The oxygen isotope composition of dissolved inorganic phosphate (d18Op) offers new opportunities to understand the sources and the fate of phosphorus (P) in freshwater ecosystems. However, current analytical protocols for determining d18Op are unable to generate reliable data for samples in which ambient P concentrations are extremely low, precisely the systems in which d18Op may provide new and important insights into the biogeochemistry of P. In this paper, we report the development, testing and initial application of a new technique that enables d18Op 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 hours. Combined with a new freeze-drying method to maximise the yield and minimise the contamination of silver phosphate generated for isotope ratio mass spectrometry, the TSM protocol is able to generate accurate and precise d18Op 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 d18Op into a much wider range of freshwater ecosystems than has been possible to date.