Technical note: Unresolved aspects of the direct vapor equilibration method for stable isotope analysis (<i>δ</i><sup>18</sup>O, <i>δ</i><sup>2</sup>H) of matrix-bound water: unifying protocols through empirical and mathematical scrutiny

Abstract. The direct vapor equilibration laser spectrometry (DVE-LS) method has been developed for obtaining matrix-bound water stable isotope data in soils, the critical zone, and bedrock, deriving therefrom subsurface flow transport processes and, ultimately, characterizing, example, groundwater recharge vulnerability. Recently, DVE-LS increasingly adopted due to its possible high sample throughput, relative simplicity, cost-efficiency. However, this come at cost of a non-unified standard operation protocol (SOP), several contradictory suggestions regarding details do exist which have not resolved date. Particularly, container material times yet agreed upon. Beside practical constraints, often limits applicability interpreting dynamics instead absolute values. It also prevents comparability among studies or laboratories, previous comparisons with other, more traditional approaches extraction subsequent analysis yielded significant discrepancies various matrices physical states. In series empirical tests, we scrutinized controversial details. Specifically, tested 10 different easily available cost-efficient inflatable bags previously employed potentially suitable collection equilibration. storage tests similar process but lasting weeks, quickly found heat-sealed made laminated aluminum (Al) sheets be superior by orders magnitude over frequently used freezer terms evaporation safety accompanying adverse effects. For first time, Al-laminated allow applied time adapted exclusively requirements accepting reduced quality trade-off shortcomings. Based on detailed considerations, further describe how calculate minimum headspace sample-contained liquid volume their ratio affects analytical precision accuracy. We are confident that these guidelines will expand improve laboratories contributing unified, physically well-founded SOP based appropriate components.