The flux ratio equation under nonstationary conditions.

The time dependent (i.e., nonstationary) unidirectional fluxes through a multilayered system consisting of sandwiched layers of arbitrary composition and exhibiting arbitrary potential and resistance profiles have been calculated, assuming that the flux is governed by the Smoluchowski equation (i.e., a flux resulting from a diffusion process superimposed upon a migration and/or a convection process, where part of the latter may arise from an active transport process). It is shown that during the building up of the concentration profile of the isotope inside the system towards the stationary value the ratio between the two oppositely directed, time-dependent unidirectional fluxes is, from the very first appearance of the isotope in the surrounding solutions, equal to the value of the stationary flux ratio. The practical implications of this result are discussed.