Fractionation in the thermal ionization source

Abstract

In this paper a model for isotopic fractionation in the thermal ionization source is presented. The samples, normally loaded as salts, are assumed to evaporate, in general, as binary vapors of two chemically different forms. The molecular species in the vapor might either dissociate before ionization and the metal might then be ionized; or, alternatively, molecular ions might be generated which then dissociate into metal ions. Whereas isotopic effects during ionization are negligible, such effects have to be considered for the dissociation process.

The dependence of the observed isotope ratio on the chemical form of the loaded sample and on the temperature of the ionization can be explained with this model, whereas the time dependence and the effects of reverse or enhanced fractionation of the observed isotope ratio are readily explained by a generalized Rayleigh distillation equation.

The application of the fraetionation model to the normalization of observed isotope ratios to an internal standard ratio shows the principal limits for the accuracy of normalization. The commonly used normalization techniques and their inherent errors are considered in the light of the fractionation model and an improved normalization formula is presented which uses the concept of the “apparent mass”. Finally, the model is used to propose experimental methods for the accurate determination of non-normalizable isotopic ratios.