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 ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios with meaningful precision has been challenging due to the much lower natural abundance of ¹⁷O than ¹⁶O and even ¹⁸O. In this study, in line with the advancement of laser-based techniques and their growing applications, we demonstrate how to optimize the determination of ¹⁷O-excess (Δ¹⁷O = ln(δ¹⁷O + 1) − 0.528 × ln(δ¹⁸O + 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 Δ¹⁷O 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 Δ¹⁷O values in Antarctic precipitation. Our approach represents an underlying analytical method that provides guidelines for determining Δ¹⁷O from various types of natural waters.