Isotope effects accompanying δ2H, δ18O and δ17O analyses of aqueous saline solutions using cavity ring-down laser spectroscopy

Rationale: The presence of substantial amounts of dissolved salts creates serious difficulties in isotope analyses of water samples using conventional isotope ratio mass spectrometry. Although nowadays laser-based instruments are increasingly used for this purpose, a comprehensive assessment of isotope effects associated with direct analyses of aqueous saline solutions using this technology is lacking.Methods: Here we report the results of laboratory experiments aimed at quantifying isotope effects associated with direct, delta H-2, delta O-18 and delta O-17 analyses of single-salt solutions and double-salt mixtures prepared with a water of known isotopic composition. Three single-salt solutions (NaCl, CaCl2 and MgSO4) and two double-salt mixtures (NaCl + CaCl2 and NaCl + MgSO4) were prepared and investigated for a wide range of molalities. The triple-isotope composition of the prepared solutions was analysed with the aid of a Picarro L2140-i Cavity Ring-Down Spectroscopy analyser.Results: The NaCl and CaCl2 solutions revealed small negative salt effects, independent of molality and comparable with measurement uncertainty. The MgCl2 solution showed the highest salt effects, reaching saturated solution ca. +2.7 parts per thousand (H-2), -3.5 parts per thousand (O-18) and -1.7 parts per thousand (O-17). Salt effects for the double-salt mixtures generally mirrored the effects observed for the single-salt solutions. The observed salt effects are discussed in the context of processes occurring during the injection of the salt solutions into the vaporizer unit of the CRDS analyser.Conclusions: The presented study has demonstrated feasibility of direct, triple-isotope analyses of aqueous salt solutions using a Picarro L2140-i CRDS analyser for a broad range of salinities up to saturated conditions. Large uncertainties of O-17-excess determinations for solutions forming hydrated salts preclude the use of this parameter for interpretation purposes.