Electron probe microanalysis
Non-destructive elemental analysis of solid materials using electron-beam-induced X-rays.
Electron probe microanalysis (EPMA) is a microanalytical and imaging technique used to non-destructively determine the chemical element composition of small volumes of solid materials. The device used, an electron probe microanalyzer, bombards the sample with a high-intensity electron beam, causing the emission of X-rays whose wavelengths are characteristic of particular elements. EPMA is distinguished from similar techniques in scanning electron microscopes by its fixed electron beam and primary focus on elemental analysis rather than imaging.
Quick Facts
- Typical sample volume
- 10-30 cubic micrometers or less
- Conventional accelerating voltage
- 15–20 kV
- Detection limit
- as low as 100 parts per million (ppm), material dependent; below 10 ppm possible with care
- Elements measurable
- lithium to plutonium
- Lithium quantification achieved
- 2008
- Beam diameter range
- approximately 5 nm to 10 μm
- Accelerating voltage range
- 3 to 30 thousand electron volts (keV)
Facts from the source article.
Principles
An electron gun produces an electron beam focused on the sample through magnetic lenses, similar to a scanning electron microscope, but the beam is fixed rather than raster scanning, preventing the production of scanning electron micrographs. The beam current is significantly higher and more highly stabilized than in a typical SEM, allowing deeper penetration and a high signal-to-noise ratio for characteristic X-rays. These X-rays are usually analyzed by wavelength-dispersive X-ray spectrometers, which use a pivoting-crystal goniometer to detect the first-order diffraction peak angle; applying Bragg's law to this angle and the crystal's lattice spacing yields the X-ray wavelength, unique to a specific element. An EPMA may also include energy-dispersive X-ray spectrometers, detectors for secondary and backscattered electrons, or a cathodoluminescence detector. This setup enables determination of elemental abundances within small sample volumes, typically 10-30 cubic micrometers or less, at a conventional accelerating voltage of 15–20 kV. Concentrations from lithium to plutonium can be measured at levels as low as 100 parts per million, material dependent, though levels below 10 ppm are possible with care; quantification of lithium became feasible in 2008.
History
The electron microprobe developed from two technologies: electron microscopy, which uses a focused high-energy electron beam, and X-ray spectroscopy, which identifies photons scattered from the beam impact. Ernst Ruska and Max Knoll are associated with the prototype electron microscope in 1931, and Henry Moseley discovered the direct relationship between X-ray wavelength and the originating atom. In the early 1940s, James Hillier and Richard Baker at RCA built an electron microprobe combining an electron microscope and an energy loss spectrometer, filing a patent in 1944; they obtained spectra of C-Kα, N-Kα, and O-Kα radiation. In 1947, Hillier patented the concept of using an electron beam to produce analytical X-rays, proposing Bragg diffraction with a flat crystal and a photographic plate, but RCA did not commercialize it. In France from 1948 to 1950, Raimond Castaing, supervised by André Guinier, built the first electron microprobe at ONERA, producing a 1-3 μm beam with ~10 nA current and using a Geiger counter. In 1950, Castaing added a quartz crystal for wavelength discrimination and an optical microscope to view the beam impact point. His 1951 PhD thesis, translated by Pol Duwez and David Wittry, laid the foundations of quantitative analysis theory, including matrix corrections for absorption and fluorescence; Castaing is considered the father of electron microprobe analysis.
Operation
A beam of electrons is fired at a sample, causing each element to emit X-rays at characteristic frequencies, which are then detected. The size and current density of the electron beam determine the trade-off between resolution and scan or analysis time. Low-energy electrons are produced from a tungsten filament, a lanthanum hexaboride crystal cathode, or a field emission source and accelerated by a positively biased anode plate to 3 to 30 keV. The anode plate has a central aperture; electrons passing through are collimated and focused by magnetic lenses and apertures, forming a beam approximately 5 nm to 10 μm in diameter. This beam may be rastered across the sample or used in spot mode, producing effects such as phonon excitation, cathodoluminescence, continuum X-ray radiation (bremsstrahlung), characteristic X-ray radiation, secondary electrons, backscattered electrons, and Auger electrons. When beam electrons or scattered electrons interact with bound electrons in the innermost shells of sample atoms, they can ionize the atom, creating a vacancy. This vacancy is filled by an electron from a higher energy shell, and the difference in binding energy is emitted as an X-ray photon. The energy of these X-rays is characteristic of the element, as the electron structure of each element is unique, though lines from different elements may overlap. For low atomic number elements (B, C, N, O, F for Kalpha; Al to Cl for Kbeta), line energies may shift due to chemical bonding effects.
Applications
The technique is commonly used for analyzing the chemical composition of metals, alloys, ceramics, and glasses, particularly for assessing individual particles or grains and chemical changes on a scale of a few micrometres to millimeters. An early scanning electron microanalyzer image of steel containing 0.23% nickel showed lighter regions at grain boundaries where nickel had concentrated at 3–4% during oxidation and diffused into the steel. The electron microprobe is widely used for research, quality control, and failure analysis. Mineralogists and petrologists most commonly use it to study rocks, which are aggregates of small mineral grains preserving chemical information from formation and alteration, illuminating processes such as crystallization, lithification, volcanism, metamorphism, orogenic events, and plate tectonics. It is also used for extraterrestrial rocks (meteorites), providing chemical data vital to understanding planetary, asteroid, and comet evolution. Changes in elemental composition from the core to the rim of a mineral can reveal the history of crystal formation, including temperature, pressure, and chemistry of the surrounding medium; for example, quartz crystals incorporate titanium as a function of these parameters. In exceptionally preserved fossils like those of the Burgess Shale, elemental mapping can distinguish features; for instance, triangular extensions in Opabinia were found to have a composition similar to the gut, favoring that interpretation. Due to the thinness of carbon films, only low voltages (5-15 kV) can be used on them.
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