A new method for phosphate purification for oxygen isotope ratio analysis in freshwater and soil extracts using solid‐phase extraction with zirconium‐loaded resin

Rationale Phosphate (PO4) oxygen isotope (δ18OPO4) analysis is increasingly applied to elucidate phosphorus cycling. Due to its usefulness, analytical methods continue to be developed and improved to increase processing efficiency and applicability to various sample types. A new pretreatment procedure to obtain clean Ag3PO4 using solid‐phase extraction (SPE) with zirconium‐loaded resin (ZrME), which can selectively adsorb PO4, is presented and evaluated here. Methods Our method comprises (1) PO4 concentration, (2) PO4 separation by SPE, (3) cation removal, (4) Cl− removal, and (5) formation of Ag3PO4. The method was tested by comparing the resulting δ18OPO4 of KH2PO4 reagent, soil extracts (NaHCO3, NaOH, and HCl), freshwater, and seawater with data obtained using a conventional pretreatment method. Results PO4 recovery of our method ranged from 79.2% to 97.8% for KH2PO4, soil extracts, and freshwater. Although the recovery rate indicated incomplete desorption of PO4 from the ZrME columns, our method produced high‐purity Ag3PO4 and accurate δ18OPO4 values (i.e., consistent with those obtained using conventional pretreatment methods). However, for seawater, the PO4 recovery was low (1.1%), probably due to the high concentrations of F− and SO42− which interfere with PO4 adsorption on the columns. Experiments indicate that the ZrME columns could be regenerated and used repeatedly at least three times. Conclusions We demonstrated the utility of ZrME for purification of PO4 from freshwater and soil extracts for δ18OPO4 analysis. Multiple samples could be processed in three days using this method, increasing sample throughput and potentially facilitating more widespread use of δ18OPO4 analysis to deepen our understanding of phosphorus cycling in natural environments.