The Basic Process
The technology of the analyzer is based on energy dispersive X-ray fluorescence using an X-ray tube as the source of excitation. The range of detectable elements varies according to the individual instrument’s configuration, but typically EDXRF covers all elements from sodium (Na) to uranium (U). Concentrations can range from ‘100%’ down to ‘ppm’ (and in some cases sub-ppm) levels. Limits of detection depend upon the specific element and the sample matrix, but as a general rule, the heavier elements will have better detection limits.
X-ray fluorescence involves the emission of characteristic fluorescent X-rays from a material which has been excited by bombarding it with high-energy X-rays or gamma rays. X-ray fluorescence can be considered as a simple, three-step process occurring at the atomic level. First, an incoming X-ray knocks out an electron from one of the orbitals surrounding the nucleus within an atom of the material. A ‘hole’ is produced in the orbital, resulting in a high- energy unstable configuration for the atom. To restore equilibrium, an electron from a higher-energy, outer orbital falls into the hole. Since this is a lower-energy position, the excess energy is emitted in the form of a fluorescent X-ray. This is the ‘Energy Dispersive’ aspect of the process and it is this energy which is measured by the equipment.
The difference in energy between the expelled and the replacement electrons is characteristic of the element atom in which the fluorescence process is occurring – thus, the energy of the emitted fluorescent X-ray is directly related to the specific element being analyzed. It is this key feature which makes XRF such a fast analytical tool for elemental composition.
It should be noted that in general the energy of the emitted x-ray for a particular element is independent of the chemistry of the material. For example, a calcium peak obtained from CaCO3, CaO and CaCl2 will be in exactly the same spectral position for all three materials.