3.3 Collagen extraction and sulphur isotope analysis

Collagen, the main protein component of bone as well as dentine, was extracted from the skeletal material in a laboratory dedicated to bone chemical analysis. The skeletal specimens were cleaned using deionised water and the outer surface was removed prior to sampling. Bone or dentine powder was obtained from each sample using a dentist's drill. Tooth samples were taken from the crown and cervix of the tooth, unless otherwise stated. Collagen was subsequently extracted following a modified Longin method (Brown et al. 1988). In short, this method includes the following steps: samples were demineralised for approximately 48 h in 0.25 M HCl. Inorganic materials were subsequently removed by filtration and the remaining organic material was rinsed in deionised water and then gelatinised in 0.01 M HCl at 58ºC for c. 16 h. The solution was filtered in a 30 kDa ultrafilter in order to eliminate fragmented collagen peptides, as well as other contaminants such as some humic substances. The residual >30 kDa fraction was frozen to -80ºC and freeze dried overnight.

The sulphur isotope analyses were performed using the EA-IRMS technique either at Iso-Analytical Limited, Crewe, UK, or at the Stable Isotope Laboratory (SIL), Department of Geological Sciences, Stockholm University, Sweden. Sulphur isotopic values are measured against the standard VCDT, and expressed in per mil, ‰. For each sample, approximately 5mg of collagen was weighed into tin capsules and combusted with an elemental analyser (unknown at Iso-Analytical and Carlo Erba NC2500 at SIL) connected to a continuous flow isotope ratio mass spectrometer (Europa 20-20 at Iso-Analytical and Finnigan MAT Delta+ or DeltaV Advantage at SIL), and the precision for δ34S, based on standard measurements, was ±0.3‰ or better at Iso-analytical, and ±0.2‰ or better at SIL. The standards used at Iso-Analytical were IA-R036 (BaSO4, +20.74‰), IA-R025 (BaSO4, +8.53‰) and IA-R026 (Ag2S, +3.96‰). At SIL they were MSS-2 (BaSO4, +21.5‰), MSS-3 (BaSO4, +3.35‰), SSS-2 (BaSO4, +22.8‰), SSS-3 (BaSO4, +3.8‰) and CDT (BaSO4, 0‰).


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