A sub-liter pretreatment method for Orbitrap–based freshwater phosphate oxygen isotope measurement

Phosphate oxygen isotope composition ( δ 18 O(PO 4 3− )) provides critical insights into the sources and transformation processes of phosphorus in aquatic environments. However, the low PO 4 3− concentration (<1 μM) in natural freshwater necessitates large sample volumes (25–100 L) and tedious procedures in the conventional δ 18 O(PO 4 3− ) analysis method that uses thermal conversion/elemental analysis linked to a gas–source isotope-ratio mass spectrometer (TC/EA–IRMS). Here, we report a method for analyzing δ 18 O(PO 4 3− ) using sub-liter volumes (∼1 L) of freshwater. The approach uses a well-established zirconium-oxide-gel method to adsorb dissolved PO 4 3− , followed by purification of PO 4 3− via anion exchange resin and isotope measurement by Electrospray Ionization Orbitrap Mass Spectrometry (ESI–Orbitrap–MS). The whole pretreatment procedure results in a δ 18 O(PO 4 3− ) accuracy of ±0.8 ‰, within the analytical uncertainty of the ESI–Orbitrap–MS measurement. Calibration with TC/EA–IRMS produced nearly a 1:1 regression (slope 0.96, residual <0.9 ‰). Tests on freshwater samples from a natural lake achieved an accuracy of ±0.3 ‰. The method reduces sample volume by three orders of magnitude compared to the TC/EA–IRMS method, opening up opportunities to trace PO 4 3− cycling and source apportionment in an unprecedented temporal and spatial resolution.