Preprints
https://doi.org/10.5194/hess-2021-255
https://doi.org/10.5194/hess-2021-255

  17 May 2021

17 May 2021

Review status: this preprint is currently under review for the journal HESS.

Technical note: Controversial aspects of the direct vapor equilibration method for stable isotope analysis (δ18O, δ2H) of matrix-bound water: Unifying protocols through empirical and mathematical scrutiny

Benjamin Gralher, Barbara Herbstritt, and Markus Weiler Benjamin Gralher et al.
  • Chair of Hydrology, University of Freiburg, Freiburg 79098, Germany

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, characterising e.g. 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 ten 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.

Benjamin Gralher et al.

Status: open (until 29 Jul 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Benjamin Gralher et al.

Benjamin Gralher et al.

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Short summary
We scrutinized the quickest currently available method for stable isotope analysis of matrix-bound water. Simulating the common protocol, we demonstrated the limits of certain materials currently used and identified a reliable and cost-efficient alternative. Further, we calculated the optimum proportions of important protocol aspects critical for precise and accurate analyses. Our unifying SOP suggestions increase data quality and comparability as well as the method’s general applicability.