1.3. Electron Microprobe Overview

Electron microprobe analysis (EMPA) is a non-destructive method for determining the chemical composition of tiny amounts of solid materials. It was developed by R. Castaing in Paris as his 1950 Ph.D. dissertation. Commercial electron microprobes became available in the 1960s, and have become standard analytical tools.

An electron microprobe (Figure 1.3) uses a high-energy focused beam of electrons to generate X-rays characteristic of the elements within a sample from volumes as small as 3 micrometers (3 x 10-6 m) across.  Low-energy thermionic electrons are produced from a tungsten filament and accelerated by a positively biased anode plate to 10-30 thousand electron volts (keV). The anode plate has a hole in its center and the electrons pass through it and are collimated and focused by a series of magnetic lenses and apertures. The X-rays resulting from beam interaction with the sample are diffracted by analyzing crystals (TAP, PET, LIF) and counted using gas-flow and sealed proportional detectors. Chemical composition is determining by comparing the intensity of X-rays from standards (known composition) with those from unknown materials and correcting for the effects of absorption and fluorescence in the sample.

The electron microprobe is designed specifically for detecting and measuring characteristic X-rays. It uses an electron beam current from 10 to 200 nanoamps (nA), roughly 1000 times greater than that in a scanning-electron microscope (SEM). These higher beam currents produce more X-rays from the sample and improve both the detection limits and accuracy of the resulting analysis. Analysis locations are selected using a transmitted-light optical microscope, which allows positioning accurate to about 1 micrometer, a feature not available on an SEM. The resulting data yield quantitative chemical information in a textural context. Variations in chemical composition within a material, such as a mineral grain or metal, can be readily determined.

The electron microprobe can quantitatively analyze elements from fluorine (Z=9) to uranium (Z=92) at routine levels as low as 100 ppm. Although principally used for geological investigations, the microprobe is available to all in the University community and outside researchers. Projects have included research in metamorphic and igneous petrology, and studies of archaeological materials such as pottery (paste and temper, glazes), glass, and lithics.

Figure 1.3. Schematic diagram of an electron microprobe showing the electron column and one wavelength-dispersive spectrometer (most microprobes have three or more situated radially around the column and sample chamber). Electron microprobes also usually have imaging systems and their associated detectors for secondary and/or backscattered electrons.