Enzymatically catalyzed CO2‐H2O equilibration for oxygen isotope analyses of aqueous samples

RATIONALE: The classic CO₂ -H₂ O equilibration method is a very popular technique for the measurement of the oxygen isotope composition of aqueous samples in stable isotope geochemistry. This study examined whether enzymatically controlled CO₂ -H₂ O equilibration by carbonic anhydrase (CA) could reduce the time for oxygen isotope equilibrium between CO₂ and H₂ O at 25°C. METHODS: Four types of aqueous samples containing CA were equilibrated with CO₂ gases using a continuous flow isotope ratio mass spectrometer equipped with an automated gas sample collection device. We examined the effect of CA concentration in an aqueous sample, the influence of drying technique for the preparation of sample vials containing dried CA, the age of CA stock solution, and the ionic strength and the oxygen isotope composition of aqueous samples. RESULTS: CA rapidly catalyzed the oxygen isotope exchange between CO₂ and H₂ O and was unaffected by drying technique or stock solution age. Compared with aqueous samples with no CA or 0.2 μmolal CA, samples containing 4 μmolal CA significantly reduced the CO₂ -H₂ O equilibration time for deionized water and artificial seawater (ionic strength = ~0.6) from ~19 h and ~23 h to ~0.30 h and ~0.77 h, respectively at 25°C. CONCLUSIONS: This enzymatically catalyzed CO₂ -H₂ O equilibration method is time-efficient, cost-effective, requires no additional data correction procedure, and can be used for most commercially available CO₂ -H₂ O equilibration devices without any modification.