Technical note: comparison of water vapor sampling techniques for stable isotope analysis

Abstract. Water vapor samples are key elements to describe the evaporation process thanks to the stable isotope signatures of δ²H and δ¹⁸O. However, its sampling is a difficult task that can introduce errors due to isotopic fractionation. This study investigates the consistency of different sampling techniques for atmospheric water vapor. The isotope signature of a parcel of air was determined with a cavity output spectroscopy device during a period of 3 hours (benchmark). This parcel of air was sampled simultaneously with 3 types of sampling bags made of different materials (metalized polyethylene -MPE-, polyvinyl fluoride -PVF-, low density polyethylene -LDPE-) and with 2 cryogenic baths running at two different pumping rates (3 L min^-1 and 50 mL min^-1). The tested water vapor sampling techniques differ in their ability to keep reliable measurements after sampling and are highly susceptible to procedural errors. MPE bags are the best option for measuring samples up to two weeks of storage after sampling. LDPE and PVF bags are only reliable if the measurement is performed on the same sampling day.