The use of electrolysis for accurate δ17O and δ18O isotope measurements in water

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 delta(17)O and delta(18)O are carefully calibrated against delta(18)O of VSMOW using our "conventional" H2O-CO2 equilibrium system. For the electrolysis system, we obtain an accuracy (including calibration) of 0.10 parts per thousand for delta(18)O, and 0.07 parts per thousand for delta(17)O. 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 O-17 and O-18 relation to be of the type: 1 + delta(17)O = (1 + delta(18)O)(lambda), and find lambda to be 0.5281 +/- 0.0015. We conclude that the O-17 fractionation in water appears to be completely analogous to that of O-18. 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 O-17 may be fruitfully exploited, e.g., in biomedical studies extending the doubly labelled water technique, or in studies of stratospheric water.