Field-Flow Fractionation of Cationic Cellulose Derivatives

The asymmetric flow field-flow fractionation (AF4) method was developed for cationic cellulose derivatives. AF4 is the method of choice especially for high-molar mass samples, which are challenging to characterize with conventional chromatographic techniques such as size-exclusion chromatography (SEC). The cationic charge of macromolecules also complicates the size-based separations where no interaction between the analytes and the column stationary phase (SEC) or membrane (AF4) should occur. However, many column matrices and membranes carry negative charge and thus preventing interactions between cationic analytes and negatively charged separation support should be taken into consideration when doing method development. In this study, two eluent compositions, neutral and acidic, were tested for AF4 separation of cationic hydroxyethyl celluloses with varying charge densities. The eluent composition with a pH below the isoelectric point of regenerated cellulose membrane, which was used in this AF4 study, enabled the size-based separation with close to 100% analysis recovery. Macromolecular parameters (molar mass and radius of gyration) and conformation were investigated by coupling a multi-angle light scattering detector and differential refractometer to the AF4 system.