Quantifying absolute carbon isotope ratios by AMS

Abstract

Our AMS produced a ratio of instrument transmissions for 14C and 13C equal to 13/14 to 0.22±0.06% accuracy at a specific charge-changing energy of 235keV and giving absolute isotope ratios with that mass correction. The ratio for 13C and 12C transmissions was 12/13 at 215keV. A differential equation model of energetic ion and atom transport through the gas collision cell was constructed with functions representing experimental cross-sections for all collision effects. This model showed cation yield dependencies that were linear with energy for the monoenergetic isotopic ions at the energies that we measured, explaining the inverse mass dependence. The model predicted multiple lower energies that have this dependence and, more importantly, two low energies at which cation yields were equal for pairs of isotopes, presaging potential absolute isotope ratio measurements at low MS energies.