Physical and Chemical Effects
1. Low fluorescence yields from these k-lines, compounded by significant matrix absorption at "normal" accelerating voltages (15-20 keV), the relatively poor reflectivity of traditional analyzing crystals (e.g., STE), and the low sensitivity of detector systems, can result in extremely low count rates. Increasing the beam current can produce unacceptably high deadtime corrections for any metal lines being measured, although Probe for EPMA now can accommodate "combined conditions" where certain low intensity elements are run at high beam currents, and other high intensity elements are run at low beam currents. Count rates can be enhanced by use of layered dispersive element (LDE) reflectors as well as decreasing the absorption path length by running at lower operating voltages.
2. Errors in the estimation or accuracy of the takeoff angle or operating voltage of the microprobe. While slight variations in these parameters normally have a little effect on a typical analysis, due to the very high absorption corrections of low energy x-rays, the effect on the ZAF correction can be much larger for the light elements and can sometimes result is large systematic errors.
3. Errors in the mass absorption coefficients (or MACs) for the x-ray absorption cross sections for these elements are considerable. See Appendix C for a table of MACs for the light elements from a variety of sources. There is considerable disagreement among investigators. Probe for EPMA allows the user to enter MACs from any source for re-calculation purposes.
4. Chemical bonding effects can results in large peak shift and/or shape changes to the analytical x-ray lines. In fact the degree of shift and/or shape changes in light element K lines and transition metal L lines can sometimes be related to the degree of oxidation in the compound. But normally these effects are a nuisance to the analyst. In a few instances there has even been documented cases of particular lines being strongly enhanced (Ni Lα in NiAl) or reduced (B Kα in nickel borides) (Pouchou and Pichoir, 1988).
5. Mobilization toward or away from the beam ("volatilization") in some glasses and a few minerals of certain light elements such as fluorine and oxygen as well as alkali elements such as sodium and potassium to a lesser extent is often the case. This requires special attention, either by operating a lower current, defocusing the beam, using a cold stage (not common), or using a robust software correction. See the section on the Time Dependent Intensity (TDI) element correction in this document.
6. Crystallographic orientation can play a role for analysis of certain phases. This has been well documented for some crystalline borides, and also when measuring fluorine in apatites.