A procedure is proposed to achieve the selective extraction of any given species from a mixture of closely related molecules exhibiting similar thermodynamic and kinetic properties. It is based on exposing the mixture to a reactive, out-of-equilibrium medium in the presence of a static and spatially periodic potential. Optimal results are obtained upon satisfying stochastic resonance conditions involving the rate constants, the diffusion coefficients of the chemical species, and the characteristic features of the periodic potential. In the case of an electric field, the corresponding relations are shown to be compatible with the current level of experimental achievement.