A simplified silver phosphate extraction method for oxygen isotope analysis of bioapatite

Abstract

Rationale: Although phosphatic materials are chemically complex and are prone to exchange oxygen isotopes with their environments, the phosphate (PO43-) component of these materials is robust and retains its original oxygen isotopic composition. As a result, there are currently several methods for the isolation of phosphate oxygen through the precipitation of silver phosphate (Ag3PO4). However, some of these techniques produce Ag3PO4 of questionable purity, while nearly all are lengthy and/or require relatively large sample sizes. Methods: Five milligrams of bioapatite from modern cow teeth (enamel and cementum) were pre-treated for removal of organic material prior to digestion in 2M HF. The digested samples were titrated with silver ammine solution at 50°C to precipitate Ag3PO4. Oxygen isotopic data were collected using a Thermal Combustion Elemental Analyzer (TC/EA) paired with a Delta VPlus isotope ratio mass spectrometer via a ConFlo III universal interface. Results: The quality of Ag3PO4 is dependent on effective removal of organic material and the volume of silver ammine solution used during titration. A two-step pre-treatment of 2.5% NaOCl, followed by a 0.125M NaOH solution is the most effective treatment for the removal of organic material from both enamel and cementum. Optimal yields of Ag3PO4 were achieved using 1.8 mL of silver ammine solution. The reproducibility of the phosphate δ18O compositions ranges from 0.3 to 0.4‰ (1σ) for modern cow teeth. Conclusions: We present a simplified method for phosphate extraction from organic-rich phosphatic material. Our method gave reproducible 18O values for enamel and cementum from cows’ teeth.