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Rationale: The oxygen (O) isotope composition of collagen proteins is a potential indicator of adult residential location, useful for provenancing in ecology, archaeology and forensics. In acidic solution, proteins can exchange O from carboxylic acid moieties with reagent O. This study investigated whether this exchange occurs during demineralisation and gelatinisation preparation of bone/ivory collagen. Methods: EDTA and HCl demineralisation or gelatinisation reagents were made up in waters with different δ18O values, and were used to extract collagen from four skeletal tissue samples. Aliquots of extracted collagen were exposed to two different atmospheric waters, at 120°C and ambient temperature, and subsequently dried in a vacuum oven at 40°C or by freeze drying. Sample δ18O values were measured by HT-EA pyrolysis/IRMS using a zero-blank autosampler. Results: Collagen samples exchanged O with both reagent waters and atmospheric water, which altered sample δ18O values. Exchange with reagent waters occurred in all extraction methods, but was greater at lower pH. Damage to the collagen samples during extraction increased O exchange. The nature of exchange of O with atmospheric water depended on the temperature of exposure: kinetic fractionation of O was identified at 120°C but not at ambient temperature. Exchange was difficult to quantify due to the high variability of δ18O values between experimental replicates. Conclusions: Studies of δ18O values in collagen proteins should avoid extraction methods using acidic solutions.
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