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 lnmm0.In this equation, D0 is the value of the vapour/residue distribution coefficient for a reference mass m0 anda=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.

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► The exponential law of isotope fractionation is modelled in terms of distillation law.
► The relationship between distribution coefficient and isotope mass is found.
► The distribution coefficient is a natural logarithmic function of isotope mass.