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 (δ¹⁸O_PO4) 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 δ¹⁸O_PO4 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 δ¹⁸O_PO4 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 δ¹⁸O_PO4 analysis. Multiple samples could be processed in three days using this method, increasing sample throughput and potentially facilitating more widespread use of δ¹⁸O_PO4 analysis to deepen our understanding of phosphorus cycling in natural environments.