Calibrated high-precision 17O-excess measurements using cavity ring-down spectroscopy with laser-current-tuned cavity resonance
High-precision analysis of the 17O / 16O isotope ratio in water and water vapor is of interest in hydrological, paleoclimate, and atmospheric science applications. Of specific interest is the parameter 17O excess (Δ 17O), a measure of the deviation from a~linear relationship between 17O / 16O and 18O / 16O ratios. Conventional analyses of Δ 17O of water are obtained by fluorination of H 2O to O 2 that is analyzed by dual-inlet isotope ratio mass spectrometry (IRMS). We describe a new laser spectroscopy instrument for high-precision Δ 17O measurements. The new instrument uses cavity ring-down spectroscopy (CRDS) with laser-current-tuned cavity resonance to achieve reduced measurement drift compared with previous-generation instruments. Liquid water and water-vapor samples can be analyzed with a better than 8 per meg precision for Δ 17O using integration times of less than 30 min. Calibration with respect to accepted water standards demonstrates that both the precision and the accuracy of Δ 17O are competitive with conventional IRMS methods. The new instrument also achieves simultaneous analysis of δ 18O, Δ 17O and δ D with precision of < 0.03‰, < 0.02 and < 0.2‰, respectively, based on repeated calibrated measurements.