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Electron probe microanalysis
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Electron probe microanalysis
Electron probe microanalysis (EPMA), also known as electron probe X-ray microanalysis, electron microprobe analysis (EMPA) or electron probe analysis (EPA) is a microanalytical and imaging technique used to non-destructively determine the chemical element composition of small volumes of solid materials. The device used for this technique is known as an electron probe microanalyzer (also abbreviated EPMA), often shortened to electron microprobe (EMP) or electron probe (EP).
In EPMA, the instrument bombards the sample with a high-intensity electron beam, which then emits X-rays. The X-ray wavelengths emitted are characteristic of particular chemical elements and are analyzed using X-ray spectroscopy. The instrument has some similarity to a scanning electron microscope (SEM), but is characterized by a fixed electron beam rather than a scanning one. An EPMA is primarily used for elemental analysis rather than imaging, and the images it produces are two-dimensional cross-sections rather than images of surface topography that would be seen in a SEM image.
An electron gun produces an electron beam focused on the sample through a series of magnetic lenses, much like a SEM. However, a key difference from a SEM is that the electron beam is fixed rather than raster scanning, which makes it incapable of producing scanning electron micrograph images. The electron beam has a significantly higher beam current than is typical of a SEM and is highly stabilized and focused using a special beam stabilization system. This allows the electrons to more deeply penetrate the sample, producing characteristic X-rays at a high signal-to-noise ratio.
The characteristic X-ray signal is typically analyzed by one or more wavelength-dispersive X-ray spectrometers (WDS), which use a pivoting-crystal goniometer to discern the angle relative to the crystal's surface at which the reflected X-ray's first-order diffraction peak is detected. Using this angle and the known distance between lattice planes of the reflecting crystal, Bragg's law can then be applied to derive the wavelength of the characteristic X-ray emitted from the sample, which is unique to a particular chemical element. An EPMA may also have a number of other detectors, such as an energy-dispersive X-ray spectrometer, detectors for secondary and backscattered electrons, or a detector for cathodoluminescence.
This enables the abundances of elements present within small sample volumes (typically 10-30 cubic micrometers or less) to be determined, when a conventional accelerating voltage of 15–20 kV is used. The concentrations of elements from lithium to plutonium may be measured at levels as low as 100 parts per million (ppm), material dependent, although with care, levels below 10 ppm are possible. The ability to quantify lithium by EPMA became a reality in 2008.
The electron microprobe (electron probe microanalyzer) developed from two technologies: electron microscopy, which uses a focused high energy electron beam to impact a target material, and X-ray spectroscopy, which identifies the photons scattered from the electron beam impact, with the energy/wavelength of the photons characteristic of the atoms excited by the incident electrons. Ernst Ruska and Max Knoll are associated with the prototype electron microscope in 1931. Henry Moseley was involved in the discovery of the direct relationship between the wavelength of X-rays and the identity of the atom from which it originated.
There have been at several historical threads to electron beam microanalysis. One was developed by James Hillier and Richard Baker at RCA. In the early 1940s, they built an electron microprobe, combining an electron microscope and an energy loss spectrometer. A patent application was filed in 1944. Electron energy loss spectroscopy 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 concept 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. However, RCA had no interest in commercializing this invention.
A second thread developed in France in the late 1940s. In 1948–1950, Raimond Castaing, supervised by André Guinier, built the first electron "microsonde électronique" (electron microprobe) at ONERA. 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 PhD thesis, translated into English by Pol Duwez and David Wittry, 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 is considered the father of electron microprobe analysis.
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Electron probe microanalysis
Electron probe microanalysis (EPMA), also known as electron probe X-ray microanalysis, electron microprobe analysis (EMPA) or electron probe analysis (EPA) is a microanalytical and imaging technique used to non-destructively determine the chemical element composition of small volumes of solid materials. The device used for this technique is known as an electron probe microanalyzer (also abbreviated EPMA), often shortened to electron microprobe (EMP) or electron probe (EP).
In EPMA, the instrument bombards the sample with a high-intensity electron beam, which then emits X-rays. The X-ray wavelengths emitted are characteristic of particular chemical elements and are analyzed using X-ray spectroscopy. The instrument has some similarity to a scanning electron microscope (SEM), but is characterized by a fixed electron beam rather than a scanning one. An EPMA is primarily used for elemental analysis rather than imaging, and the images it produces are two-dimensional cross-sections rather than images of surface topography that would be seen in a SEM image.
An electron gun produces an electron beam focused on the sample through a series of magnetic lenses, much like a SEM. However, a key difference from a SEM is that the electron beam is fixed rather than raster scanning, which makes it incapable of producing scanning electron micrograph images. The electron beam has a significantly higher beam current than is typical of a SEM and is highly stabilized and focused using a special beam stabilization system. This allows the electrons to more deeply penetrate the sample, producing characteristic X-rays at a high signal-to-noise ratio.
The characteristic X-ray signal is typically analyzed by one or more wavelength-dispersive X-ray spectrometers (WDS), which use a pivoting-crystal goniometer to discern the angle relative to the crystal's surface at which the reflected X-ray's first-order diffraction peak is detected. Using this angle and the known distance between lattice planes of the reflecting crystal, Bragg's law can then be applied to derive the wavelength of the characteristic X-ray emitted from the sample, which is unique to a particular chemical element. An EPMA may also have a number of other detectors, such as an energy-dispersive X-ray spectrometer, detectors for secondary and backscattered electrons, or a detector for cathodoluminescence.
This enables the abundances of elements present within small sample volumes (typically 10-30 cubic micrometers or less) to be determined, when a conventional accelerating voltage of 15–20 kV is used. The concentrations of elements from lithium to plutonium may be measured at levels as low as 100 parts per million (ppm), material dependent, although with care, levels below 10 ppm are possible. The ability to quantify lithium by EPMA became a reality in 2008.
The electron microprobe (electron probe microanalyzer) developed from two technologies: electron microscopy, which uses a focused high energy electron beam to impact a target material, and X-ray spectroscopy, which identifies the photons scattered from the electron beam impact, with the energy/wavelength of the photons characteristic of the atoms excited by the incident electrons. Ernst Ruska and Max Knoll are associated with the prototype electron microscope in 1931. Henry Moseley was involved in the discovery of the direct relationship between the wavelength of X-rays and the identity of the atom from which it originated.
There have been at several historical threads to electron beam microanalysis. One was developed by James Hillier and Richard Baker at RCA. In the early 1940s, they built an electron microprobe, combining an electron microscope and an energy loss spectrometer. A patent application was filed in 1944. Electron energy loss spectroscopy 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 concept 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. However, RCA had no interest in commercializing this invention.
A second thread developed in France in the late 1940s. In 1948–1950, Raimond Castaing, supervised by André Guinier, built the first electron "microsonde électronique" (electron microprobe) at ONERA. 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 PhD thesis, translated into English by Pol Duwez and David Wittry, 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 is considered the father of electron microprobe analysis.
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