Carbon can take various forms depending on the function of the molecule in which it is placed, each with its own distinct ‘species’. The ‘speciation’ of carbonaceous materials is generally obtained by methods requiring destructive sampling or through surface analysis methods such as optical Raman spectrometry, infrared microscopy or X-ray absorption spectroscopy (XAS). The last technique is routinely available at synchrotron facilities, however a limitation remains in the use of the XAS method for many carbonaceous compounds: the strong interaction of these soft X-rays at the carbon absorption edge imposes severe limitations on the sample and its environment. In particular, its application is limited for complex, heterogeneous, fragile and/or rare samples, such as heritage objects or paleontological specimens, as these systems might have contaminated surfaces and cannot be put in a vacuum environment.
An unexpected application of X-ray Raman scattering
An international team of researchers from France (Paris–Saclay, Sorbonne Universités, and Grenoble), Manchester in the United Kingdom and the United States (Stanford, Columbia, and Charleston) have adapted X-ray Raman scattering (XRS) to these systems. This method, which has been previously applied to study the structure of carbon speciation related to fossil fuels, utilizes more penetrating harder (higher energy) X-rays. In XRS, a small fraction of the X-ray energy is transferred to the electrons by inelastic scattering, yielding a detectable signal to diagnose compositional chemistry of a sample. Therefore, it is possible to carry out measurements similar to that achievable with XAS but at an energy more than 20 times higher. This makes it possible to work at ambient conditions and with bulk probe depth avoiding many of the limitations of the conventional soft X-ray methods like XAS.
Characterizing historical and ancient materials
The results are published in the journal Analytical Chemistry and involve three international synchrotron facilities: SOLEIL, where experiments were carried out on the GALAXIES beamline, as well as the ESRF in France and the Stanford Synchrotron Radiation Lightsource in the US, where preliminary studies were performed. The scientists conducted the work during the invited visit of one of the team members, Uwe Bergmann, to IPANEMA (Paris-Saclay), funded by Fondation des Sciences du Patrimoine and SLAC National Accelerator Laboratory. The team demonstrated the use of XRS for the study of artistic samples (carbon-based pigments), archaeological, and paleontological samples (fossils). This research shows how XRS successfully provides a new way to image ancient worlds. The pivotal role that carbon plays in life can now be further scrutinized using the extreme bright X-rays from modern synchrotron light sources.
Pierre Gueriau, Jean-Pascal Rueff, Sylvain Bernard, Josiane A. Kaddissy, Sarah Goler, Christoph J. Sahle, Dimosthenis Sokaras, Roy A. Wogelius, Phillip L. Manning, Uwe Bergmann & Loïc Bertrand, Noninvasive synchrotron-based X-Ray Raman scattering discriminates carbonaceous compounds in ancient and historical materials, Analytical Chemistry, 2017. doi : 10.1021/acs.analchem.7b02202