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.
Highlights
The direct vapor equilibration laser spectrometry (DVE-LS) method first published by Wassenaar et al (2008) has facilitated a way for fairly convenient, high-throughput stable isotope analysis of water bound to the soil matrix, rocks, or plant tissue
In the second part of the study we focused on quantitatively assessing the effects of selected storage bags on DVE-LSbased stable isotope analysis of matrix-bound water
The average area-normalized weight loss rates of the 10 tested bags varied by 3 orders of magnitude, ranging from 0.006 to 1.415 g (m2 d)−1. They were highest for the transparent low-density polyethylene (LDPE) bag of low strength (G&G) and lowest for the heat-sealed bag that included one layer of aluminum (Al) foil (Al3z_hs)
Summary
The direct vapor equilibration laser spectrometry (DVE-LS) method first published by Wassenaar et al (2008) has facilitated a way for fairly convenient, high-throughput stable isotope analysis of water bound to the soil matrix, rocks, or plant tissue. Instead of physically extracting water, the method employs analysis of a corresponding vapor phase and thereby bypasses many of the previously necessary, laborious sample preparation steps. It increases the number of samples that can be processed per day. It employs inflatable sample containers into which evaporation-susceptible soil, rock, or plant samples of interest are quickly collected. The containers are commonly inflated with a dry inflation atmosphere and sealed.
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