Oxygen isotope analysis of phosphate by Electrospray Orbitrap mass spectrometry

Oxygen isotopic composition of phosphate has been widely used to reconstruct paleo-environment and to trace biogeochemical phosphorous cycling. Traditional oxygen isotope measurement of phosphate is through the purification and precipitation of solution PO43- as solid Ag3PO4 which is then converted to CO gas through pyrolysis. The δ18O value of the CO is measured in a gas-source isotope ratio mass spectrometer. The extraction and purification of phosphate involve many cumbersome steps, which introduce large uncertainties in the measured δ18O values. In addition, the existing method requires the amount of Ag3PO4 at micro-mole levels, which limits the applications of oxygen isotopic composition of phosphate. Recent efforts in using electrospray quadrupole-Orbitrap mass spectrometry to directly analyze isotope composition of nano-mole levels of sulfate and nitrate in a liquid phase have shown great promises (Neubauer et al., 2020; Hilkert et al., 2021). Here, based on a similar framework of isotopic measurements using Orbitrap, we report our effort and result on oxygen isotope analysis of phosphate. Calibrated using international and in-house standards, we have achieved an analytical precision for δ18O of phosphate better than ±0.8‰ (1 SD, N=20), which is comparable to or better than that of the existing method. Further improvement of the precision has met with challenges.

Reference:

Hilkert, A., Böhlke, J.K., Mroczkowski, S.J., Fort, K.L., Aizikov, K., Wang, X.T., Kopf, S.H. and Neubauer, C., 2021. Exploring the Potential of Electrospray-Orbitrap for Stable Isotope Analysis Using Nitrate as a Model. Analytical Chemistry 93, 9139-9148.

Neubauer, C., Crémière, A., Wang, X.T., Thiagarajan, N., Sessions, A.L., Adkins, J.F., Dalleska, N.F., Turchyn, A.V., Clegg, J.A., Moradian, A., Sweredoski, M.J., Garbis, S.D. and Eiler, J.M., 2020. Stable Isotope Analysis of Intact Oxyanions Using Electrospray Quadrupole-Orbitrap Mass Spectrometry. Analytical Chemistry 92, 3077-3085.