Influence of ionic strength and organic modifier on performance in capillary electrochromatography on phenyl silica stationary phase

The influence of three physicochemical parameters (temperature, ionic strength and organic modifier content of the hydro-organic buffer) upon electrophoretic (electroosmotic flow, EOF, chromatographic (retention factor) and separation (retention time, peak efficiency) performances has been carefully investigated in capillary electrochromatography (CEC) on a phenyl bonded silica column. Five benzodiazepines (diazepam, lorazepam, oxazepam, temazepam, tofisopam) have been selected as test solutes. From our CEC results, an increase of the organic modifier content induces an increase of EOF and peak efficiency and a decrease of retention factor. Concerning the ionic strength parameter, an increase of the ionic strength undergoes a decrease of EOF and retention factor and an increase of peak efficiency. Finally, higher temperature of the column involves an increase of EOF and peak efficiency and a decrease of retention factor. So, the modification of ionic strength and temperature in CEC can mainly be interpreted as a CE-like behavior at the opposite of organic modifier content which acts as a LC-like behavior. At last, the CEC separation of these benzodiazepines has been achieved in 18 min, using Tris . HCl (pH 8)-acetonitrile (60:40) mixture, ionic strength 5 mM as mobile phase, and a 3 mu m phenyl-bonded silica as stationary phase. High peak efficiencies (200000 theoretical plates/meter) and resolutions of 1.5 are easily obtained. (C) 1999 Elsevier Science B.V. All rights reserved.