Double-chained surfactants for semipermanent wall coatings in capillary electrophoresis

The double-chained cationic surfactant didodecyldimethylammonium bromide (DDAB) was found to form more stable coatings onto the walls of CE capillaries than similar single-chained surfactants such as cetyltrimethylammonium bromide (C16TAB). After removing DDAB from the buffer, the reversed EOF decreased only 3% over 75 min under continuous electrophoretic conditions. Also, the reversed EOF is 60% greater for DDAB than for C16TAB at pH 2. This greater coating stability is associated with a different aggregate structure for the surfactant at the capillary surface. The more homogeneous coating and greater surface coverage provided by DDAB allows the excess surfactant to be flushed from the capillary prior to performing electrophoretic separations. Separations of a basic protein mixture yielded quantitative recoveries, efficiencies ranging from 560,000 to 750,000 plates/m, and migration time reproducibility of 0.8-1.0% RSD (n = 10). This performance is similar to that of adsorbed cationic polymers (Polybrene, polyethyleneimine) but is achieved using a coating procedure that is over 10 times faster.