X-ray Fluorescence (XRF) Spectroscopy

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Basics

In X-ray fluorescence (XRF) spectropscopy, X-rays are applied to a sample material, dislodging electrons from the atoms. However, if the ejected electron comes from one of the tightly-bound inner shells of electrons of an atom, a very unfavourable “hole” is left in the electron shell. Another of the atom’s electrons then fills this hole, and the change in energy is accompanied by emission of a new photon of radiation - this is known as fluorescence. XRF spectroscopy involves measuring the energy of the outgoing radiation, and since the energy of fluorescent radiation is element-specific, the amount of a certain element in the sample can be determined.

Benefits

XRF spectroscopy can be used to accurately measure both the major constituents of a material and its trace elements. The high penetration of X-ray radiation means that internal structures and buried features of materials can be probed, and in some cases sample containers can be used without affecting the measurement. Due to its high intensity X-ray source, synchrotron-based XRF spectroscopy is able to measure element concentrations much more accurately than conventional XRF spectrometers, even down to the parts-per-billion range. Another advantage of the technique is the small sample area required for measurement, allowing elements to be “mapped” over a surface of the material with a resolution of a few micrometers. For instance, certain paints can be detected in painted-over layers in historical artworks, allowing the hidden artwork to be seen. A variation on the XRF technique is X-ray fluorescence tomography, which provides this analysis of a sample in three dimensions. Confocal XRF is a related technique in which a very finely focused X-ray beam is applied to a sample and a detector at an angle to this allows the measurement of very small, and buried, spots of samples. This technique is particularly useful for determining structures inside geological samples.

Types of samples

Almost any solid or liquid sample can be measured. As this is not a diffraction technique, the sample materials need not be crystalline or ordered, although for elemental analysis purposes, the sample must be completely homogeneous. The high intensity of the X-ray source at DESY means that much smaller samples are needed than in conventional XRF spectroscopy. However, the accuracy of analysis of the lighter elements (i.e. anything lighter than sodium) is limited, and the technique is unable to detect distinguish between different isotopes of an element.

Applications of X-ray fluorescence spectroscopy at DESY

The element-specificity of XRF means that it is a particularly useful technique for geological applications or any application where the identity and amount of the elements in a sample is required, e.g. elemental analysis. The penetrating yet non-destructive nature of the radiation source also makes XRF a popular technique for cultural heritage applications such as paintings with hidden layers.

Examples of specific applications:

  • Determining the amount of valuable metals (e.g. copper, gold, uranium) in mineral deposits, rocks, etc.
  • Determining the constituents of inks and paints in historical documents and artwork.
  • Detection of hidden layers in historical artwork.
  • Determination of trace element content in cells, microorganisms, environmental samples, or minerals.
  • Determination of structures of specific, buried environments of geological samples.
  • The position-resolved measurement of trace elements in semiconductors and microchips down to the parts-per-million level.