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 17Oexcess (117O), a measure of the deviation from a linear relationship between 17O / 16O and 18O / 16O ratios. Conventional analyses of 117O of water are obtained by fluorination of H2O to O2 that is analyzed by dual-inlet isotope ratio mass spectrometry (IRMS). We describe a new laser spectroscopy instrument for highprecision 117O measurements. The new instrument uses cavity ring-down spectroscopy (CRDS) with laser-currenttuned 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 117O using integration times of less than 30 min. Calibration with respect to accepted water standards demonstrates that both the precision and the accuracy of 117O are competitive with conventional IRMS methods. The new instrument also achieves simultaneous analysis of δ18O, δ17O and δD with precision of < 0.03 ‰, < 0.02 and < 0.2 ‰, respectively, based on repeated calibrated measurements.