Noninvasive Synchrotron-Based X-ray Raman Scattering Discriminates Carbonaceous Compounds in Ancient and Historical Materials

Abstract

Carbon compounds are ubiquitous and occur in a diversity of chemical forms in many systems including ancient and historic materials ranging from cultural heritage to paleontology. Determining their speciation cannot only provide unique information on their origin but may also elucidate degradation processes. Synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy at the carbon K-edge (280-350 eV) is a very powerful method to probe carbon speciation. However, the short penetration depth of soft X-rays imposes stringent constraints on sample type, preparation, and analytical environment. A hard X-ray probe such as X-ray Raman scattering (XRS) can overcome many of these difficulties. Here we report the use of XRS at ∼6 keV incident energy to collect carbon K-edge XANES data and probe the speciation of organic carbon in several specimens relevant to cultural heritage and natural history. This methodology enables the measurement to be done in a nondestructive way, in air, and provides information that is not compromised by surface contamination by ensuring that the dominant signal contribution is from the bulk of the probed material. Using the backscattering geometry at large photon momentum transfer maximizes the XRS signal at the given X-ray energy and enhances nondipole contributions compared to conventional XANES, thereby augmenting the speciation sensitivity. The capabilities and limitations of the technique are discussed. We show that despite its small cross section, for a range of systems the XRS method can provide satisfactory signals at realistic experimental conditions. XRS constitutes a powerful complement to FT-IR, Raman, and conventional XANES spectroscopy, overcoming some of the limitations of these techniques.

Carbon speciation in organic fossils using 2D to 3D x-ray Raman multispectral imaging

“…There is only a limited number of characteristic resonances at the carbon K-edge ( 20 , 25 ), and the spectra at this edge from the Lepidodendron trunk are well explained by a combination of two Gaussians centered at the 1s-π* and 1s-σ* energies and of an arctangent-shaped contribution to account for transitions to the continuum (Fig. 2A).…”

“…XRS has demonstrated a great potential to probe carbon speciation in homogeneous liquid and solid carbon–based samples that are poor in heavier elements (absorption of x-ray from the latter represents the main limitation of this technique). XRS has been shown to be a promising means to identify the chemical speciation of light elements in a range of systems from oil cuts to artists’ pigments ( 20 – 22 ). The proof of concept of imaging has been established on a model object ( 19 ), yet XRS imaging (XRI) of real-life materials has never been studied.…”

“…The 3D amber volume presented here was collected in a high-flux mode (10 13 ph/s), resulting in darkening of the exposed area after a dwell time of 2 min. Nevertheless, the spectra still carry the chemical information of the sample original chemistry, and none of the samples show signal corresponding to a substantially altered material ( 20 ). Optimization to further reduce the impact of irradiation is beyond the scope of this paper; however, we did carry out tests in a low-flux mode (20 times less) and a higher energy resolution, which shows promise to mitigate radiation damage.…”

“…Taking advantage of the capability of nonresonant x-ray Raman scattering (XRS) for direct tomography with chemical bond contrast ( 19 ), we develop 2D and 3D XRS spectral imaging for cultural heritage and geosciences. The large penetrative power of hard x-rays enables the measurement to be done in a noninvasive way, with no particular preparation nor specific experimental conditions, in air, and provides information that is not compromised by surface contamination by ensuring that the dominant signal contribution is from the bulk of the probed material ( 20 ). XRS 2D and 3D imaging are demonstrated against a fragment of Lepidodendron trunk from the Upper Carboniferous [ca.…”

Nouvelles spectroscopies Raman X du carbone pour les matériaux anciens

“…1). À ces énergies, les rayons X peuvent traverser jusqu'à plusieurs millimètres de matière ; il est alors possible d'étudier bien au-delà de leur surface la spéciation chimique de systèmes anciens [4,5]. Le spectre obtenu peut être analysé pour identifier des absorptions caractéristiques des différents groupements chimiques contenant du carbone composant l'échantillon.…”