A new technique to determine the phosphate oxygen isotope composition of freshwater samples at low ambient phosphate concentration

The oxygen isotope composition of dissolved inorganic phosphate (δ18Op) offers new opportunities to understand the sources and the fate of phosphorus (P) in freshwater ecosystems. However, current analytical protocols for determining δ18Op are unable to generate reliable data for samples in which ambient P concentrations are extremely low, precisely the systems in which δ18Op may provide new and important insights into the biogeochemistry of P. In this Article, we report the development, testing and initial application of a new technique that enables δ18Op analysis to be extended into such ecosystems. The twist spinning mode (TSM) protocol described here enables >1000 L of sample with a P concentration <0.016 mg P L–1 to be initially processed within the field in approximately 24 h. Combined with a new freeze-drying method to maximize the yield and minimize the contamination of silver phosphate generated for isotope ratio mass spectrometry, the TSM protocol is able to generate accurate and precise δ18Op d...