Technical note: Water vapor sampling for the analysis of water stable isotopes in trees and soils – optimizing sampling protocols

Abstract. The study establishes an optimized, minimally invasive sampling protocol for water vapor – a semi-in situ method crucial to ecohydrology that addresses the limitations of destructive techniques. The key innovation is an integrated, multifactor assessment of how container type, flow rate, temperature, and storage time affect the isotopic stability of δ¹⁸O and δ²H. The optimal configuration uses 250 ml infusion glass bottles (ND 32), which provided the highest isotopic stability (±0.5 ‰ for δ¹⁸O and ±1‰ for δ²H over 24 hours), outperforming FlexFoil and aluminum-zip bags. The best results were obtained with flow rates of 100–125 ml/min and storage not exceeding 24 hours under ambient conditions (20–25 °C). δ¹⁸O was highly stable under nearly all conditions (<0.6 ‰), whereas δ²H was more sensitive (±0.3 ‰ to ±1.5 ‰); its variability increased markedly with storage beyond 24 hours or at extreme temperatures (4 °C or 40 °C), underscoring the need to minimize the time between sampling and measurement. This validated protocol provides an accessible and reliable methodology that expands the toolkit for high-temporal-resolution ecohydrological studies, particularly in remote areas or settings with limited infrastructure.