Shifted-excitation Raman difference spectroscopy and charge-shifting detection coupled with spatially offset Raman spectroscopy for heritage science†

Abstract

In situ measurements have great importance since in many scientific fields certain samples cannot be moved because of diverse reasons (excessive dimensions or weight, security, logistics etc.). In heritage science, this is a crucial requirement due to the high value of art objects, requiring non-invasive and in situ analyses. Therefore, it is important to have analytical methods capable of providing relevant information also outside laboratory environments. Such measurements face multiple challenges: for example, interference from ambient light or formation of artefacts due to undesired motions of the instruments. In Raman spectroscopy, a number of solutions have been demonstrated to mitigate these effects. For instance, Shifted Excitation Raman Difference Spectroscopy (SERDS) has proven efficient in removing the fluorescence of the sample and ambient light interference, and a charge-shifting detection approach was shown to be valuable in dealing with varying ambient light. In this study, we provide a comparison of conventional Raman spectroscopy, Shifted Excitation Raman Difference Spectroscopy (SERDS), charge-shifting detection technology and a combined SERDS and charge-shifting approach, in order to evaluate their effectiveness in mitigating fast evolving interfering backgrounds (e.g., varying ambient light). Further investigations were also carried out into the potential of coupling of these methods with Spatially Offset Raman Spectroscopy (SORS) to facilitate more effective non-invasive investigations of subsurface sample components (e.g. paint layers). The study was carried out using samples mimicking cultural heritage materials with different degrees of complexity and in the presence of fluorescence and ambient light interference. The results are, nevertheless, applicable more generally to other areas such as forensics or biomedical fields, where both dynamic and static interferences can hinder measurements.