The Use of Electrolysis for Accurate δ17O and δ18O Isotope Measurements in Water

Abstract

We present a new system to measure the relative isotopic abundances of both rare isotopes of oxygen in water. Using electrolysis with CuSO4 as electrolyte, water is transformed into oxygen gas. This gas is subsequently analyzed with a standard Isotope Ratio Mass Spectrometer. We investigated the features of the system in detail. The results for δ17O and δ18O are carefully calibrated against δ18O of VSMOW using our “conventional” H2O-CO2 equilibrium system. For the electrolysis system, we obtain an accuracy (including calibration) of 0.10‰ for δ18O, and 0.07‰ for δ17O. Finally, we demonstrate the usefulness and accuracy of the system by analyzing a large set of natural waters (both fresh and salt waters). We establish the 17O and 18O relation to be of the type: 1 + δ17O = (1 + δ18O)λ, and find λ to be 0.5281 ± 0.0015. We conclude that the 17O fractionation in water appears to be completely analogous to that of 18O. Still, measurements concerning the equilibrium and kinetic fractionation of the evaporation process are needed. The new method is an excellent alternative to the traditional H2O-CO2 equilibrium method. Moreover, its capability to also measure 17O may be fruitfully exploited, e.g., in biomedical studies extending the doubly labelled water technique, or in studies of stratospheric water.