An optimal strategy for determining triple oxygen isotope ratios in natural water using a commercial cavity ring-down spectrometer

Triple oxygen isotope ratios have been increasingly acknowledged as useful hydrological tracers but measuring both O/O and O/O ratios with meaningful precision has been challenging due to the much lower natural abundance of O than O and even O. In this study, in line with the advancement of laser-based techniques and their growing applications, we demonstrate how to optimize the determination of O-excess (ΔO = ln(δO + 1) − 0.528 × ln(δO + 1)) in natural water using a commercial wavelength-scanned cavity ring-down spectrometer (WS-CRDS). We particularly focus on how to decide the injection numbers of samples and standard solutions utilized for normalization of sample results to the VSMOW-SLAP reference scale. With a measurement strategy aimed at an uncertainty better than 10 per meg (1σ), the ΔO of Greenland Ice Sheet Precipitation (GISP) is determined to be 24 ± 9 per meg (n = 104), in agreement with previous literature values. By applying this method to Antarctic glacial ice, it is shown to be useful in detecting the seasonality of ΔO values in Antarctic precipitation. Our approach represents an underlying analytical method that provides guidelines for determining ΔO from various types of natural waters.