Comparison of ODS-modified silica gels as stationary phases for electrochromatography in packed capillaries

The electroosmotic mobilities of the eluent generated during electrochromatography with a range of commercial ODS-bonded silica gel stationary phases packed into fused-silica capillary columns of 50 mu m I.D. have been determined using thiourea as an unretained marker compound. The electrochromatography was carried out using acetonitrile-25 mM 3-cyclohexylamino-2-hydroxy-1-propanesulfonic acid buffer (pH 9.53) (70:30, v/v) as the mobile phase with electric field strengths in the range of 0.2 to 1.5 kV/cm. The experiments were carried out without pressurisation of the column, but the temperature of the column was controlled with a liquid coolant. Nucleosil 5 C-18 and LiChrospher RP-18 showed the highest electroosmotic mobilities. However, these stationary phases had low chemical stabilities in the alkaline medium and only retained their properties for 2-10 days. Spherisorb S5 ODS2, Zorbax BP-ODS and Hypersil ODS all demonstrated high chemical stability but showed only moderate or low electroosmotic mobilities. No electroosmotic how was detected with Partisil 5 ODS3 or with Purospher RP-18, highly base-deactivated column materials. The electroosmotic mobilities have been compared with the carbon loading and surface areas of the stationary phases and with the silanol activities determined chromatographically on the same stationary phases packed into polyether ether ketone (PEEK) micro columns (0.5 mm I.D.) using an aniline/phenol test mixture and elution with methanol-water (45:55, v/v). However, there was little correlation between the electroosmotic flow-rate and the silanol activity or carbon loading of the stationary phases. It appeared that materials specially suited for good liquid chromatography, such as Purospher, may not be useful in electrochromatography because they may create only a limited electroosmotic flow-rate.