In X-ray fluorescence (XRF) a material is exposed to X-rays with a relatively high energy. These photons are capable of exciting (ejecting) the electrons in the core levels of the material under investigation. The induced excited state relaxes under emission of a X-ray photon with a smaller energy. This emitted light is analysed in a spectrometer. Because the core levels have very different energies for different elements the XRF spectrum contains information on the elemental composition of the sample under investigation.
Typically the lightest element that can be analysed is beryllium (Z = 4), but due to instrumental limitations and low x-ray yields for the light elements, it is often difficult to quantify elements lighter than sodium (Z = 11).
There are two types of spectrometer:
- wavelength dispersive spectrometers (WDX or WDS): the photons are separated by diffraction on a single crystal before being detected;
- energy dispersive spectrometers (EDX or EDS): the detector allows the determinaion of the energy of the photon when it is detected; the EDX spectrometers are smaller (even portable), cheaper, the measurement is faster, but the resolution and the detection limit is far worse than the WDX spectrometers.
Other spectroscopic methods using the same principle
It is also possible to create a characteristic secondary X-ray emission with other incident radiation to excite the sample:
- electron beam : electron microprobe (or Castaing microprobe);
- ion beam: particle induced X-ray emission (PIXE).
When radiated by an X-ray beam, the sample also emits other radiations that can be used for analysis:
The de-excitation also gives ejection of Auger electrons, but the Auger electron spectroscopy (AES) uses an electron beam as primary beam.
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