Calibration of Water Vapour Isotope Ratios for High Precision Measurements in the Tropics

Due to the recent development of isotope ratio infrared spectroscopy (IRIS) analysers, measurements of δ 18O and δ2H in water vapour can be made in real time with a high temporal resolution. This study is focussed on the development of a calibration device used to characterise such an analyser for measurement of the ambient H2O concentration observed in the Tropical Maritime location of Darwin NT, AUS. The calibration instrument was via experiments associated with injection tube material and vaporiser temperature. Initial findings indicated that system using a copper plate as an immediate injection surface was the most suitable due to both precision measurements and evaporative stability. With an appropriate measurement averaging time (10 to 20 minutes) the vaporiser system was able to meet precision measurements of δ18O and δ2H at 0.1‰ and 2‰ respectively over a H2O mixing ratio range of 5-45 mmol mol-1. The calibration device designed in this study was compared against a calibration correction from a former calibration instrument utilised at the study site of Darwin. Data was contrasted between the two systems which revealed a discrepancy of up to ~11% for δ18O.