Calibrated high-precision 17O-excess measurements using cavity ring-down spectroscopy with laser-current-tuned cavity resonance

High-precision analysis of the O-17/O-16 isotope ratio in water and water vapor is of interest in hydrological, paleoclimate, and atmospheric science applications. Of specific interest is the parameter (17)Oexcess (Delta O-17), a measure of the deviation from a linear relationship between O-17/O-16 and O-18/O-16 ratios. Conventional analyses of Delta O-17 of water are obtained by fluorination of H2O to O-2 that is analyzed by dual-inlet isotope ratio mass spectrometry (IRMS). We describe a new laser spectroscopy instrument for high-precision Delta O-17 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 Delta O-17 using integration times of less than 30 min. Calibration with respect to accepted water standards demonstrates that both the precision and the accuracy of Delta O-17 are competitive with conventional IRMS methods. The new instrument also achieves simultaneous analysis of delta O-18, delta O-17 and delta D with precision of < 0.03 parts per thousand, < 0.02 and < 0.2 parts per thousand, respectively, based on repeated calibrated measurements.