We point out the specificities of the separation process at work during liquid chromatography of polymers in a gradient of composition of the eluent. Tile main feature is that, after a transient regime, the concentration profile tends to move as a front, i.e., without diffusive spreading in time. This results in relatively sharp peaks and the uselessness of long columns. We propose a simple analytical model for this self-focusing process that allows us to anticipate optimal conditions of operation. This analysis is supported by experimental tests.