Throughout the history of electron-beam X-ray microanalysis, analysts have made good use of the strong dependence of electron range on incident energy (R ≈ E1,7) to optimize the analytical volume when attacking certain types of problems, such as inclusions in a matrix or layered specimens. The “conventional” energy range for quantitative electron beam X-ray microanalysis can be thought of as beginning at 10 keV and extending to the upper limit of the accelerating potential, typically 30 - 50 keV depending on the instrument. The lower limit of 10 keV is selected because this is the lowest incident beam energy for which there is a satisfactory analytical X-ray peak excited from the K-, L-, or M- shells (in a few cases, two shells are simultaneously excited, e.g., Fe-K and Fe-L) for every element in the Periodic Table that is accessible to X-ray spectrometry, beginning with Be (Ek =116 eV) and extending to the transuranic elements. This criterion is based upon establishing a minimum overvoltage U = E0/Ec > 1.25, which is the practical minimum for useful excitation.