A new automated system for the rapid analysis of atmospheric water vapor samples for stable isotope composition.

An automated technique for measuring the isotopic composition (δ D) of atmospheric water vapor is being developed at the University of New Mexico. Air is sampled using 12 mL glass vials with screw-on caps. Our analytical system flushes the sample vial with He, isolates the water vapor in a cold trap, and then reduces the water with carbon at 1300° C to form H₂ and CO. Isotopic ratios are then measured in continuous flow using a Finnigan Delta plus XL mass spectrometer. With Albuquerque-area air, sample size is approximately 100 nanomoles of H₂O. Our system is robotic and interfaces with a commercially available autosampler. This enables us to analyze ~100 air samples per day with little supervision. Standards are prepared by sampling water vapor in equilibrium with waters of known isotopic composition within a glovebox. The advantage of such a system is that it will allow us to analyze atmospheric water vapor at temporal and spatial resolutions not practical with more time-consuming traditional techniques. Sample vials are compact, inexpensive, and easy to use, enabling air samples to be acquired with little effort. One potential application for this technique includes problems requiring high temporal resolution (and a high number of samples) such as diurnal cycles of evapotranspiration. Other applications include studies of the three-dimensional spatial distribution of δ D in water vapor to assess water vapor transport pathways.