Analytical strategies to evaluate the in-depth degradation processes in protein-based heritage substrates exposed to ion beam techniques

This study is aimed at proposing an analytical protocol to study the in-depth effects of ion beams technique on protein-based heritage materials such as parchment and silk, exploiting the penetration of Near-Infrared (NIR) and of confocal micro-Raman (μ-Raman) spectroscopy. The objective is to verify if the two techniques are suitable to identify the main chemical modifications induced by varying proton beam fluences. As a proof of concept a series of parchment and silk samples were exposed to varying doses of proton beam irradiation (from 0.125, up to 20 μC/cm2, provided by ATOMKI) and then submitted to micro-FTIR spectroscopy (μ-FTIR) in mapping mode (7000–675 cm−1) and μ-Raman spectroscopy. By collecting three-dimensional dataset of irradiated versus unirradiated regions, the extent of degradation in response to different levels of proton beam dosage was identified employing a multivariate statistical approach based on principal component analysis (PCA). PCA proved efficient in reducing the dimensionality of the large spectroscopic datasets and in highlighting the most relevant spectral features responsible for the irradiation-induced modifications across the near-infrared and mid-infrared regions. Results from this study indicate that μ-FTIR mapping in the NIR spectral region enables intuitive identification of modifications induced by proton beam fluence range from 0.5 to 20 μC/cm2 for parchment and 1 to 20 μC/cm2 for silk. In addition, the μ-Raman analysis revealed that deeper layers of the samples are susceptible to damage at doses as low as 0.125 μC/cm2. This finding provides valuable insights into the vulnerability of protein-based materials when subjected to ion beam analysis (IBA), contributing to the determination of safe analytical conditions for performing such analyses.