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

Abstract. 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 (Δ¹⁷O), a measure of the deviation from a~linear relationship between ¹⁷O / ¹⁶O and ¹⁸O / ¹⁶O ratios. Conventional analyses of Δ¹⁷O 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 high-precision Δ¹⁷O 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 Δ¹⁷O using integration times of less than 30 min. Calibration with respect to accepted water standards demonstrates that both the precision and the accuracy of Δ¹⁷O 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.