Comparing bioapatite carbonate pre-treatments for isotopic measurements: Part 1-Impact on structure and chemical composition

A large variety of bioapatite pre-treatments are found in the literature for a wide range of applications going from mineralogy to diagenesis. Additionally, stable carbon and oxygen isotopes in bioapatite carbonates are often measured after pre-treatment to study ancient diets and reconstruct past environments. It is common to pre-treat the samples to remove contributions from competing sources associated to organic material and secondary carbonates. Several chemical methods are described in the literature to pre-treat archaeological and fossil bioapatites (bone and teeth) for isotopic measurements with no consistency on their efficiency. Infrared and elemental analyses are combined in this study to highlight the effects of the different pre-treatments on the structure and chemistry of bioapatite. The different pre-treatments can variably affect the organic and carbonate contents of bioapatites, with some, such as sodium hypochlorite, apparently adding carbonate to the system. This may lead to a change in the biological isotopic signature of bioapatite carbonate, with effects on the correct interpretation of the results. According to the findings of our study, hydrazine hydrate seems to be the best chemical to remove organic matter without affecting the carbonate content of bioapatites, while the use of acetate buffered acetic acid solutions is recommended for removing adsorbed carbonates, when necessary. Sodium hypochlorite, instead, induces the adsorption of exogenous carbonates, while hydrogen peroxide does not remove organic matter efficiently even at high temperatures. These two chemicals should not be used to remove organic matter, especially from bone and dentine.