A new sampling method with zirconium‐loaded resin for phosphate oxygen isotope analysis in oligotrophic freshwater systems

• Published in: Rapid communications in mass spectrometry 2022-11, Vol.36 (22), p.e9393-n/a
• Main Authors: Ishida, Takuya, Tayasu, Ichiro, Onodera, Shin‐ichi, Ban, Syuhei, Okuda, Noboru
• Format: Article
• Language: English
• Subjects: Adsorption; Amberlite (trademark); Desorption; Fractionation; Isotope ratios; Isotopes; Oxygen isotopes; Potassium phosphates; Resins; Rivers; Sampling methods; Water sampling; Zirconium
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.9393

Rationale The phosphate oxygen isotope ratio (δ18OPO4) is a useful technique to trace the sources and biogeochemical cycles of phosphorus (P) in aquatic ecosystems. However, δ18OPO4 has not been widely used in oligotrophic freshwater systems due to technical and methodological difficulties in collecting sufficient phosphate (PO4) for the δ18OPO4 analysis, which sometimes requires hundreds of liters of the water sample. In this study, a new approach (PaS‐Zir) was developed for the δ18OPO4 analysis in oligotrophic freshwater systems using zirconium (Zr)‐loaded (ZrIRC) resin, which has a high affinity for PO4. Methods ZrClO2 was added to Amberlite IRC748 to obtain the ZrIRC resin. The adsorption/desorption experiment using KH2PO4 with a known value of δ18OPO4 was conducted to determine the adsorption/desorption properties of the resin and the likelihood of isotopic fractionation. By installing mesh bags filled with the resin, the PaS‐Zir approach was used in two rivers with low PO4 concentrations (0.2 and 5.3 μmol/L). A conventional sampling method was also performed in the study river with a higher PO4 concentration to validate the efficacy of the PaS‐Zir method. Results The adsorption/desorption experiment demonstrated that the ZrIRC resin possessed a sufficient adsorption capacity (153 μmol/resin‐mL) and exhibited little isotopic fractionation during the adsorption/desorption processes. Using the PaS‐Zir method, we were able to collect sufficient PO4 samples for the δ18OPO4 analysis from the rivers within at least 4 days of mesh bag installation. The δ18OPO4 values (14.2‰ ± 0.2‰) obtained using the PaS‐Zir method were comparable to those obtained using the conventional method (14.0‰ ± 0.03‰). Conclusion We proved that the PaS‐Zir method is applicable to oligotrophic freshwater systems and is generally more efficient than the conventional method. In addition, our method is useful for improving the understanding of the P dynamics of oligotrophic ecosystems because of the extremely low concentration of PO4 commonly found in them, which are often prone to P pollution.