Separation of enantiomers by on-line capillary isotachophoresis-capillary zone electrophoresis

The ability of capillary zone electrophoresis (CZE) coupled on-line with ca pillary isotachophoresis (ITP) sample pretreatment in the column-coupling c apillary electrophoresis equipment to separate trace enantiomers present in samples of complex ionic matrices and enantiomers present in their mixture s at significantly differing concentrations has been studied. Enantiomers o f 2,4-dinitrophenyl labeled norleucine (DNP-Nleu) and tryptophan enantiomer s were employed as model analytes in this work while urine and mixtures of tryptophan enantiomers of differing concentrations served as model samples. Experiments performed with urine samples spiked with the DNP-Nleu racemate at sub-mu mol/L concentrations demonstrated excellent sample pretreatment capabilities of ITP (concentration of the analytes, in-column and post-colu mn sample clean up) when coupled online with chiral CZE separations, In the CZE separations of enantiomers present in the samples at trace concentrati ons the sample pretreatment could be performed in both achiral and chiral I TP electrolyte systems, The use of a chiral electrolyte system was found to be essential in the ITP pretreatment of the samples containing the enantio mers at very differing concentrations. For example, a 2 x 10(-7) mol/L conc entration of L-tryptophan could be detected in the CZE separation stage of the ITP-CZE combination in samples containing about a 10(4) excess of D-try ptophan only when the ITP pretreatment was carried out in the electrolyte s ystem providing the resolution of enantiomers (alpha-cyclodextrin served fo r this purpose in the present work), A post-column ITP sample clean up was found effective in enhancing the destacking rate of the trace enantiomer in the CZE stage when the migration configuration of the enantiomers was less favorable (the trace constituent migrating behind the major enantiomer).