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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Electron probe microanalysis | 4/4 | https://en.wikipedia.org/wiki/Electron_probe_microanalysis | reference | science, encyclopedia | 2026-05-05T10:04:28.363360+00:00 | kb-cron |
=== Detailed description === Low-energy electrons are produced from a tungsten filament, a lanthanum hexaboride crystal cathode or a field emission electron source and accelerated by a positively biased anode plate to 3 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 electron beam (approximately 5 nm to 10 μm diameter) may be rastered across the sample or used in spot mode to produce excitation of various effects in 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. When the beam electrons (and scattered electrons from the sample) interact with bound electrons in the innermost electron shells of the atoms of the various elements in the sample, they can scatter the bound electrons from the electron shell producing a vacancy in that shell (ionization of the atom). This vacancy is unstable and must be filled by an electron from either a higher energy bound shell in the atom (producing another vacancy which is in turn filled by electrons from yet higher energy bound shells) or by unbound electrons of low energy. The difference in binding energy between the electron shell in which the vacancy was produced and the shell from which the electron comes to fill the vacancy is emitted as a photon. The energy of the photon is in the X-ray region of the electromagnetic spectrum. As the electron structure of each element is unique, the series X-ray line energies produced by vacancies in the innermost shells is characteristic of that element, although lines from different elements may overlap. As the innermost shells are involved, the X-ray line energies are generally not affected by chemical effects produced by bonding between elements in compounds except in low atomic number (Z) elements (B, C, N, O and F for Kalpha and Al to Cl for Kbeta) where line energies may be shifted as a result of the involvement of the electron shell from which vacancies are filled in chemical bonding. The characteristic X-rays are used for chemical analysis. Specific X-ray wavelengths or energies are selected and counted, either by wavelength-dispersive X-ray spectroscopy (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. While EDS yields more information and typically requires a much shorter counting time, WDS is generally more precise with lower limits of detection due to its superior X-ray peak resolution and greater peak to background ratio. Element composition is determined by comparing the intensities of characteristic X-rays from the sample with intensities from standards of known composition. Counts from the sample must be corrected for matrix effects (depth of production of the X-rays, absorption and secondary fluorescence) to yield quantitative elemental compositions. The resulting chemical data is gathered in textural context. Variations in chemical composition within a material (zoning), such as a mineral grain or metal, can be readily determined. The interaction volume from which chemical information is gathered (volume of X-rays generated) is 0.3–3 cubic micrometers.
=== Limitations === WDS cannot determine elements below number 5 (boron). This restricts WDS when analyzing geologically important elements such as H, Li, and Be. Despite the improved spectral resolution of elemental peaks, some peaks exhibit significant overlap that causes analytical challenges (e.g., VKα and TiKβ). WDS analyses are unable to distinguish the valence states of elements (e.g. Fe2+ vs. Fe3+) which must be obtained by other techniques such as Mössbauer spectroscopy or electron energy loss spectroscopy. Element isotopes cannot be determined by WDS, and are most commonly determined with a mass spectrometer.
== Applications ==
=== Materials science and engineering ===
The technique is commonly used for analyzing the chemical composition of metals, alloys, ceramics, and glasses. It is particularly useful for assessing the composition of individual particles or grains and chemical changes on the scale of a few micrometres to millimeters. The photograph to the right is an output image from an early scanning electron microanalyzer of a sample of steel containing nickel at 0.23%. The lighter regions, at the grain boundaries between iron crystals, are actually created in this image by the raised concentrations of Nickel, which had concentrated at the surface of the sample during oxidation at a high temperature, and then diffused down the boundaries between the iron crystals into the steel. This concentration in the boundaries was measured by the electron microprobe at 3–4%. The electron microprobe is widely used for research, quality control, and failure analysis.
=== Mineralogy and petrology === This technique is most commonly used by mineralogists and petrologists. Most rocks are aggregates of small mineral grains. These grains may preserve chemical information acquired during their formation and subsequent alteration. This information may illuminate geologic processes such as crystallization, lithification, volcanism, metamorphism, orogenic events (mountain building), and plate tectonics. This technique is also used for the study of extraterrestrial rocks (meteorites), and provides chemical data which is vital to understanding the evolution of the planets, asteroids, and comets. The change in elemental composition from the center (also known as core) to the edge (or rim) of a mineral can yield information about the history of the crystal's formation, including the temperature, pressure, and chemistry of the surrounding medium. Quartz crystals, for example, incorporate a small, but measurable amount of titanium into their structure as a function of temperature, pressure, and the amount of titanium available in their environment. Changes in these parameters are recorded by titanium as the crystal grows.
=== Paleontology ===
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 planar film, it can be difficult to distinguish the features: a famous example is the triangular extensions in Opabinia, which were interpreted as either legs or extensions of the gut. Elemental mapping showed that their composition was similar to the gut, favoring that interpretation. Because of the thinness of carbon films, only low voltages (5-15 kV) can be used on them.
=== Meteorite analysis === The chemical composition of meteorites can be analyzed quite accurately using EPMA. This can reveal much about the conditions that existed in the early Solar System.
== References ==
== Further reading ==
== See also == Electron microscope Electron spectroscopy Thin section
== External links ==
=== Online tutorials === Jim Wittke's class notes at Northern Arizona University John Fournelle's class notes at the University of Wisconsin–Madison John Donovan's class notes at the University of Oregon Media related to Electron microprobes at Wikimedia Commons