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An electron microprobe (EMP), also known as an electron probe microanalyser (EPMA) 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, in which 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 micrometres or less to be determined.[1] Elements from boron to plutonium can be quantitatively analysed at levels as low as 100 parts per million (ppm).
History
Development 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, MIT 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-Ka, N-Ka and O-Ka 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 électronique” (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 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 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.
Elemental mapping is a microscopic technique where the distribution of elements over the surface of a sample is determined. It requires the use of an electron microprobe, using Energy-Dispersive X-Ray Spectroscopy or Wavelength-Dispersive X-Ray Spectroscopy[2]
How it works
A beam of electrons is fired at a sample. The size of this beam determines the trade off between resolution and scan time.[2]
The beam causes elements to emit X-rays at a characteristic frequency which can be detected by the electron microprobe.[3]
The technique contrasts with the simpler and quicker back-scatter electron microscopy (BSE) which measures the electron reflectivity of a surface; this gives an indication of its composition but does not make it possible to determine the distribution of individual elements.[2]
Uses
[] Mineralogy
The technique is most commonly used by mineralogists; the change in elemental composition from the centre to the edge of a mineral can yield information about the history of the crystal's formation, including the temperature, pressure, and chemistry of the surrounding medium.
Palaeontology
In exceptionally preserved fossils, such as those of the Burgess shale, soft parts of organisms may be preserved. Since these fossils are often compressed into a 2D fil, it can be difficult to deduce what features were what: a famous example is that of triangular extensions in Opabinia, which were interpreted as either legs or extensions of the gut. Elemental mapping showed that they had a similar composition to the gut, favouring the second interpretation.[4]
- For more information about element abundance in the Burgess shale, see Burgess_shale_type_preservation#elemental mapping
See also
References
- ^ Wittry, David B. (1958). "Electron Probe Microanalyzer", US Patent No 2916621, Washington, DC: U.S. Patent and Trademark Office.
- ^ a b c http://serc.carleton.edu/research_education/geochemsheets/elementmapping.html
- ^ Jansen, W.; Slaughter, M. (1982), "Elemental mapping of minerals by electron microprobe" (free full text), American Mineralogist 67 (5-6): 521–533, http://www.minsocam.org/ammin/AM67/AM67_521.pdf
- ^ 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. doi:10.1111/j.1502-3931.2007.00013.x, http://www3.interscience.wiley.com/journal/117996987/abstract. Retrieved on 20 August 2008.
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