Effects of organic mobile phase modifiers on elution and separation of beta-blockers in micellar electrokinetic capillary chromatography.

A study was made of the effect of organic modifiers (acetone, acetonitrile, ethanol, ethylene glycol, methanol and 2-propanol) in phosphate buffer (0.08 M) containing 15 mM cetyltrimethylammonium bromide as surfactant on the elution and separation of eleven common beta-adrenergic blocking agents. The amount of the modifier was varied from 0.1 to 10.0% (v/v). At maximum addition, the organic solvents increased the viscosity of the buffer solution as follows: acetone 16%, acetonitrile 9%, ethanol 26%, ethylene glycol 27%, methanol 20% and 2-propanol 29%. In contrast to the migration time of the other beta-blockers, that of labetalol was not increased by the addition of organic solvent to the buffer solution. Rather, labetalol eluted more quickly with increase in the amount of modifier, and thereby effected changes in the elution order of the beta-blockers. The addition of modifiers also affected the resolution, and the best resolution values were achieved with the following amounts of organic solvent in MEKC buffer: acetone 0.1%, acetonitrile 0.1-0.5%, ethanol 5.0-7.5%, ethylene glycol 1.0-2.5%, methanol 5.0% and 2-propanol 1.0-2.5% (v/v). No significant relationship was found between the elution order and separation and the structure of the beta-blockers in micellar electrokinetic capillary chromatography with an organic modifier in buffer solutions.