Isotopic Fractionation in a Thermal Ion Source

Isotopic fractionation behavior in a thermal ion source was theroretically studied. An equation relating to the observed isotope ratio with the evaporated fraction of the sample was derived under conditions where two kinds of chemical species, atomic (atomic ions and neutral atoms) and molecular (molecular ions and molecules) species, simultaneously evaporate from the filament. It was demonstrated that integrations of all ion currents for two isotopes do not necessarily give a true ratio of the two isotopes in the sample when two evaporating chemical species are ionized at different rates. Isotopic fractionation curves for halides and elements of lithium, potassium, and rubidium are presented.