EDX is usually done on a Scanning Electron Microscope and can be used to determine chemical composition for regions as small as 0.1µm at low voltage, even smaller for elements of high atomic number. The depth of analysis also can be as little as 0.1µm or, again, much less for elements of high atomic number. These two parameters are functions of the beam voltage and the sample composition. In this process, the sample is bombarded with a beam of electrons.

Occasionally, one of the incoming electrons will knock a core shell electron loose and a second electron will fall into its place, losing some energy in the process which is given off as a photon of energy in the X-ray range. The energy with which it leaves the atom is a function of only the energy levels of the 2 electons of the atom and so is characteristic of the particular set of interactions and the element. Thus, by knowing the energy of the X-ray photon, one can determine from what element it was emitted. The system we use can detect all elements of atomic number greater than 4, beryllium. The detection limits are about 0.1% for most elements except fluorine (0.5%), oxygen (0.5%), nitrogen (2%), carbon (0.5%) and boron (2%). Quantitative determinations are done using well - established theoretical models that in most cases provide good accuracy. The accuracy will be best if the sample is flat and smooth and standards are available. It is generally accepted that in such cases, the accuracy is ~± 0.5% relative. This is lower for samples of low atomic number, samples that are not smooth and for samples without standards.