Basics
In X-ray fluorescence (XRF) spectroscopy, X-rays are directed at a sample, dislodging electrons from the atoms. When an electron is ejected from one of the tightly bound inner electron shells of an atom, a “hole” is created in the 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 measures the energy of this emitted radiation. Since the energy of fluorescent radiation is element-specific, the amount of specific elements in the sample can be determined.
Benefits
XRF spectroscopy can accurately measure both major and trace elements in a material. The high penetration of X-ray radiation allows for the examination of internal structures and buried features of materials, and in some cases sample containers can be used without affecting the measurement. Synchrotron-based XRF spectroscopy, due to its high-intensity X-ray source, provides much more accurate measurements of element concentrations compared to 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 micrometres. For instance, certain paints can be detected in painted-over layers in historical artworks, allowing the hidden artwork to be seen. A variation of 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 to determine structures inside geological samples.
Types of samples
Almost any solid or liquid sample can be measured. As XRF is not a diffraction technique, the sample material does not need to be crystalline or ordered, though for elemental analysis, the sample must be completely homogeneous. The high intensity of the X-ray source at DESY allows for analysis of much smaller samples 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 distinguish between different isotopes of an element.
Applications of X-ray fluorescence spectroscopy at DESY
The element-specific nature of XRF makes it particularly valuable for geological studies or any application requiring detailed elemental analysis. Its penetrating yet non-destructive radiation is also beneficial for cultural heritage applications, such as analysing hidden layers in paintings.
Examples of specific applications:
- Determining the concentration of valuable metals (e.g., copper, gold, uranium) in mineral deposits, rocks, etc.
- Identifying the constituents of inks and paints in historical documents and artwork.
- Detecting hidden layers in historical artwork.
- Determining trace element content in cells, microorganisms, environmental samples, or minerals.
- Analysing the structure of specific buried environments of geological samples.
- Performing position-resolved measurements of trace elements in semiconductors and microchips down to the parts-per-million level.
