X-ray fluorescence (XRF) is the phenomenon where a material is exposed to X-rays of high energy, and as the X-ray (or photon) strikes an atom (or a molecule) in the sample, energy is absorbed by the atom. If the energy is high enough, a core electron is ejected out of its atomic orbital.

An electron from an outer shell then drops into the unoccupied orbital, to fill the hole left behind. This transition gives off an X-ray of fixed, characteristic energy that can be detected by a fluorescence detector. The energy needed to eject a core electron is characteristic of each element, and so is the energy emitted by the transition. The transition of an L shell electron dropping into the K shell is termed a Kα transition, while an M shell electron dropping into the K shell is a Kβ transition.

When the energy source is a synchrotron, or the X-ray are focussed by a optic, like a polycapillary, the X-ray beam can be very small and very intense, and atomic information on the sub-micrometer scale can be obtained.

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 determination 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

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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:

• electrons ejected by photoelectric effect: X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA)

The de-excitation also gives ejection of Auger electrons, but the Auger electron spectroscopy (AES) uses an electron beam as primary beam.

See also

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• List of surface analysis methods

External links

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• xrf-l@listserv.syr.edu mailing list
• The Science of Spectroscopy - supported by NASA. Spectroscopy education wiki and films - introduction to light, its uses in NASA, space science, astronomy, medicine & health, environmental research, and consumer products.

References

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Van Grieken, R. E.; Markowicz, A. A., Handbook of X-Ray Spectrometry. 2nd ed.; ISBN 0824706005 Marcel Dekker Inc: New York, 2002; Vol. 29.

Atomic physics | Molecular physics | Spectroscopy X-rays

Spectrométrie de fluorescence X | Spettrofotometria XRF | Röntgenfluoreszenzanalyse | Röntgenfluorescentiespectrometrie