Separation of neutral compounds by capillary electrokinetic chromatography using polyethyleneimine as replaceable cationic pseudostationary phase

Polyethyleneimine (PEI, molecular weight 6 x 10(5) - 1 x 10(6)) is applied as a positively charged pseudostationary phase for electrokinetic chromatography (EKC) of uncharged mono- and oligophenols. EKC is carried out in PEI-coated fused-silica capillaries (with electroosmotic flow directed towards the anode) in 2-(N-morpholino)ethanesulfonic acid (MES) buffer (pH 7.0, 20 mM) with PEI added to the solution in concentrations up to 0.70% w/v. The pseudostationary phase leads to a retardation of the solutes mainly according to the number (and the position) of the OH-groups of the separands, and is not influenced significantly by methyl groups. For 0.70% w/v PEI solution, for instance, the relative retention, rho, has values between 0.33 and 0.53. For the systems with the highest resolution of the separands (0.25-0.30% PEI) 190 000 plates per meter are observed. The results indicate that the separation selectivity is mainly caused by ion-dipole interactions between the OH-groups of the solutes and the pseudostationary phase.