X-Ray Emission Spectrometry

Stylianos Aspiotis, Olivier Bonnerot

Elemental analysis by X-ray emission techniques relies on the study of characteristic patterns of X-ray emissions from atoms irradiated with high-energy X-rays or particles: X-ray fluorescence (XRF), particle-induced X-ray emission (PIXE), electron microprobe analysis (EMPA), and energy-dispersive X-ray spectroscopy (EDX) (Hitachi S-520, Microbeam SX 100 SEM System, Cameca). They are the ultimate analytical methods for a qualitative (e.g. XRF) and quantitative (e.g. EMPA) determination of the elemental composition of various solid materials. When the external excitation beam interacts with an atom within the sample, an electron is ejected from the atom's inner shell, creating a vacancy. In the next step, another electron from an outer shell fills the vacancy, emitting X-rays in the process. The energy of the emitted X-ray fluorescence is characteristic for a certain element, whereas the signal intensity is related to the amount of the element in the sampled volume. It is noteworthy that each technique has its limitations such as different penetration depths, so that excitation by electrons (EDX or EMPA), conventionally used with electron microscopy, and by protons (PIXE) is limited to the study of surfaces and is capable of detecting light elements. Excitation by X-rays (XRF) has greater penetration power and allows the detection of elements heavier than aluminium (Z ≽ 13), meaning that is not well suited for the study of organic materials which are mostly constituted of carbon (Z = 6), oxygen (Z = 8) and nitrogen (Z = 7). EMPA, on the other hand, can be used for the qualitative chemical analysis of compounds containing elements from Be to O and quantitative analysis of materials containing elements from fluorine (F) to uranium (U). Since EMPA (EDX or WDX) is performed in a vacuum chamber, the analysis of objects which can fit in the chamber and can withstand vacuum conditions and electron bombardment limits the application field of this method. On the contrary, XRF and PIXE are particularly well-suited to analyse cultural heritage objects, as the spectrometers can be operated in an open-beam configuration. However, PIXE requires a particle accelerator as an excitation source, which is found only in a selection of facilities. XRF, on the other hand, benefits from the availability of a variety of transportable instruments ranging from single-spot measurements to high-resolution scanning equipment, allowing measurements to happen directly on the objects in museums, libraries or even archaeological sites. For that reason, XRF has become one of the most used techniques for elemental analysis of cultural heritage artefacts, despite its limitations.