Intercomparison of four commercial analyzers for water vapor isotope measurement

The 18O and 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 18O and 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 mo-1 and the isotopic ratios spanned about 90 and 13 for D and 18O, respectively, these analyzers tracked the natural variability in ambient conditions very well and achieved an average difference between one another within 2 for D and within 0.1 for 18O 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 D and 0.1 for 18O. The Rayleigh distillation technique appeared to be acceptable as a calibration standard for D but not for 18O. The D measurements were less prone to concentration dependence errors than the 18O measurements. The concentration dependence underscores the importance of using a calibration procedure at multiple mixing ratios to bracket the range of natural variability. 2012 American Meteorological Society