Technical note: Efficiency of various evaporation barriers for use in automated water samplers for subsequent water isotope analysis

Abstract. High temporal and spatial resolution water sampling of stream water or precipitation for subsequent water stable isotope analysis (δ2H and δ18O) is commonly conducted with self-made or commercial automated samplers. However, prolonged storage of water samples in open bottles inside autosamplers can lead to isotopic alterations caused by evaporative fractionation and diffusion driven by vapor pressure gradients. In this study, we tested the effectiveness of different evaporation barriers – tube-dip-in, ball valves, and siphons – under controlled laboratory conditions. Experiments were conducted using glass bottles with varying filling levels (50 to 250 mL in 250 mL bottles) and exposure times of up to 52 d. Our results demonstrate that all tested barriers effectively reduce isotopic alterations. Among these, the tube-dip-in emerged as the simplest and most efficient solution, reliably preserving isotopic data even over extended storage durations. We also observed isotopic differences between the water in the tube-dip-in and the main bottle. This phenomenon becomes particularly relevant when sample volumes are very low, as water from the tube-dip-in may influence the overall isotopic composition of the sample. This finding highlights the need for careful consideration of bottle design and sample handling, especially with low water volumes. These findings provide valuable insights for the design of cost-effective, automated water sampling systems for stable isotope applications, emphasizing the importance of evaporation barriers to ensure reliable and accurate isotopic analyses.