Diffusion of reactive species tuned by modulated external fields: Application to high performance chromatography

In order to improve the separation of any given chemical species from a mix ture of compounds with close thermodynamic and kinetic properties, we propo se a new chromatography procedure in the presence of a uniform time-periodi c field. In the framework of a macroscopic reaction-diffusion model in an e xternal field, we prove that the apparent motion of the chemical species is of diffusion type and determine an approximate analytical expression for t he effective diffusion coefficient. Considering this coefficient as a funct ion of the rate constants and maximizing it leads to specific relations bet ween rate constants and field properties interpreted as stochastic resonanc es. In the case of an electric field, we show that these constraints are co mpatible with typical experimental values. (C) 2000 American Institute of P hysics. [S0021- 9606(00)51219-7].