Elemental Analysis Using X-Ray Fluorescence
X-ray fluorescence is an analytical technique that can be used to determine the elemental composition of solid substances. A beam of x-rays strikes the surface of the sample and a core electron is ejected from the atom that absorbed the x-ray photon. When an outer electron falls into the hole created by the ejected electron, it gives off energy in the form of light (see Figure 1). This light is called fluorescence and a characteristic pattern exists for each element.
Different types of x-ray fluorescence lines are observed for all elements:
- K radiation, produced when the electron falls into a hole in the n=1 level
- L radiation, produced when the electron falls into a hole in the n=2 level
- M radiation, produced when the electron falls into a hole in n=3
and so on. (Click the figure to view an animation of fluorescence.)
The K, L, or M describes the level in which the electron comes to a stop. A Greek letter describes how many levels the electron fell before stopping, for one level, ß for two levels and so on. For example, if the electron filling a hole in the K level drops from the L level the resulting line is called K. Similarly, an L line is produced when an electron falls from M to L. If the electron fell from M to K, however, the line produced is Kß.
The x-rays are generated by applying 80,000 V at 30mA to a Mo/Sc source. The emitted x-rays are diffracted using a LiF crystal with 2d = 4.0267 Å. Under these conditions there is sufficient excitation energy to observe the K radiation for the elements from potassium to barium and the L radiation for the elements from palladium to uranium. No elements below potassium in the Periodic Table will be observed.
Potential problems with the x-ray fluorescence technique include: the existence of second and third order peaks, spectral lines from two different elements that overlap, and the presence of broad bands which result from inelastic photon scattering by electrons. To improve accuracy, the experiment is reproduced using different crystals, sources, and voltages, or by using a filter.
The Bragg equation (shown below) relates the wavelength, , of the x-ray fluorescence to , the angle between the incident light and the surface. In this expression, n is the order of the diffracted light (usually first order) and d is the spacing of the diffraction grating.