An easy-to-use water vapor sampling approach for stable isotope analysis using affordable membrane valve multi-foil bags

Abstract. Water-stable isotopes are commonly used in hydrological and ecological research. Until now, measurements were obtained either by taking a destructive sample in the field (such as a soil or plant sample) and extracting its water in the laboratory, or by directly measuring it in the field using semi-permeable membranes. These methods, however, present challenges in achieving high-resolution measurements across multiple sites since they require significant effort and resources. Gasbag sampling offers the advantage of non-destructive, cost-efficient, easy to perform, in-situ measurements without the need to bring a Cavity Ring-Down Spectroscopy (CRDS) analyzer into the field. Gas permeable membranes (GPM) were utilized to extract samples of water vapor from the soil, which were then stored in specialized gas bags (multi-layer foil bags). The bags were tested using laboratory isotope standards for their maximum storage time, potential memory effects, and reusability. To demonstrate their applicability in field experiments, in-situ measurements using gas bags were compared to measurements directly connecting a water stable isotope laser. The storage experiment demonstrated the ability to store water vapor samples for up to seven days while maintaining acceptable results for δ²H and δ¹⁸O, although the relative uncertainty was higher for δ¹⁸O. A “Memory experiment” revealed that reusing bags can lead to previous samples influencing subsequent ones. The experiment on “Combined storage and memory” showed that the duration of storage increases the effect on memory. The field experiment demonstrated an overall measurement precision of 0.23 ± 0.84 for δ¹⁸O [‰] and 0.94 ± 2.69 for δ₂H [‰] using the gas bags. Together, laboratory and field experiments confirmed that the proposed water vapor sampling system and procedure for stable water isotope analyses using GPM and re-usable gas bags is a simple, cost-effective, and versatile approach allowing for various applications. We were able to demonstrate that both 1) storage is possible, and that 2) gas bags can be reused, since memory effects caused by previous samples can be prevented by appropriate treatment. This makes the gas bags suited for field collection of water vapor samples for many applications.