Rayleigh fractionation
Rayleigh fractionation refers to the fractional distillation of mixed liquids. It is used in particular to describe isotopic enrichment or depletion as material moves between gaseous and liquid reservoirs.
The Rayleigh equation
The isotopic literature abounds with different approximations of the Rayleigh equations, including the equations below. These equations are so-named because the original equation was derived by Lord Rayleigh for the case of fractional distillation of mixed liquids. This is an exponential relation that describes the partitioning of isotopes between two reservoirs as one reservoir decreases in size. The equations can be used to describe an isotope fractionation process if: (1) material is continuously removed from a mixed system containing molecules of two or more isotopic species (e.g., water with 18O and 16O, or sulfate with 34S and 32S), (2) the fractionation accompanying the removal process at any instance is described by the fractionation factor a, and (3) a does not change during the process. Under these conditions, the evolution of the isotopic composition in the residual (reactant) material is described by:
(R / Rº) = (X1 / X1º)^(a-1)
where R = ratio of the isotopes (e.g., 18O/16O) in the reactant, Rº = initial ratio, Xl = the concentration or amount of the more abundant (lighter) isotope (e.g.,16O), and X1º = initial concentration. Because the concentration of Xl >> Xh, Xl is approximately equal to the amount of original material in the phase. Hence, if ƒ = Xl/X1º = fraction of material remaining, then:
R = Rº ƒ(a-1)
For large changes in concentration, such as they occur during e.g. distillation of heavy water, these formula's need to be integrated over the distillation trajectory. For small changes such as occur during transport of water vapour through the atmosphere, the differentiated equation will usually be sufficient.