Anion exchange resin and slow precipitation preclude the need for pretreatments in silver phosphate preparation for oxygen isotope analysis of bioapatites

Preventing the inclusion of oxygen bearing compounds from the organic fraction of skeletal tissues is often considered key to obtaining faithful δ ¹⁸O measurements of the mineral fraction, which are widely used across the archaeological, forensic and geochemical sciences. Here we re-explore the contentious issue of organic removal pretreatments by establishing how different silver phosphate preparation methods perform in producing pure silver phosphates with a faithful biogenic isotopic signal. We then compare this baseline performance to a pretreatment based approach. Our results show that anion exchange purification combined with slow precipitation of silver phosphate consistently produces silver phosphates of high purity without prior pretreatment. Rapid precipitation protocols without additional purification, while effective and time-efficient for low organic samples such as enamel, suffer from the inclusion of substantial amount of organic matter in silver phosphates from bone or dentine samples. However, despite substantial organic contamination in such samples, δ ¹⁸O values do not necessarily show substantial shifts. Further study is needed to clarify the reason for this, but for now the use of an anion exchange based protocol represents the most cautious approach to processing bone and dentine samples and we recommend its use for such samples. Confirming previous work we find H ₂O ₂ pretreatment to be only partially effective at removing higher amounts of organic matter. Both H ₂O ₂ and NaOCl pretreatments show unpredictable side effects on δ ¹⁸O values of both bones and inorganic samples. We additionally find no indication that the presence of organic material hinders the dissolution of bioapatite samples.