Maria Rowena N. Monton

Separation and on-line preconcentration by sweeping of charged analytes in electrokinetic chromatography with nonionic micelles

The separation and on-line preconcentration by sweeping of charged analytes in electrokinetic chromatography using a neutral pseudostationary phase is described. Under neutral or basic conditions, the electrophoretic migration of anionic analytes towards the anode is overcome by a high, cathode-directed electroosmotic flow; hence, they experience net migration towards the cathode, and the system is run at positive polarity mode. The separation and the retention factor, k, are dependent on both the analytes electrophoretic mobility and its interaction with the pseudostationary phase. The versatility of the sweeping mechanism is then shown in this system. The charged analyte, prepared in a matrix free of the pseudostationary phase, penetrates the pseudostationary phase zone upon application of voltage. Analyte molecules are consequently accumulated and concentrated. As a demonstration, the separation and preconcentration of phenol derivatives using nonionic surfactants of the alkyl polyoxyethylene ether type (Brij 35 and Brij 58) yielded peak height enhancements up to 100-fold. The efficiency of sample stacking was also found to be improved with the use of a high viscosity background solution.

On-line sample preconcentration in micellar electrokinetic chromatography by sweeping with anionic–zwitterionic mixed micelles

On-line preconcentration by sweeping in micellar electrokinetic chromatography using mixed micelles of sodium dodecyl sulfate (SDS)-SB-12 is presented. Because of their large micelle radius, they permit increased partitioning of hydrophobic analytes into the core. In addition, they also possess lower negative surface charge relative to pure SDS micelles so anionic analytes can be retained better due to decreased electrostatic repulsion. As the efficiency of sweeping is predicated on the magnitude of retention factors, these advantages translated to better focusing. As much as a 370-fold improvement in detector response, in terms of peak height, was obtained for some neutral steroids, while about a 360-fold improvement was obtained for some phenol derivatives, which were previously not amenable to sweeping by pure SDS micelles.