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

Although phosphatic materials are chemically complex and are prone to exchange oxygen isotopes with their environments, the phosphate (PO₄^3- ) 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 (Ag₃ PO₄ ). However, some of these techniques produce Ag₃ PO₄ of questionable purity, while nearly all are lengthy and/or require relatively large sample sizes.

Five milligrams of bioapatite from modern cow teeth (enamel and cementum) were pre-treated for removal of organic material prior to digestion in 2 M HF. The digested samples were titrated with silver ammine solution at 50°C to precipitate Ag₃ PO₄ . Oxygen isotopic data were collected using a Thermal Combustion Elemental Analyzer (TC/EA) paired with a Delta V^Plus isotope ratio mass spectrometer via a ConFlo III universal interface.

The quality of Ag₃ PO₄ 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 0.125 M NaOH solution, is the most effective treatment for the removal of organic material from both enamel and cementum. Optimal yields of Ag₃ PO₄ were achieved using 1.8 mL of silver ammine solution. The reproducibility of the phosphate δ¹⁸ O compositions ranges from 0.3 to 0.4‰ (1σ) for modern cow teeth.

We present a simplified method for phosphate extraction from organic-rich phosphatic material. Our method gave reproducible δ¹⁸ O values for enamel and cementum from cows' teeth.