ATR-FTIR spectroscopy and chemometric complexity: unfolding the intra-skeleton variability

Spectroscopy of skeletal tissues is increasingly used in various fields, including legal medicine, forensic science and archaeology. As it is fast and technically and financially affordable yet accurate and widely applicable, spectroscopy is often used when investigating skeletal tissues. Despite its usefulness, the heterogeneity of skeletal tissues highly affects their chemical composition and complicates the interpretation of the spectroscopy results. Though the research on the use of spectroscopy and skeletal tissues from various contexts is growing, little is known regarding the differences in the chemometric indices caused by intra-skeleton variability. The objectives were a comparison of the chemometric indices between teeth and bones, between different bone classes and between the skeletal elements within a bone class as well as an attempt to correlate the observed similarities or differences in chemometric indices to the functional or structural differences of skeletal elements. Different skeletal elements of three individuals were analysed with attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy and compared. Significant differences between the chemometric indices of the bones and teeth were observed and meaningfully correlated with their structural differences. Though more subtle and harder to understand, significant differences were also observed between and within the bone classes and were tentatively correlated with their structural and functional differences. The observed variability agrees with other studies that stress the importance of intra-skeleton variability, which should be acknowledged when using spectroscopy to investigate skeletal tissues.