Enantiomeric separation of methadone by cyclodextrin-based capillary and recycling isotachophoresis

The separation of methadone enantiomers by cationic capillary isotachophoresis (CITP) and recycling isotachophoresis (RITP) having (2-hydroxypropyl)-beta-cyclodextrin (OHP-beta-CD) as chiral selector in the leading electrolyte is described. Sodium acetate/acetic acid (pH between 4 and 5) served as leading electrolyte (catholyte) and acetic acid as terminator (anolyte). Complete separation of the enantiomers was obtained by CITP in a 50 microm internal diameter (ID) fused-silica capillary and in a 500 microm ID Teflon capillary. In the first approach, enantiomeric separation could be monitored via UV absorbance detection at low wavelength. With the second instrumental setup, an additional conductivity sensor permitted the visualization of the enantiomeric separation and the characterization of the buffer system employed. A 10 mM sodium acetate/acetic acid leading buffer of pH 4.3, containing 5 mM OHP-beta-CD, was found to provide best enantiomeric separation and was thus chosen for RITP. With RITP processing of a few mg of racemic methadone, partial separation of methadone enantiomers was obtained. R-(-)-methadone and S-(+)-methadone were found to be significantly (up to about 80%) enriched at the front and back side, respectively, of the isotachophoretic zone. The enantiomeric composition of methadone in the collected fractions was assessed by chiral capillary zone electrophoresis (CZE) and circular dichroism spectroscopy. CZE was found to represent a simple and efficient method for the determination of the enantiomeric excess, whereas the latter technology was noted to be the superior approach for properly characterizing fractions that contain similar amounts of the two enantiomers. Furthermore, chiral RITP and analysis of the collected fractions by circular dichroism spectroscopy is shown to be potentially useful for identification of single enantiomers in absence of pure chiral standards.