Expanding the horizons: Raman probe development and spectra preprocessing evaluation for recognition of large hydroxyapatite-based samples

This paper addresses the challenge posed by the size of certain objects that do not conform to the constraints of microscope-coupled Raman spectrometers, making sample collection impossible due to their inherent value or nature. Specifically, materials like hydroxyapatite-based substances used in artistic and ornamental carvings, such as bone or ivory, fall within this problematic category. The focus of this study is the enhancement of analytical capabilities in the context of large objects using a Raman microscope system. The study details the innovation involving a remote probe integrated with fiber optics, elaborating on the design and performance aspects, and emphasizing the appropriateness of chosen components in the analysis of ivory artifacts belonging to private collectors. In order to assess the robustness of our discriminative approaches, an archaeological bone and the exposed dentine in a human tooth were also evaluated and compared. Results showed that using an 805-nm longpass dichroic mirror successfully directed the near-infrared laser onto the samples and significantly suppressed the Rayleigh scattering contribution to the spectrum. Regarding the preprocessing methods to spectra evaluation essayed, the most promising approach was the use of principal component analysis for dimension reduction followed by k-means cluster analysis. By leveraging the complementary strengths of PCA and k-means clustering, the robustness and interpretability of clustering analyses are enhanced.