Intercomparison of Four Commercial Analyzers for Water Vapor Isotope Measurement

Abstract

he δ18O and δD of atmospheric water vapor are important tracers in hydrological and ecological studies. Isotope ratio infrared spectroscopy (IRIS) provides an in situ technology for measuring δ18O and δD in ambient conditions. An intercomparison experiment was carried out with four commercial IRIS analyzers to characterize their performance and transferability of calibration methods. Over a 15-day atmospheric measurement, during which the water vapor concentration ranged from 14 to 27 mol mol?1 and the isotopic ratios spanned about 90? and 13? for δD and δ18O, respectively, these analyzers tracked the natural variability in ambient conditions very well and achieved an average difference between one another within 2? for δD and within 0.1? for δ18O after calibration at appropriate frequencies. Two of the calibration methods (discrete liquid water injection and continuous dripping) agreed with each other within the tolerance thresholds of 2? for δD and 0.1? for δ18O. The Rayleigh distillation technique appeared to be acceptable as a calibration standard for δD but not for δ18O. The δD measurements were less prone to concentration dependence errors than the δ18O measurements. The concentration dependence underscores the importance of using a calibration procedure at multiple mixing ratios to bracket the range of natural variability.