COMPARISON OF SEPARATION SELECTIVITY IN AQUEOUS AND NONAQUEOUS CAPILLARY ELECTROPHORESIS

The use of non-aqueous capillary electrophoresis in free solution in uncoated fused-silica capillaries was investigated and compared with capillary electrophoresis using aqueous systems. Cationic drug substances differing only in the amine functionallity or even having the same charge-to-mass ratio were used as test solutes. Some of these test substances are difficult to separate in plain aqueous buffers and even when using micellar electrokinetic chromatography or by adding cyclodextrins to the electrophoresis buffer it is difficult to obtain baseline separations. Very high separation selectivity may be obtained in non-aqueous capillary electrophoresis systems without adding surfactants or complexing agents to the electrophoresis medium. Major selectivity changes may be obtained using different organic solvents (formamide, N-methylformamide, N,N-dimethylformamide, N,N-dimethyiacetamide, dimethyl sulfoxide, methanol and acetonitrile) for the electrophoresis medium, e.g., the relative electrophoretic mobilities of a primary, a secondary and a tertiary amine with otherwise identical structures are reversed on replacing acetonitrile with methanol as solvent.