Oxygen Isotope Analysis of Saline Solutions by a Carbonic anhydrase-Catalyzed CO2–H2O Equilibration Method (C3HEM) with an Improved Drying Technique

The recently introduced carbonic anhydrase-catalyzed CO2–H2O equilibration method (or C3HEM) for oxygen isotopes of aqueous samples has remarkably improved the analytical efficiency of the classic and most popular stable isotope analytical method in geochemistry. However, the applicability of this novel analytical method has not yet been tested for saline/hypersaline solutions despite their significance in various scientific problems. In this study, 1 m chloride and sulfate solutions of geochemical importance were used to investigate the kinetics of oxygen isotope exchange between CO2 and H2O at 25 °C. For these saline solutions, oxygen isotope equilibrium times (EQTs), rate constants, and oxygen isotope salt effects (OISEs) were determined with 4 or 16 μm carbonic anhydrase (CA). Our experimental results reveal that EQTs, ranging from 0.73 to 4.48 h, were much shorter than the typical EQT, confirming the greatly improved analytical efficiency of the C3HEM for saline solutions. In addition, a new simpler CA drying technique was developed, further improving the analytical efficiency of the C3HEM by significantly reducing the required labor time for sample vial preparation. Detailed analytical protocols and practical guidelines of the C3HEM are provided for prospective users.