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

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

Influence of sample preparation procedures on water stable isotopes in plant organs using the water‐vapour equilibrium method

“…The amounts of the present study were based on Millar et al ( 2018), and we used 5 g of leaves, 5 g of stems and twigs, 3 g of roots and 3 g of fruit sample. We agree with previous suggestions (Gralher et al, 2021;Hendry et al, 2015;Wassenaar et al, 2008) that researchers should not aim at collecting a certain standard sample volume or weight but instead collect samples containing a minimum volume of water necessary to equilibrate the bag's headspace. Wassenaar et al (2008) and Hendry et al (2015) suggested a minimum of 3 ml, which was based on observations using 1 L doublefreezer-bagged samples of various artificially produced moisture contents.…”

“…Wassenaar et al (2008) and Hendry et al (2015) suggested a minimum of 3 ml, which was based on observations using 1 L doublefreezer-bagged samples of various artificially produced moisture contents. Gralher et al (2021) calculated the minimum necessary absolute water content to be 1.47 ml when using evaporation-safe bags of 1 L headspace volume. Their suggested minimum volume accounts for the fact that depending on the equilibration temperature a defined amount of sample water will evaporate to saturate a given bag's headspace.…”

“…A previous study compared different methods of measuring plant water stable isotopes and concluded that the WVE method is the most appropriate to analyse xylem water isotope ratios (Millar et al, 2018). While a recently published review aimed at standardizing the WVE method for pore water samples (Gralher et al, 2021), nothing similar exists for plant samples yet. Millar et al (2018) highlighted that the effect of different equilibration times, the kind of gas used to equilibrate with the liquid water, and the type of air-tight containers for storage and equilibration still need to be tested for a reliable plant sample measurement.…”

On the urgent need for standardization in isotope‐based ecohydrological investigations

“…Glass vials with polypropylene (PP) screw caps, thickwalled HDPE bottles, and perfluoroalkoxy-teflon (PFA) containers all preserved the original sample isotope values (Böttcher & Schmiedinger, 2021;Spangenberg, 2012). Other research assessed storage media choices and equilibration times for the direct vapour equilibration-IRIS approach (DVE-IRIS) (Gralher et al, 2021). They transmission rates of 0.07 cm 3 /m 2 /day (Schwab et al, 2016).…”

“…Freezing or cooling samples during transport and storage is thought to mitigate temperature related fractionation effects, limit sample decomposition, limit generation of reducing environments, and reduce microbial activity (typically an issue in soils) which thereby limits CO 2 buildup (Fischer et al, 2019;Gralher et al, 2021). For IRIS analysers, changes in the gas matrix (i.e., caused by CO 2 build up), can affect measured isotopic values (Gralher et al, 2016(Gralher et al, , 2021. However, freezing samples may destroy soil microstructures (Gralher et al, 2021), and causes bursting of cell walls in plant samples (Fischer et al, 2019;Millar et al, 2018).…”