Rationale: Stable isotopes of water in precipitation are powerful tracers of atmospheric processes. Automatic rain samplers are valuable for high temporal resolution isotopic studies but building them from scratch requires significant financial and material resources. A commercial water autosampler has been modified to prevent evaporation after sampling and to allow precipitation sampling during an event. Methods: New sampling bottles were created by reducing the original volume and opening area. Evaporation tests were carried out on different volumes of water for 72 hours under laboratory conditions. These were used to determine the minimum amount of rain to collect to minimize the impact of isotopic fractionation by evaporation. The impact of the autosampler’s air moisture saturation was also tested. Samples were analyzed by dual-inlet isotope ratio mass spectrometry and cavity ring-down spectroscopy. Results: For samples larger than 10 mL, evaporative heavy isotope enrichment due to Rayleigh distillation remains negligible compared to the overall analytical uncertainty. Intentional saturation of the autosampler’s atmosphere significantly reduces post-sampling evaporation but leads to equilibration of the samples with the added water. We have investigated the maximum time that samples must be left for this fractionation to remain negligible. Under these conditions, this autosampler is suitable for intra-event rainfall sampling for isotopic analysis. Conclusions: It is now possible to perform low-cost high-resolution precipitation sampling for isotopic analysis. The intentional air saturation of the sampler, which effectively prevents post-sampling evaporation, had never been proposed before. All instructions for modifying this sampler are now available in open access so the scientific community can easily repeat them.

Stable isotopes of the water molecule have emerged as powerful tracers of the sources and trajectories of water leading to precipitation, at different spatial and temporal scales. However, the high cost of commercially available rain samplers for isotopic analysis, have made using them for high spatial resolution networks and for studies being conducted in developing countries prohibitively expensive. We have designed a low-cost, simple, and robust rain sampler capable of sampling precipitation for isotopic analysis on the event and monthly scale, based on the existing designs provided in the literature. The event rain samplers were tested to determine the minimum amount of rainfall to minimize isotopic fractionation, both from post-sampling evaporation and equilibration. These new rain samplers will enable isotopic sampling of precipitation at high spatial resolutions. All the instructions for constructing and using these samplers are made openly accessible to the scientific community so they can easily be repeated and adapted to the needs of each project. This open access and low-cost methodology will help democratize the use of isotopes for hydrological studies in developing countries.