Due primarily to the absence of Bremsstrahlung radiation, fluorescent spectra obtained with x-ray excitation of a sample have a lower background than those obtained with electron excitation. This can be used to detect elements in liquids at very low concentrations under certain conditions. The technique consists of placing a small measured volume of liquid to be analyzed for trace elements on a thin, low atomic number film; evaporating the liquid to dryness; placing it under an x-ray beam; and obtaining a fluorescent spectra. The residue from the evaporated liquid contains any trace elements present, which then produce characteristic peaks in the fluorescent spectrum. These peaks can be used to measure concentrations quantitatively.

This technique has been around for some time, but with detection limits at the parts per million (ppm) level. Although there are many definitions of detection limit, the most commonly used one is from Burton: