Intercomparison of Four Commercial Analyzers for Water Vapor Isotope Measurement

The delta O-18 and delta D of atmospheric water vapor are important tracers in hydrological and ecological studies. Isotope ratio infrared spectroscopy (IRIS) provides an in situ technology for measuring delta O-18 and delta D 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 parts per thousand and 13 parts per thousand for delta D and delta O-18. respectively, these analyzers tracked the natural variability in ambient conditions very well and achieved an average difference between one another within 2 parts per thousand, for delta D and within 0.1 parts per thousand for delta O-18 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 parts per thousand for delta D and 0.1 parts per thousand for delta O-18. The Rayleigh distillation technique appeared to be acceptable as a calibration standard for delta D but not for delta O-18. The delta D measurements were less prone to concentration dependence errors than the delta O-18 measurements. The concentration dependence underscores the importance of using a calibration procedure at multiple mixing ratios to bracket the range of natural variability.