Migration behavior and separation of tetracycline antibiotics by micellar electrokinetic chromatography

The migration behavior and separation of six tetracyclines (TCs) were investigated by micellar electrokinetic chromatography (MEKC) in the pH range 5.0-9.0 using ammonium acetate buffer with the addition of sodium dodecyl sulfate (SDS). Mixed SDS-Brij 35, sodium cholate (SC) and tetradecyltrimethylammonium bromide (TTAB) were also used as surfactants. The influences of surfactant concentration and buffer pH on the separation of TCs were examined and the separations of TCs were optimized. Complete separation of six TCs was achieved within 8 min with 15 mM ammonium acetate buffer containing 20 mM SDS, with or without the addition of Brij 35 (0.135%, w/v), at pH 6.5 using a fused-silica capillary (42 cm x 75 microns I.D.) at 15 kV. In general, good linear correlations of the logarithm of migration factor (log k') versus the logarithm of octanol-water partition coefficient (log P(ow)) in these micellar systems, except for the TTAB-MEKC system, were obtained. The results indicate that the migration of TCs in MEKC is mainly based on hydrophobic interactions. However, hydrogen bonding interactions also play a significant role in influencing the chemical selectivity of TCs. In addition, the micelle-water partition coefficients (Pmw) of TCs, which are pH-dependent in the SDS-MEKC micellar system, are reported.