Fractionation correction and multiple collectors in thermal ionization isotope ratio mass spectrometry

Abstract

A comprehensive theory for the correction of time- and mass-dependent evaporation-related mass fractionation effects is developed. The Rayleigh distillation equation and the Langmuir equation describing evaporation are combined and are formulated here as the “Rayleigh-Langmuir” fractionation model, which provides the only currently available fundamental or “causal” description of evaporation of isotopes in a vacuum. This “Rayleigh law” relates the depletion of isotopes in a sample (fractionation) to the amount of sample left in a reservoir. Other, commonly used fractionation laws (exponential law, power law, linear law, etc.) are more or less ad hoc, being based on empirical evidence. The relevant observational facts on which such laws are based are reviewed, and the laws are compared concerning their ability to fit the experimental data. Important time-independent (static) discrimination effects are also discussed. Fractionation correction algorithms for static and dynamic multiple Faraday collector data acquisition schemes are given, applying causal and empirical laws. It is shown that the fundamental limit to the ability to measure the true isotope ratio in a solid sample by thermal ionization mass spectrometry is currently determined by incomplete knowledge of the evaporation process, insufficient reproducibility of the evaporation process, and an inability to measure with sufficient accuracy the parameters that define static discriminations.