X ray spectrometry

X ray spectrometry (XRS) techniques are used for the elemental, chemical, crystalline, structural and dynamic analysis of a broad range of materials fulfilling a wide variety of requirements.

These techniques provide exceptional spatial resolution (down to few tens of a nanometre), but can also be used to analyse and produce images of large areas, up to the size of several square metres. Their application is generally not restricted by the physicochemical state of a sample (liquid, frozen or heated) or by other environmental factors since measurements can be performed either in a vacuum or under atmospheric pressure, in specialized experimental chambers or, using optimized portable spectrometers, even in the field.

Progress of the technology over the years

Two factors have largely motived and driven the various developments in the analytical performance of XRS techniques since the early 1970s: advances made in developing X ray instrumentation, such as sources, detectors and focusing devices; and the analytical requirements of a more integrated characterization of complex, 3-D heterogeneous materials. An example for this is the introduction of liquid cooled semiconductor detectors, which slowly helped establish X ray fluorescence (XRF) as an analytical technique for the quantitative elemental analysis of different kinds of samples.

In the mid-1990s, the miniaturization of components and the development of thermoelectric cooling opened up the possibility to conduct field analyses and to use the technology for planetary exploration. At the same time, synchrotron sources became more widely used to characterize materials, based on the unique properties of synchrotron radiation. This in turn helped further the development of a plethora of advanced XRS methodologies.

Today, detection systems made by large arrays of sensors are used to markedly improve the throughput in laboratory- or synchrotron-based experiments, employing micro- or nano-focused excitation X ray beams. Some of these come even close to the diffraction limit, in the region of a few tens of nanometres.

A number of other uses

Beyond its use for energy dispersive detection systems, high resolution X ray spectrometry supports a wide range of other applications. These are related to materials science, chemistry, solid state physics (in particular the study of phase transitions), physical chemistry and studies on fundamental atomic physics.

Cryogenic particle detectors operating at very low temperatures and crystal spectrometers with spherically bent crystals also offer a number of advantages. Crystal-based high resolution spectrometers used in conjunction with a position sensitive detector can, for example, offer ultimate energy resolution at the bandwidth of X rays. Those spectrometers are used together with specialized methodologies, such as the resonance inelastic X ray scattering and X ray Raman techniques, and offer unique information on chemical environment, ligand bonding and the delocalization of valence electrons, among others.

More recent trends are to integrate various X ray spectrometry-related techniques and methodologies in the same laboratory or synchrotron set-up, with the effect that they can be deployed more effectively for various interdisciplinary uses.

One example is the grazing incidence XRF analysis that, in combination with X ray reflectometry and absorption techniques, are used to characterize heterogeneous micro- and nano-scaled materials, for example for batteries, fuel cells or photovoltaic systems. Another are confocal micro-XRF set-ups, which in combination with X ray transmission and XRF micro-tomography, can be used to investigate the elemental distribution and structure of a broad range of 3-D heterogeneous materials. These find important uses in biology, cultural heritage analysis and geology.