X-rays form part of the electromagnetic spectrum and are expressed in terms of their energy (kilo electron volts - keV) or wavelength (nanometers - nm). X-ray fluorescence is a consequence of changes that take place within an atom.
A stable atom comprises a nucleus and the electrons orbiting it. Orbiting electrons are organized into shells: each shell is made up of electrons with the same energy.
When a high energy incident (primary) X-ray collides with an atom it disturbs this stability. An electron is ejected from a low energy level (eg K shell: see diagram) and a vacancy is created. As a result an electron from a higher energy level (e.g. L shell) falls into this vacancy. The difference in energy produced as the electron moves between these levels is released as secondary X-rays which are characteristic of the element. This process is called X-ray fluorescence. By measuring the energy of the secondary X-ray the identity of the element can be determined. By counting the number of X-rays emitted at each energy the concentration of the element can be determined.
