Effect of the particle size in capillary electrochromatography.

Abstract

The application of the electro-osmotic flow to force the mobile phase in packed capillary liquid chromatography looks very attractive from a theoretical point of view. Due to the plug flow profile less broadening of the bands by convective mixing will occur and larger efficiencies can be expected than under pressure conditions where the flow profile is parabolic. Moreover, the electro-osmotic flow is independent of the channel width and thus an acceptable flow-rate can be obtained in columns of common length packed with very small particles. The effect of the particle size of the reversed packing on the performance was investigated. The particle size ranged from 1.5 to 5 μm. The efficiency of the columns was measured under pressure and electrically driven conditions. All of the experiments with 3 and 5 μm ODS-Hypersil particles were performed with a home made system, with the electrode vials at ambient pressure. The experiments with 1.5 μm ODS-Chromspher particles were performed with a HP 3DCE (Hewlett Packard) system in which both electrode vials were equally pressurised at 10 bar. In Fig. 1 the effect of particle size on obtained plate height of pyrene as neutral compound with CEC is presented. As can be seen the plate height decreases with decreasing particle size in according with theory. With 3 and 5 μm particles the reduced plate height was about 2, but with the non-porous 1.5 μm ODS- Chromspher reduced plate height of 1.3 was observed. In order to avoid bubble formation with the 1.5 μm particles the two electrode vials has to be pressurised at 10 bar and 1–5 mmol/L sodium dodecyl sulfate (SDS) has to be added to the mobile phase. Fig. 2 shows a very fast and efficient separation of a mixture alkylbenzenes on the 1.5 μm ODS-Chromspher column. The capillaries packed with 1.5 μm ODS non-porous particles appeared to be extremely efficient and on a 24 cm long column about 120,000 plates were generated (∼ 500,000 plates/m). The columns are quite stable and have been used for more than two months without loss in efficiency. © 1998 John Wiley & Sons, Ltd.