Problems with the Measurement Techniques
1. Higher order interferences from metal lines are everywhere and must be avoided when possible or corrected for in software when not. The elements Ti, Cr, Mn, Zr and Nb are well known in this respect. See Donovan et al (1993). These interferences are especially severe for minor and trace level measurements. Use of pulse height analysis to reduce those interferences that are caused by high order interferences (>1) is a logical choice; however it can introduce further problems if not used with care. In particular, narrow windows required for discrimination of second order peaks may be so narrow that a shift in pulse location ("pulse height depression", as pulse voltage drops with high count rate) will produce significant errors. Similarly, the pulse location may shift between the standard (pure element) and the unknown in extreme cases.
2. Carbon contamination is a serious problem for quantitative analysis of light elements, especially of course carbon, but also other light elements. Contamination is produced by the polymerization of hydrocarbons at the point of impact of the electron beam. Carbon compounds present may include mainly rotary but also some diffusion pump oils, polishing agents, cleaning solvents, and "canned air" propellants, among others. The use of an air jet and cold finger has been shown to dramatically reduce carbon contamination, though the use of a Freon chilled baffle at the top of the diffusion pump will completely eliminate back streaming of oil from a diffusion pump. A turbo pumped system is expensive but also eliminates oil contamination from the pumping system. It is suggested that the analyst perform measurements to ascertain the actual carbon contamination rate in their microprobe for each element being analyzed.
3. Trace analysis of light elements is particularly difficult, especially oxygen and carbon. Keeping a metallic sample surface from oxidizing is not a trivial matter. The use of an ion mill inside the probe to remove the very top layer of the sample, just prior to the analysis, if available, would be ideal. Note that the increase in temperature of non-thermally conducting samples can be significant enough to cause additional oxidation of the surface. Trace carbon analyses can be difficult without the use of an air jet to reduce the carbon contamination at the point of impact of the beam; however, it has been found to be counterproductive for oxygen analysis (i.e., it enhances oxidation under the beam).
4. Another problem which has not been adequately addressed is x-ray production in non-conductive samples. Effects on x-ray production are seen even when a conductive coating has been applied, possibly due to internal charge buildup below the conductive layer. This effect may also be implicated in the "Time Dependent Intensity (TDI) element" behavior observed in glasses and some minerals.
5. The selection of standards is also critical due to the already mentioned peak shift and/or shape changes for light element x-rays. The use of Area Peak Factors (APFs) (Bastin and Heijligers, 1986) can provide the analyst with a practical method for treatment of these effects. Probe for EPMA supports the use of APF factors for analysis of light elements. See the previous sections on using APFs for correcting for peak shape and shift effects when analyzing light elements.
Many analysts use high purity metals as standards for various elements, without realizing that many metals develop oxide skins, which will reduce the actual number of x-rays of the particular element of interest (e.g., Al, Ti, Mn being among the worst). One either needs to repolish immediately prior to probing, or spend some time estimating the oxide content, or find an alloy that doesn't oxidize as quickly.
6. Be aware also that carbide, boride and nitride standards are often pressed and sintered powders and are not 100% dense, resulting in significant errors in the analysis. If single crystals, or arc-melted pieces (e.g., boron) can be obtained, so much the better.