What is an 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.
The "Electron Probe" part
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.
The "Micro" part
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.
The "Analyzer" part
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.
How does the Electron Microprobe work?
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What is the Electron Microprobe for?
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.
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