We point out the specificities of the separation process at work during liquid chromatography of polymers in a gradient of composition of the eluent. The 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.

SYNOPSISIn this paper, the shape of chromatograms obtained by elution of copolymers with a mixture of solvent-nonsolvent of variable composition has been studied from a theoretical point of view. It is assumed that the thermodynamic quality of the mixture a t which the copolymer is eluted depends only on its composition and not on its mass and structure. If one characterizes the polydispersity in composition, assuming that each constituent obeys a molar mass distribution of the Zimm-Schulz type, one can draw the following curves: concentration of the copolymer versus eluant composition. They depend on three parameters, including composition of the copolymer and the polydispersity of each constituent. The most striking result is that the shape of the chromatogram changes dramatically when one increases the polydispersity of either constituents. For low polydispersities, one obtains a classical peak. For large polydispersities, the chromatogram has a minimum for the intermediate values of composition and presents peaks in the vicinity of the pure homopolymers. This can be explained easily by a qualitative argument.