Quantification of the degree-of-isotopic-equilibrium of carbon and oxygen isotopes in mixtures of CO2 gases

A model has been developed enabling to quantify the degree of scrambling of carbon and oxygen isotopes in mixtures Of CO2 gases with different isotopic compositions. If such isotope mixtures are quantitatively prepared ('synthesized') they are 'embodiments' ('realizations') of SI units, and can serve as 'calibrators' of measurements of C and O isotope amount ratios, thus establishing metrological traceability of the measurement results to the SI. Absence of complete equilibrium will cause further isotope scrambling reactions during the isotope amount ratio measurements in the mass spectrometer until the realization of full isotopic equilibrium, but biasing the measurement results continuously before the equilibrium is attained. The concept of isotope ratio space has been conceived to describe this process. This model fits the experimental data adequately and enables to determine the degree-of-isotopic-equilibrium in the gaseous mixture at any time during the mixing process. This creates the possibility to verify the achievement of the isotopic equilibrium prior to the (ion current ratio) measurements. (C) 2007 Elsevier B.V. All rights reserved.