Studies on the analytical performance of a non-covalent coating with N,N-didodecyl-N,N-dimethylammonium bromide for separation of basic proteins by capillary electrophoresis in acidic buffers in 25- and 50-μm capillaries

Capillaries (25-and 50-mu m inner diameter) coated with a double-alkyl-chain cationic surfactant N,N-didodecyl-N,N-dimethylammonium bromide (DDAB) were used for the separation of four basic standard proteins in buffers of pH 4 at various ionic strengths. The choice of buffer is critical for the analytical performance. Ammonium ions must be avoided in the buffer used in the non-covalent coating procedure owing to competition with the surfactant. Phosphate buffer gave a better separation performance than some volatile buffers; the reason seems to be a complex formation between the proteins and dihydrogenphosphate ions, which decreases tendencies for adsorption to the capillary surface. The DDAB coating was easy to produce and stable enough to permit, without recoating, consecutive separations of the proteins for up to 100 min with good precision in migration times and peak areas. A strong electroosmotic flow gives rapid separations, which is of special importance when commercial instruments are used, since the choice of the length of the capillary is restricted.