Investigation of the causes of reduced efficiency in micellar liquid chromatography

Abstract

Reduced chromatographic efficiency is a major drawback of micellar liquid chromatography (MLC). The Knox equation h  Av 1/3 + B/v + Cv, was used to determine the individual contribution of the flow anisotropy ( A term), molecular band broadening ( B term) and mass transfer processes ( C term) to the final band broadening. Knox plots of h, the reduced plate height, versus v, the reduced linear flow-rate, were determined on the same column (i) with an aqueous—organic mobile phase, (ii) with a micellar mobile phase and (iii) with the same aqueous—organic phase. The changes in the A, B and C terms are discussed. Two stationary phases were used: a classical C 18 monomer phase and a densely grafted (3.5 μmol/m 2) C 14 phase. Two micellar solutions were used: a non-ionic micellar solution of Brij 35 and an anionic solution of sodium dodecyl sulfate(SDS). Test solute diffusion coefficients were measured in each mobile phase used. The increase in A is mainly responsible for reduced MLC efficiency. However, the B and C terms also increased significantly with micellar solutions. It is shown that the observed changes in the knox parameters can be explained by the change in the stationary phase produced by surfactant adsorption; 6% of the adsorbed SDS (0.14 μmol/m 2) was irreversibly adsorbed on the C 14 phase whose initial efficiency could not be restored. Such a small amount of adsorbed surfactant was able to degrade completely the initial efficiency of the C 14 stationary phase. A model explaining how that irreversible adsorption may occur with ionic long-chain surfactants and densely grafted stationary phases with long-chain (C 8) bonding moieties is proposed.