  We model the evolution of the concentration field of macromolecules in a
symmetric field-flow fractionation (FFF) channel by a one-dimensional
advection-diffusion equation. The coefficients are precisely determined from
the fluid dynamics. This model gives quantitative predictions of the time of
elution of the molecules and the width in time of the concentration pulse. The
model is rigorously supported by centre manifold theory. Errors of the derived
model are quantified for improved predictions if necessary. The
advection-diffusion equation is used to find that the optimal condition in a
symmetric FFF for the separation of two species of molecules with similar
diffusivities involves a high rate of cross-flow.
