Isotopic analyses based on the mass spectra of carbon dioxide

Abstract

Measurements of carbon and oxygen isotopic abundances are commonly based on the mass spectrum of CO2, but analysis of that spectrum is not trivial because 3 isotope ratios (17O/16O, 18O/16O and 13C/12C) must be determined from only 2 readily observable ion-current ratios (45/44 and 46/44). Approaches to the problem were reasessed in the light of new information regarding the distribution of oxygen isotopes in natural samples. It was shown that methods of calculation conventionally employed can lead to systematic errors in the computed abundance of 13C and that these errors may be related to incorrect assessment of the absolute abundance of 17O. Problems arising during the analysis of samples enriched by admixture of 18CO-labeled materials were discussed and it was shown that serious inaccuracies arise in the computed abundance of 17O and 13C if methods of calculation conventionally employed in the analysis of natural materials are applied to material labeled with 18O and that computed fractional abundances of 18O are always within 0.4% of the correct result. Methods for exact calculation of 2 isotope ratios when the 3rd is known were presented and discussed and a more exact approach to the computation of all 3 isotope ratios in natural materials was given.