Enzymatically catalyzed CO -H O equilibration for oxygen isotope analyses of aqueous samples.

Rationale: The classic CO -H 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 CO -H O equilibration by carbonic anhydrase (CA) could reduce the time for oxygen isotope equilibrium between CO and H O at 25°C.

Methods: Four types of aqueous samples containing CA were equilibrated with CO 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 CO and H 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 CO -H 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 CO -H O equilibration method is time-efficient, cost-effective, requires no additional data correction procedure, and can be used for most commercially available CO -H O equilibration devices without any modification.