Self-assembled coating for modification of the electro-osmotic flow in nonaqueous capillary electrophoresis using formamide

Abstract

The use of pure nonaqueous solvents in capillary electrophoresis (CE) can alter the separation selectivity and enhance the solubility of hydrophobic compounds and enables the use of higher voltages. However, control of the electro-osmotic flow (EOF) is essential. In this work, we report the use of self-assembled coatings for EOF modification and elimination of analyte adsorptions onto silica capillaries in pure formamide. Bilayer capillary coatings derived from the double chain cationic surfactant dimethyldioctadecylammonium bromide (2C18DAB) reverses the EOF in buffers such as acetate, formate, and phosphate. Reversed EOF of >1.1 × 10–4 cm2/Vs enables the separation of the pharmaceutical drugs propranolol, metoprolol, chloroquine, and chloropheniramine in less than 5 min with efficiencies of 0.2–0.5 million plates/m (66 000 to 165 000 plates). Chemical and physical factors affecting the coating stability and their influence on separation speed and efficiency of the cationic drugs in formamide are also investigated.Key words: capillary electrophoresis, cationic surfactants, electro-osmotic flow, formamide, nonaqueous solvents.