Water vapor δ2H and δ18O measurements using off-axis integrated cavity output spectroscopy

Abstract

We present a detailed assessment of a commercially available water vapor isotopeanalyzer (WVIA, Los Gatos Research, Inc.) for simultaneous in-situ measurements ofδ2H andδ18O in water vapor. This method, based on off-axis integrated cavity outputspectroscopy, is an alternative to the conventional water trap/isotope ratio mass spectrometry (IRMS) techniques. We evaluate the analyzer in terms of precision, memoryeffects, concentration dependence, temperature sensitivity and long-term stability. Acalibration system based on ink jet technology is used to characterize the performanceand to calibrate the analyzer. Our results show that the precision at an averagingtime of 15 s is 0.16‰ forδ2H and 0.08‰ forδ18O. The isotope ratios are strongly dependent on the water mixing ratio of the air. Taking into account this concentrationdependence as well as the temperature sensitivity of the instrument we obtained along-term stability of the water isotope measurements of 0.38‰ forδ2H and 0.25‰forδ18O. The accuracy of the WVIA was further assessed by comparative measure-ments using IRMS and a dew point generator indicating a linear response in isotopic composition and H2O concentrations. The WVIA combined with a calibration systemprovides accurate high resolution water vapor isotope measurements and opens newpossibilities for hydrological and ecological applications