AgF desalination procedure for the routine determination of oxygen and hydrogen isotopic composition of saline waters and brines

The routine methods for stable oxygen and hydrogen isotope analysis of water involve water–CO₂ gas equilibration and water reduction on hot metal (e.g. Zn, Cr, U) and subsequent mass spectrometric analysis of the evolved gases of CO₂ and H₂ for ¹⁸O/¹⁶O and ²H/¹H ratios, respectively. Precise determination of the isotopic composition of water in brines with application of these standard methods is still problematic and technically often impossible due to detrimental influence of dissolved salts. The new method of brine desalination presented in this study overcomes the problem of the isotope salt effects encountered during the application of the routine techniques for the determination of the isotopic composition of high saline waters. The procedure combines two technical steps: (i) the chemical precipitation of Mg and Ca ions as insoluble non-hydroscopic fluorides, and (ii) the vacuum distillation of water from solution–precipitate mixture. The application of simple vacuum distillation allows full extraction of water and dehydration of remaining salts in a temperature range from 300 to 350?°C without hydrogen and oxygen isotope fractionation. The precision and accuracy of δ¹⁸O and δ²H determination of saline waters and brines with prior application of AgF desalination procedure is comparable with that usually obtained for fresh waters.