Electron probe microanalysis (EPMA), also known aselectron probe X-ray microanalysis,electron microprobe analysis (EMPA) orelectron probe analysis (EPA) is amicroanalytical andimaging technique used to non-destructively determine thechemical element composition of small volumes of solid materials. The device used for this technique is known as anelectron probe microanalyzer (also abbreviated EPMA), often shortened toelectron microprobe (EMP) orelectron probe (EP).
In EPMA, the instrument bombards the sample with a high-intensityelectron beam, which then emitsX-rays. The X-raywavelengths emitted are characteristic of particular chemical elements and are analyzed usingX-ray spectroscopy. The same principle is also employed inwavelength- orenergy-dispersive X-ray spectroscopy (WDX, EDX) commonly used inscanning electron microscopes (SEM), but EPMA is characterized by a fixed electron beam rather than a scanning one and primarily used for elemental analysis rather than imaging.
Anelectron gun produces anelectron beam focused on the sample through a series ofmagnetic lenses, much like a SEM. However, a key difference from a SEM is that the electron beam is fixed rather thanraster scanning, which makes it incapable of producing scanning electronmicrograph 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, producingcharacteristic X-rays at a highsignal-to-noise ratio.
The characteristic X-ray signal is typically analyzed by one or morewavelength-dispersive X-ray spectrometers (WDS), which use a pivoting-crystalgoniometer to discern the angle relative to the crystal's surface at which the reflected X-ray'sfirst-order diffraction peak is detected. Using this angle and the known distance betweenlattice 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 anenergy-dispersive X-ray spectrometer, detectors forsecondary andbackscattered electrons, or a detector forcathodoluminescence.
This enables the abundances of elements present within small sample volumes (typically 10-30 cubicmicrometers or less) to be determined,[2] when a conventional accelerating voltage of 15–20kV is used.[3] The concentrations of elements fromlithium toplutonium may be measured at levels as low as 100parts per million (ppm), material dependent, although with care, levels below 10 ppm are possible.[4] The ability to quantify lithium by EPMA became a reality in 2008.[5]
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, andX-ray spectroscopy, which identifies thephotons scattered from the electron beam impact, with the energy/wavelength of the photons characteristic of the atoms excited by the incident electrons.Ernst Ruska andMax 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.[6]
There have been at several historical threads to electron beam microanalysis. One was developed byJames Hillier and Richard Baker atRCA. In the early 1940s, they built an electron microprobe, combining an electron microscope and an energy loss spectrometer.[7] 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 usingBragg 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 byAndré Guinier, built the first electron "microsonde électronique" (electron microprobe) atONERA. This microprobe produced an electron beam diameter of 1-3 μm with a beam current of ~10 nanoamperes (nA) and used aGeiger 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 aquartz 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,[8] translated into English byPol Duwez and David Wittry,[9] 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.
The 1950s was a decade of great interest in electron beam X-ray microanalysis, following Castaing's presentations at the First European Microscopy Conference inDelft in 1949[10] and then at theNational Bureau of Standards conference on Electron Physics[11] inWashington, DC, in 1951, as well as at other conferences in the early to mid-1950s. Many researchers, mainly material scientists, developed their own experimental electron microprobes, sometimes starting from scratch, but many times using surplus electron microscopes.
Concurrently, Pol Duwez, a Belgian material scientist who fled the Nazis and settled at theCalifornia Institute of Technology (Caltech) and collaborated withJesse DuMond, encounteredAndré Guinier on a train in Europe in 1952, where he learned of Castaing's new instrument and the suggestion that Caltech build a similar instrument. David Wittry was hired to build such an instrument as his PhD thesis, which he completed in 1957. It became the prototype for the ARL[12] EMX electron microprobe.
During the late 1950s and early 1960s there were over a dozen other laboratories in North America, the United Kingdom, Europe, Japan and the USSR developing electron beam X-ray microanalyzers.
The first commercial electron microprobe, the "MS85" was produced byCAMECA (France) in 1956.[citation needed]. It was soon followed in the early-mid 1960s by microprobes from other companies; however, all companies exceptCAMECA,JEOL andShimadzu Corporation went out of business. Significant subsequent improvements and modifications to microprobes included the addition of solid state EDS detectors (1968) and the development of synthetic multilayer diffracting crystals for analysis of light elements (1984). One breakthrough of particular note, however, was the development, from the late 1950's onwards, of scanning microprobes; that is, devices which could scan the electron beam across a sample to make X-ray maps. These found great application in metallurgy, see section below.
Later, CAMECA pioneered manufacturing a shielded electron microprobe fornuclear applications. Several advances in CAMECA instruments in recent decades expanded the range of applications inmetallurgy,electronics,geology,mineralogy,nuclear plants,trace elements, anddentistry.
