In vitro application of carbonic anhydrase to accelerate the equilibration of 18O between H2O and CO2 for the rapid measurement of 18O/16O isotope ratios in aqueous samples

Abstract

ABSTRACT A novel method of the accelerated equilibration of 18O between CO2 and H2O for the measurement of the 18O/16O isotope ratios in aqueous samples with natural isotope abundances is presented. This rapid equilibrium method is based on the in vitro application of the enzyme carbonic anhydrase (CA). The CA from bovine erythrocytes was adsorptively fixed to 3-mm glass beads with an etched surface. After the addition of this carrier-fixed CA catalyst to the water sample, the isotope equilibrium was already reached after 1 h. The previously used non-catalysed 18O isotope exchange in water samples needs about 24 h. Whole blood samples also showed fast 18O isotope equilibration, which definitely results from the native presence of CA in erythrocytes. By shortening the time for sample preparation, the CA catalysed technique can significantly increase the throughput of the samples to be measured, and also 18O and 2H measurement by means of isotope ratio mass spectrometry (IRMS) may be synchronized. The 2H and 18O sample preparation can be performed in the same reaction vessel because cross-effects at the simultaneous use of Pt and CA catalysts do not occur.