ZAF is not very good for elements with X-ray energies less than 1 keV because of a lack of knowledge of the factors discussed below. For these elements it is best to use a standard of similar composition to minimize matrix effects. For example, when analyzing F in apatite, use a fluorapatite standard rather than a fluorphlogopite standard. The atomic number effect controls the amount of incident energy from the electron beam that is actually put into the sample. Both are a function of average Z and, to a lesser degree, the accelerating voltage.
Backscattered electrons leave the sample carrying energy without producing X-rays.
At lower Z, more electrons stay within the sample to produce X-rays. The factor rarely exceeds 1.2 for Z less than 30. The characteristic intensity per unit concentration of a heavy element is less when combined with a light element than in a sample of the pure element. The effects of backscattering and stopping power are opposing and can be canceling. For example, in the analysis of Fe 2 O 3 with pure iron as a standard, the stopping power of oxygen reduces the intensity of Fe-K a from the oxide relative to that from the pure standard.