Isotope effects accompanying δ 2 H, δ 18 O and δ 17 O analyses of aqueous saline solutions using cavity ring-down laser spectroscopy

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. Here we report the results of laboratory experiments aimed at quantifying isotope effects associated with direct, δ H, δ O and δ O analyses of single-salt solutions and double-salt mixtures prepared with a water of known isotopic composition. Three single-salt solutions (NaCl, CaCl and MgSO ) and two double-salt mixtures (NaCl + CaCl and NaCl + MgSO ) 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. The NaCl and CaCl solutions revealed small negative salt effects, independent of molality and comparable with measurement uncertainty. The MgCl solution showed the highest salt effects, reaching saturated solution ca. +2.7‰ ( H), -3.5‰ ( O) and -1.7‰ ( O). 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. 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-excess determinations for solutions forming hydrated salts preclude the use of this parameter for interpretation purposes.