Distillation law and exponential model of isotope fractionation

The exponential law is an empirical model which is used to describe the isotopic fractionation both in instrumental (thermal ionization- and induced coupled plasma-source mass spectrometry) and in natural processes. It is shown that isotopic fractionation which occurs according to this law can be interpreted in terms of a Rayleigh-type distillation process, in which the value of the vapour/residue distribution coefficient for the various isotopes of an element is a natural logarithmic function of the isotope mass: D = D0 + a ln(m(m0) . In this equation, D0 is the value of the vapour/residue distribution coefficient for a reference mass m0 and a = p/(1+ln f ) , where p is the instantaneous value of the so called "fractionation exponent" (or "fractionation coefficient") of the exponential law, and f is the residual mass fraction of the sample at the same instant.