Determination of collagen by pyrolysis/GC-MS. Evaluation of the degree of conservation of archeological bones from Vicenne (Italy) by comparison with XRD, TGA and FTIR analysis

Human bones and teeth are frequently recovered at archaeological sites. Their state of preservation may depend on the mode and the burial environment. The content of collagen and the degree of crystallinity of carbonate hydroxyapatite (HA) are among the indicators adopted to evaluate the conservation status of bones. Analytical pyrolysis (Py) [1] together with X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA), were used at this scope. In this work, a new quantitative procedure in Py was employed to characterise residual proteins in five bone samples from the medieval necropolis of Vicenne-Campochiaro (Molise, Italy)[2]. The yields of cyclic dipeptides (2,5-diketopiperazines, DKPs) evolved form the pyrolysis of the samples, including the cyclo(proline-hydoxyproline) as distinctive marker of collagen, were determined by GC-MS with and without silylation. The detection of DKPs enabled the identification of collagen in all the analysed samples, in accordance to the FTIR spectra showing the characteristic amide peak. The presence of organic matter along with that of carbonatic phases was confirmed and estimated by TGA. XRD data showed that the samples mainly contained HA having different degrees of crystallinity; small amounts of quartz and calcite were also detected in some samples. The quantitative experimental data were combined to provide a relative estimate of the degree of conservation of the bone samples. The bones of an adult young female (t.139) and an aged male (t.165) resulted to be the worst and best preserved, respectively. The tombs were located in the same area where the acidity of the soil has damaged nearly all the skeletons. The skeleton from t.165 was almost complete, whereas the one from t.139 was lacking in many bones. Therefore biological (age-at-death, sex) and ritual (care, depth) factors as well as specific conditions of each burial could be involved in the observed different preservation state.