of large samples, larger than conventional SEMs. The quality of images and analyses, however, is reduced compared with conventional SEMs.