Technical note: Mobile, discrete in situ vapor sampling for measurements of matrix-bound water stable isotopes

Abstract. Methodological advancements have been made in in situ observations of water stable isotopes that have provided valuable insights in ecohydrological processes. The continuous measurement capabilities of laser-based analyzers allow for high temporal resolutions and non-destructive, minimally invasive study designs of such in situ approaches. However, isotope analyzers are expensive, heavy, and require shelter and access to electrical power which impedes many in situ assays. Therefore, we developed a new, inexpensive technique to collect discrete water vapor samples in the field via diffusion-tight inflatable bags that can later be analysed in the lab. In a series of structured experiments, we tested different procedural settings, bag materials, and closure types for diffusion-tightness during storage as well as for practical handling during filling and extraction. To facilitate re-usage of sampling bags, we present a conditioning procedure using ambient air as primer. In order to validate our method, direct measurements through hydrophobic in situ probes were compared to repeated measurements of vapor sampled with our bags from the same source. Performing pertinent calibration procedures, accuracy was better than 0.4 ‰ for δ¹⁸O and 1.9 ‰ for δ²H after one day of storage. Our technique is particularly suitable in combination with minimal invasive water vapor sampling in situ probes that have already been employed for soils and tree xylem. It is an important step towards monitoring stable isotope distributions and also time-series in soils and trees without the need to have field-access to an analyzer and therefore a promising tool for many applications in eco-hydrology and meteorology.