COMPARISON OF ODS-MODIFIED SILICA-GELS AS STATIONARY PHASES FOR ELECTROCHROMATOGRAPHY IN PACKED CAPILLARIES

The electroosmotic mobilities of the eluent generated during electroch romatography with a range of commercial ODS-bonded silica gel stationa ry phases packed into fused-silica capillary columns of 50 mu m I.D. h ave been determined using thiourea as an unretained marker compound. T he electrochromatography was carried out using acetonitrile-25 mM 3-cy clohexylamino-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 pressuri sation 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 phase s had low chemical stabilities in the alkaline medium and only retaine d their properties for 2-10 days. Spherisorb S5 ODS2, Zorbax BP-ODS an d Hypersil ODS all demonstrated high chemical stability but showed onl y moderate or low electroosmotic mobilities. No electroosmotic how was detected with Partisil 5 ODS3 or with Purospher RP-18, highly base-de activated column materials. The electroosmotic mobilities have been co mpared with the carbon loading and surface areas of the stationary pha ses and with the silanol activities determined chromatographically on the same stationary phases packed into polyether ether ketone (PEEK) m icro columns (0.5 mm I.D.) using an aniline/phenol test mixture and el ution with methanol-water (45:55, v/v). However, there was little corr elation 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, m ay not be useful in electrochromatography because they may create only a limited electroosmotic flow-rate.