Two common pitfalls in the analysis of water-stable isotopologues with cryogenic vacuum extraction and cavity ring-down spectroscopy

Abstract

Water stable isotopologue analysis is widely used to disentangle ecohydrological processes. Yet, there are increasing reports of measurement uncertainties for established and emerging methods, such as cryogenic vacuum extraction (CVE) or cavity ring-down spectroscopy (CRDS). With this study, we investigate two pitfalls, that potentially contribute to uncertainties in water-stable isotopologue research. To investigate fractionation sources in CVE, we extracted pure water of known isotopic composition with cotton, glass wool or without cover and compared the isotopologue results with non-extracted reference samples. To characterise the dependency of δ²H and δ¹⁸O on the water mixing ratio in CRDS, which is of high importance for in-situ applications with large natural variations in mixing ratios, we chose samples with a large range of isotopic compositions and determined δ²H and δ¹⁸O for different water mixing ratios with two CRDS analysers (Picarro, Inc.). Cotton wool had a strong fractionation effect on δ²H values, which increased with more ²H-enriched samples. δ²H and δ¹⁸O values showed a strong dependency on the water mixing ratio analysed with CRDS with differences of up to 34.5‰ (δ²H) and 3.9‰ (δ¹⁸O) for the same sample at different mixing ratios. CVE and CRDS, now routinely applied in water stable isotopologue research, come with pitfalls, namely fractionation effects of cover materials and water mixing ratio dependencies of δ²H and δ¹⁸O, which can lead to erroneous isotopologue results and thus, invalid conclusions about (ecohydrological) processes. These practical issues identified here should be reported and addressed adequately in water-stable isotopologue research.