What is EPMA

• Electron microprobe analysis -- or electron probe microanalysis -- or electron microbeam probe analysis -- is a technique developed by R. Castaing in the late 1940's as his Ph.D. dissertation in Paris.
• A beam of energetic electrons, of say, 15,000 volts (but there is nothing sacred about 15 KeV; sometimes I use 7 KeV, other times 30 KeV) is focused upon an object of interest. The electrons are scattered elastically, resulting in an 'excitation volume' much larger than the actual diameter of the beam -- this volume usually having a diameter on the order a several microns.
• A variety inelastic scattering events also occur, yielding information about the chemistry and structure of the target. The products that we mainly are concerned with give us chemical information at the ~micron spatial level:
  • X-rays
  • Back-scattered electrons
  • Light (cathodoluminescence)
  • and to some extent, secondary electrons, which tell us about surface topography.
• I shouldn't need to remind you, but we can look at x-rays (and other packets of energy) from either their particle description (so many electron volts of energy) or from their wave-like feature (wavelength, in say angstroms or nanometers).
• Electron microprobes are outfitted with wavelength dispersive spectrometers (WDS), and in many if not most cases have an energy dispersive one (EDS) as well.
  • WDS operates with a small fraction of the x-rays that escape from the sample, reaching a crystal, and a particular wavelength will be defracted at a particular angle (the Bragg angle) into a gas-filled amplification tube, where a pulse is produced that is proportional to the energy of the x-ray. A WDS spectrometer is tuned to one wavelength and sits there and counts the numbers of x-rays. You might compare it to a serial information source, getting one channel of information at a time.
  • EDS, on the other hand, looks at all the x-rays coming off the sample, and stretching the analogy, is a parallel information gathering device, collecting all the different wavelengths. The EDS detector is a solid-state device, where electron-hole pairs are produced for each x-ray that hits it, with the number of pairs a function of the total x-ray energy divided by the energy needed to produce one pair. This small signal is amplified, and then becomes one bit of information in a spectrum gathered for the sample ('multi-channel analyzer').
  • WDS and EDS each have their strengths and their weaknesses (more below)
• Standards and calibration: the quantitative analytical routine, in a nutshell, is in two parts: first you determine the x-ray count rate on standards (should be well characterized and homogeneous material), then you collect x-rays on your unknown usually the same conditions, and then ratio the two. That gives you the "k-ratio", which is roughly equivalent to the weight fraction of your unknown, if the standard were pure element. The second part is that there must be a matrix correction: that is, the x-rays you collect reflect several processes, with a major one being absorption as they exit the sample. This correction has several formulations, with 'ZAF'' a standard type.
• Good standards are essential, and you may be called upon to produce them yourself if we do not have appropriate ones. Always ask first.
• Images are very important, both for determining locations to analyze, as well as documenting what you analyzed. Backscattered electron (BSE) images are one main image collected, and are produced rapidly for areas up to 1-2 mm in dimension. Larger areas are possible but might take several hours. X-ray maps can also be obtained, and generally take significantly longer to produce, due to the lower efficiency of X-ray acquisition by WDS. Alternately, x-ray maps using EDS can also be obtained, but the thru-put of x-rays in the channels per element is not much better than WDS, so again it takes some time for larger areas.
• BSE images can give important information about the spatial relationships of adjacent phases, plus about zoning and inclusions within phases. BSE intensity is, to a first approximation, a function of the chemical composition: the brighter an area, the greater the mean atomic number of that area relative to adjacent areas. Thus ilmenite, magnetite and chromite will be brighter than most adjacent silicate minerals. Apatite or zircon similarly. Olivine or pyroxene are brighter than adjacent feldspars, though the Fe/Mg ratio is of course important.
• Spatial resolution of a chemical analysis: because of the elastic scattering of the electrons, the x-rays are produced in an 'interaction volume' that can be several microns across and deep. This area is a function of two factors: the composition itself, so that in high atomic number materials the scattering is less than in lower atomic number ones; and the accelerating voltage: the more energy the electrons have, the greater distance they can scatter. It is possible to utilize minimal accelerating voltage, to minimize the scattering. I may be possible, in specifica cases, to constrain most of the electrons and thus most of the generated x-rays, to a layer of say 1 micron. But this is not typical....everything is in the details, like the exact composition of your sample.
• Light element analysis: this is somewhat complicated, and is do-able in theory, and in reality in some cases. Appropriate standards, conductive samples (no carbon coating necessary) and low accelerating voltages are essential.
• Trace element analysis: what do you mean by trace? We can measure down to the hundred ppm level in many samples, depending on their nature and how well they can stand up to high current. In some samples, 50 ppm or below is possible, but again "it all depends on what you got". Trace element mapping is virtually impossible.
• Coating of specimens -- if your samples are not conductors, they need to be coated, and ONLY coated in our lab if you are using our standards. We have had too many problems with samples coated elsewhere.

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