Analysis of oxygen isotopes of inorganic phosphate (δ18Op) in freshwater: A detailed method description for obtaining oxygen isotopes of inorganic phosphate in environmental water samples.

The ability to identify the origin of phosphorus and understand processes controlling P cycling is essential for designing effective mitigation and restoration of eutrophic freshwater ecosystems. The oxygen isotope composition of orthophosphate (δ18Op) has significant potential as a tracer for P entering freshwater ecosystems. However, methods of analysis of δ18Op are still in their preliminary stages and have proven challenging to implement for new practitioners. In order to achieve progress in developing the application of δ18Op signatures as a tracing tool, there is a need to eliminate the methodological challenges involved in accurately determining δ18Op. This protocol article describes the various steps needed to concentrate and isolate orthophosphate in freshwater samples into an adequately pure Ag3PO4 analyte, without isotopic alteration during processing. The protocol compiles the disperse experiences from previous studies, combined with our own experience. The twofold aim of the paper is toprovide a baseline for an increasing standardisation of the silver phosphate purification method associated with analysis of the oxygen isotope composition of orthophosphate (δ18Op), and to foster new research in the applicability of δ18Op signatures for P source tracing in catchment science.