Using isotopic disequilibrium of CO2 to model gas adsorption in mass spectrometric measurements

Mass spectrometric measurements were carried out on a CO2 sample prepared by mixing known amounts of 'natural' CO2 with CO2 enriched in C-13 and, to a lesser extent, in O-17 and O-18. The sample was in isotopic disequilibrium, i.e. with the isotopic species distribution not obeying the statistical probability. The results indicate that isotope scrambling reactions occur during the measurement which tend to shift the gas isotopic composition towards the equilibrium. The isotope exchange reactions take place via gas adsorption in the mass spectrometer inlet system. A Langmuir-type adsorption model, developed independently to describe SiF4 adsorption in the mass spectrometer inlet system, is applied to the CO2 scrambling reactions. The model is capable of fitting the experimental data adequately, and sets of values for the isotope exchange kinetics can be derived from it. Small deviations from the model prediction may be due to isotope fractionations which have not been taken into account and require further investigation. (C) 1997 Elsevier Science B.V.