Choosing optimal acquisition conditions for XRF analysis is a complex and critical partof the art. Selecting the proper acquisition conditions can mean the difference between measuring an element at PPM levels, or not seeing it at all. There are two fundamental principles that must be met to achieve optimal analysis conditions.
A) There must be a significant source peak above absorption edge energy of the element of interest. This may be either the K or L edge depending on which one is within the measurable range of the instrument with the preference usually going to K line measurements. The closer the source energy is to the absorption edge, the higher the intensity and sensitivity (counts/sec/PPM) will be for the element of interest. The ideal energy would be precisely at the absorption edge energy, but that is usually not possible.
B) The other fundamental principle is that the background x-rays within the element of interest region should be reduced as much as practical.