About the Electron Microprobe

The electron microprobe, more formally called the Electron Probe Micro Analyzer (EPMA), is based upon the electron optical column of a conventional Scanning Electron Microscope (SEM), but incorporates a hardware addition specifically designed for the accurate, quantitative chemical analysis of solid materials. The application of this instrument can be most easily explained by breaking down the component parts of its acronym.

Like the SEM, the EPMA uses a primary electron beam to stimulate signal emission. An important capability of the EPMA, however, is the ability to fix the beam into an immobile "spot" or probe of user-defined size and automatically monitored and regulated current. This permits the selection of single locations for irradiation at a constant electron flux over time.

With our instrument, the diameter of the fixed spot can be varied in the range of 0.2 to 20.0 m m. [Although larger spot sizes are available, they typically are not recommended. Quantitative analysis of larger areas is commonly done by either replicate analyses of small spots or by moving the sample beneath a fixed beam.] Thus, the EPMA does not produce a bulk chemical analysis, but rather provides information on small areas.

Chemical analysis with the EPMA is performed by the detection and counting of fluorescent x-rays that are produced by electron transitions (from outer to inner orbitals) in atoms of the sample, the transitions being stimulated by electron bombardment (by the primary beam). Because the energy levels of electron orbitals for the atoms of a given element are intrinsic, the fluorescent x-rays also have characteristic energies. As a form of electromagnetic radiation, x-rays exhibit both particle- and wave-like properties, permitting two different methods of detection. The particle-like properties allow separation on the basis of energies, using a solid state detector in a device known as the Energy-Dispersive X-ray Analyzer (EDXA). Many modern SEMs, and our microprobe, are equipped with an EDXA, which has the advantage of rapid analysis stemming from the simultaneous acquisition of the entire x-ray spectrum. The rapidity of this process makes it an invaluable qualitative tool for phase identification, and it can be used in a quantitative capacity as well. Most elements, however, give rise to fluorescent x-rays of several different energies, and very often the energy of the x-ray emission from one element is similar enough to that of another that the two cannot be distinguished (called x-ray "overlap" or "interference") by EDXA.

The EPMA also can sort fluorescent x-rays on the basis of their wave-like properties utilizing one or more Wavelength-Dispersive Spectrometers (WDS): these are the "added hardware" alluded to above. The WDS resolve x-rays via diffraction through regular periodic solids in a manner very similar to the way a prism can separate component colors from white light. Hence by selecting the position and inter-planar spacing of the diffraction element, a single x-ray emission line can be resolved and sent to a gas-filled, "scintillation-type", detector for counting. WDS have far superior x-ray resolution compared to the EDXA, and thus represent a much better tool for the analysis of materials having elements with overlapping x-ray lines. Superior peak/background intensity ratios for WDS also make them the tool of choice for minor- to trace-level components and for light elements (which emit low-energy x-rays), and yield minimum levels of detection commonly 1-2 orders of magnitude lower than by EDXA.

Electron Microprobe Diagram (PDF)

The very nature of the EPMA makes it ideally suited to quantifying chemical compositions and compositional heterogeneity within complex solid materials. Among others, this includes such tasks as determining the compositions of individual phases in fine-grained multi-component materials or characterizing chemical heterogeneity within large continuous grains. Combined with capacities to image a material on the basis of its composition (see Imaging Capabilities) and digital image acquisition, you get a very powerful tool for characterizing and documenting both compositions and phase distributions in complex and heterogeneous solids. In addition, availability of the secondary electron imaging mode used in conventional SEMs provides topographic (surface morphology) characterization for such applications as phase discrimination, surface reaction mechanisms, and component failure analysis.

The Electron Microprobe Laboratory (EMPL) was established in 1988 through a grant from the U.S. Department of Energy (#DE-FG22-87FE1146). It was initially built around a CAMECA SX50 electron probe microanalyzer that was the second such instrument in the world, and first in North America, to have a maximum configuration with five wavelength-dispersive spectrometers. In August-September of 1995 upgrades to computer automation, backscattered electron detection system, and imaging systems were accomplished via a National Science Foundation grant (#EAR-9404658), with matching funds supplied by the University of Oklahoma Vice President for Research and from user fees accrued by the laboratory. Further upgrades to the computing systems, Energy-Dispersive X-ray Analyzer automation, digital image capture system, display monitors, and printers were made in the first half of 1999, again via funds from OU VPR and accrued user fees. The GATAN cathodoluminescence detector was added in June, 2000, made possible from by funds from the OU VPR.  The sample stage was replaced in 2004 with funds from a National Science Foundation grant (#EAR-0124179). Further upgrades to the computer automation, image and EDXA acquisition systems, and other hardware components within the microprobe were enabled by a 2007-2008 National Science Foundation grant (#EAR-0649001), with additional funds furnished by the University of Oklahoma Vice President for Research.

The SX50 was retired from service in March of 2015 in favor of a newer CAMECA SX100 that was obtained by donation from Sandia National Laboratory through the Department of Energy’s Laboratory Equipment Donation Program. Installation and automation of this instrument, including updating the Energy-Dispersive X-Ray Analyzer to a system based on a new silicon drift detector, was supported by funds from a National Science Foundation grant (#EAR-1401940) and, again, by generous support of the University of Oklahoma’s Vice President for Research. The new instrument is also a five-WDS machine with configuration that provides even greater flexibility for light element analysis and greater sensitivity for many trace elements than the previous system. The cathodoluminescence detector, developed as a prototype for the SX50, was transferred to the SX100.

The EMPL occupies three rooms on the lower level of Sarkeys Energy Center, comprising a total area of near 1000 ft². The microprobe itself is situated in a spacious room (#E106) of nearly 600 ft². Controlled atmosphere and dedicated uninterruptable power supply provide a superbly stable operating environment year-round. Complete sample preparation facilities, including grinding and polishing equipment, binocular microscope, and research grade petrographic microscope are available in the adjoining laboratory. Support equipment (rotary mechanical vacuum pumps and the dedicated water chiller for the microprobe) are located in the adjoining sample preparation laboratory and in a nearby equipment room, to minimize noise, vibration, and excess heat around the microprobe itself. Throughout its existence, operation of the facility has graciously been supported by the Office of Research Administration at the University of Oklahoma.