At the end of the 1950's, Castaing's innovative work was complemented by an instrument that scanned the electron beam and thus enabled the distribution of trace and alloying elements in a sample of metal to be imaged. From a metallurgist's point of view this constituted the biggest advance inmetallography sinceHenry Clifton Sorby had invented thereflected light microscope a hundred years earlier. For while it is helpful to be able to detect the presence of an element on themicron scale, it is even more valuable to be able to image its distribution. This ability to detect for the first time the presence ofalloying ortrace elements dissolved in a host metal, and image their distribution advanced the science of metallurgy itself. It enabled the identification of non-metallic inclusions, revealed segregation during solidification, and allowed identification of the sources of grain boundary weakness as well as many other problems. The instrument that first did this, the scanning electron probe microanalyzer, emerged from research at Cambridge University, and development work at the nearby laboratories of British engineering firmTube Investments (TI). It is one of the early examples of a breakthrough borne of the close collaboration between university and industry in what became known as theCambridge Phenomenon.
One of the organizers of the 1949 Delft Electron Microscopy conference had beenVernon Ellis Cosslett at theCavendish Laboratory atCambridge University, a center of research on electron microscopy.[13] Concurrently, in the Department of Engineering at Cambridge,Charles Oatley had been working on the related but distinct field scanning electron microscopy, and Bill Nixon onX-ray microscopy. In 1957Peter Duncumb, then a young physicist and research fellow, combined all three technologies to produce a prototype scanning electron X-ray microanalyzer for his PhD thesis.
Meanwhile, ten miles south of Cambridge, British engineering group Tube Investments (TI) had recently opened (1954) a group research laboratory; the Tube Investments Research Laboratory (TIRL) atHinxton Hall,[14] and in 1957 had recruitedDavid Melford, a metallurgist from Cambridge who had just completed his own PhD. They set him the task of finding the distribution of trace elements dissolved in steel in regions on the scale of microns.[15] Melford was quickly directed toDuncumb, back at the university, and on August 7, 1957, the pair examined a piece of steel in the instrument Duncumb had built. It proved an ideal demonstration of the potential value of this equipment as a research tool.
TIRL at once recruited Duncumb as a consultant and tasked Melford to design whatever it took to embody the demonstrator Duncumb had developed into an instrument for metallurgical use. Melford's pencil sketch, drawn on Christmas Day 1957 and now in the Cambridge University library, defined the layout of the instrument, although no engineering drawings had yet been made. Crucially, the instrument included an optical metallurgical microscope, essential in selection of the field of view, and allowing both optical and X-ray images of the sample to be captured and studied alongside each other. Duncumb and he then produced around a 100 dimensioned sketches which the well-equipped workshop at Hinxton Hall converted into a finished instrument.[16] It was commissioned shortly before Christmas 1958 and is now in the reserve collection of theScience Museum, London.
There had been no thought so far of building anything other than a valuable research tool, but, in January 1959, H. C. Pritchard the Managing Director of the Cambridge Instrument Company visited TIRL and saw the instrument in action. In March of that year the Company, with the agreement of TI and the Cavendish Laboratory, decided to build a copy – the first commercial scanning electron probe microanalyzer. With the help of Duncumb and Melford's drawings, they soon started manufacture and the first instrument was on show at theInstitute of Physics meeting in January 1960. This early example (pictured at the head of this page) is now in theCambridge Museum of Technology.
A beam of electrons is fired at a sample. The beam causes each element in the sample to emitX-rays at a characteristic frequency; the X-rays can then be detected by the electron microprobe.[17] The size and current density of the electron beam determines the trade-off between resolution and scan time and/or analysis time.[18]
Low-energy electrons are produced from atungstenfilament, alanthanum hexaboride crystalcathode or afield emission electron source and accelerated by a positively biasedanode plate to 3 to 30 thousandelectron volts (keV). The anode plate has central aperture and electrons that pass through it arecollimated and focused by a series ofmagnetic 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 produceexcitation 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, andAuger electron production.
When the beam electrons (and scattered electrons from the sample) interact with bound electrons in the innermostelectron shells of the atoms of the various elements in the sample, they canscatter 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 theelectromagnetic 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 lowatomic number (Z) elements (B, C, N, O and F forKalpha and Al to Cl forKbeta) 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 bywavelength-dispersive X-ray spectroscopy (WDS) orenergy-dispersive X-ray spectroscopy (EDS). WDS utilizesBragg 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 statesemiconductor 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 formatrix effects (depth of production of the X-rays,[19][20]absorption and secondaryfluorescence)[21][22] to yield quantitative elemental compositions. The resulting chemical data is gathered in textural context. Variations in chemical composition within a material (zoning), such as amineral grain or metal, can be readily determined.
Theinteraction volume from which chemical information is gathered (volume of X-rays generated) is 0.3–3 cubic micrometers.
The technique is commonly used for analyzing the chemical composition of metals, alloys, ceramics, and glasses.[24] 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 containingnickel at 0.23%.[25] 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.
This technique is most commonly used by mineralogists andpetrologists. 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), andplate 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.
In exceptionally preserved fossils, such as those of theBurgess 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 inOpabinia, 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.[26] Because of the thinness ofcarbon films, only low voltages (5-15 kV) can be used on them.[27]
The chemical composition of meteorites can be analyzed quite accurately using EPMA. This can reveal much about the conditions that existed in the earlySolar System.[citation needed]
Media related toElectron microprobes at Wikimedia Commons
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