Electron probe microanalyser

An electron probe microanalyser (EPMA) or electron micro probe analyser (EMPA), is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. It works similarly to a scanning electron microscope: the sample is bombarded with an electron beam, and signals that come from the sample are collected. This enables the elements present within sample volumes of 10-30 cubic micrometers or less to be determined. The concentrations of elements from boron to plutonium can be measured at levels as low as 100 parts per million (ppm). Development of the electron microprobe was preceded by that of the closely related analytical technique of X-ray fluorescence spectrometry (XRF). A beam of electrons is fired at a sample. The beam causes each element in the sample to emit X-rays at a characteristic frequency; the X-rays can then be detected by the electron microprobe. The size of the electron beam determines the trade-off between resolution and scan time.Low-energy electrons are produced from a tungsten filament cathode and accelerated by a positively biased anode plate to 10 to 30 thousand electron volts (keV). The anode plate has central aperture and electrons that pass through it are collimated and focused by a series of magnetic lenses and apertures. The resulting approximately 1 micrometre diameter electron beam may be rastered across the sample or used in spot mode to excite various effects from the sample. Among these effects are: phonon excitation (heat), cathodoluminescence (visible light fluorescence), continuum X-ray radiation (bremsstrahlung), characteristic X-ray radiation, secondary electrons (plasmon production), backscattered electron production, and Auger electron production. The characteristic X-rays are used for chemical analysis. Specific X-ray wavelengths are selected and counted, either by wave-length dispersive spectrometry (WDS) or energy dispersive X-ray spectroscopy (EDS). WDS utilizes Bragg diffraction from crystals to select X-ray wavelengths of interest and direct them to gas-flow or sealed proportional detectors. In contrast, EDS uses a solid state semiconductor detector to accumulate X-rays of all wavelengths produced from the sample. Chemical composition is determined by comparing the intensities of characteristic X-rays from the sample material with intensities from known composition (standards). Count from the sample must be corrected for matrix effects (absorption and secondary fluorescence) to yield chemical compositions. The resulting chemical information is gathered in textural context. Variations in chemical composition within a material (zoning), such as a mineral grain or metal, can be readily determined.

Further information:

1. Wittry, David B. (1958). "Electron Probe Microanalyzer", US Patent No 2916621, Washington, DC: U.S. Patent and Trademark Office.
2. Jansen, W.; Slaughter, M. (1982), "Elemental mapping of minerals by electron microprobe" (free full text), American Mineralogist 67 (5-6): 521–533.
3. Zhang, X. & Briggs, D.E.G. (2007). "The nature and significance of the appendages of Opabinia from the Middle Cambrian Burgess Shale". Lethaia 40 (2): 161–173.
4. Мальцев П. П., и др. «Нанотехнологии. Наноматериалы. Наносистемная техника», Москва, изд. Техносфера, 2008 г., 430 с.
5. Article Electron probe microanalyser  from Wikipedia, the Free Enciclopedia. Available under the license Creative Commons Attribution-Share Alike.

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