Separation of ergot alkaloids by micellar electrokinetic capillary chromatography using cationic Gemini surfactants

A micellar electrokinetic capillary chromatographic method using novel cationic dimeric (Gemini) surfactants has been developed for the separation, in less than 8 min, of 17 dihydroergotoxines, aci-alkaloids and oxidation products. Both 1,3-bis(dodecyl-N,N-dimethyl ammonium)-2-propanol (20 mM) and 1,3-bis(tetradecyl-N,N-dimethyl ammonium)-2-propanol (40 mM) surfactants in 50 mM phosphate buffer, pH 3.0, at 20 degrees C: produced separations that could not be achieved using single chain tetradecyl- or hexadecyl-trimethyl ammonium micelles. The retention factors (k') increased linearly with surfactant concentration in accordance with theory. Similarly, the net mobilities generally decreased nonlinearly with surfactant concentration. The smallest alkaloids, i.e. the 6-methylergoline-8-carboxylic acids, and the more polar aci-alkaloids with a Z-type conformation, have the smallest k' values. For the larger alkaloids, the k' values with the Gemini surfactants were smaller than with the single chain species, possibly because the more rigid dimeric molecules hinder the partitioning of the alkaloids into the micelle as well as the greater hydrophilicity of the hydroxylated micelles. Increasing buffer pH mainly affects the more strongly acidic and basic alkaloids, causing a marked increase and decrease in net mobilities, respectively, k' values generally increased with phosphate buffer concentration because of the decreasing solubility of the hydrophobic alkaloids with increasing ionic strength. (C) 1998 Published by Elsevier Science B.V. All rights reserved.