A New Technique to Determine the Phosphate Oxygen Isotope Composition of Freshwater Samples at Low Ambient Phosphate...

The oxygen isotope composition of dissolved inorganic phosphate (δ¹⁸O_p) offers new opportunities to understand the sources and the fate of phosphorus (P) in freshwater ecosystems. However, current analytical protocols for determining δ¹⁸O_p are unable to generate reliable data for samples in which ambient P concentrations are extremely low, precisely the systems in which δ¹⁸O_p 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 δ¹⁸O_p 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 δ¹⁸O_p data. We evaluated the TSM protocol using synthetic test solutions and subsequently applied the protocol to samples from locations around the Saint Lawrence River in Montreal, Canada. We believe that the novel technique reported here offers the methodological basis for researchers to extend the application of δ¹⁸O_p into a much wider range of freshwater ecosystems than has been possible to date.