X-ray fluorescence (XRF) is an analytical technique which uses the interaction of x-rays with a target material to determine its elemental composition (i.e. the range of elements present and their proportions). XRF is a totally safe, non-destructive method. 

The Basic Process

The technology of the analyser 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 orbital’s 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 analysed. 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. 

Qualitative & Quantitative Analysis

XRF is mostly a quantitative technique – the peak-height for any element is directly related to the concentration of that element within the sampling volume. However, extreme care must be taken because two or more elements can interact with each other, resulting in contamination and thus skewed results. For example, chlorine atoms strongly absorb fluorescent X-Rays from lead – thus, if chlorine is present, the observed lead signal will be much less intense than expected for a particular concentration. 

The ability of fluorescent X-rays to penetrate through and escape from the sample itself depends on their energy, which directly relates to which elements are being detected. The lighter elements all have very low energy X-rays (e.g. Na, Mg, Al and Si) and thus it is difficult to detect these even at relatively shallow depths within the sample. Heavier elements (e.g., Cu, Ag, and Au) have higher energy X-rays, which are able to traverse larger distances within the sample. 

The sample composition itself is also an important factor- the higher the concentration of the heavier elements which absorb strongly, the lower is the chance of X-rays escaping from deep within the sample. 

Physics of X-ray fluorescence, in a schematic representation