Electron Microprobe
The electron microprobe is a non-destructive analytical tool used to determine the chemical composition of small volumes of solid materials. It uses a high-energy focused beam of electrons to generate X-rays characteristic of the elements present within a sample volumes 1–3 micrometers (μm) across and can quantitatively analyze elements from boron to the plutonium at routine levels as low as 100 parts per million (ppm).
History
Developement of the electron microprobe was preceded by that of the closely related analytical technique of X-ray fluorescence spectrometry (XRF). This technique was first proposed by Georg von Hevesy in 1923 and applied by other workers in the following few years.
In 1944, Hillier and Baker (1944) at the Radio Corporation of America (RCA) Labs at Princeton, New Jersey, built an electron microprobe, combining an electron microscope and an energy-loss spectrometer. Electron energy-loss spectrometry is very good for light element analysis and they obtained spectra of C-Kα, N-Kα and O-Kα radiation. In 1947, Hiller patented the idea of using an electron beam to produce analytical X-rays, but never constructed a working model. His design proposed using Bragg diffraction from a flat crystal to select specific X-ray wavelengths and a photographic plate as a detector.
In 1948–1950, Raymond Castaing, supervised by André Guinier, built the first electron microsonde electronique (electron microprobe) at the University of Paris. This microprobe produced an electron beam diameter of 1–3 μm with a beam current of ~10 nanoamperes (nA) and used a Geiger counter to detect the X-rays produced from the sample. However, the Geiger counter could not distinguish X-rays produced from specific elements and in 1950, Castaing added a quartz crystal between the sample and the detector to permit wavelength discrimination. He also added an optical microscope to view the point of beam impact. The resulting microprobe was described in Castaing's 1951 Ph.D. thesis, in which he laid the foundations of the theory and application of quantitative analysis by electron microprobe, establishing the theoretical framework for the matrix corrections of absorption and fluorescence effects. Castaing (1921–1999) is considered the "father" of electron microprobe analysis.
Cameca (France) produced the first commercial microprobe, the MS85, in 1956. It was soon followed by many microprobes from other companies; however, all companies except Cameca and JEOL, are now out of business. In addition, many researchers build electron microprobes in their labs. Significant subsequent improvements and modifications to microprobes included scanning the electron beam to make X-ray maps (1960), the addition of solid state EDS detectors (1968) and the development of synthetic multilayer diffracting crystals for analysis of light elements (1984).
Process
Low-energy electrons are produced from a tungsten filament cathode and accelerated by a positively biased anode plate to 10–30 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 μm diameter electron beam may rastered across the sample or used in spot mode to excite various effects from the sample. Among these 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 determining 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.
Geojim56 01:10, 22 Mar 2005 (UTC) J.H. Wittke