Separations of enantiomers by preparative capillary isotachophoresis

The use of capillary isotachophoresis (ITP), operating in a discontinuous f ractionation mode, for preparative separations of enantiomers of chiral com pounds was studied. The ITP separations were carried out in the column-coup ling configuration of the separation unit provided with the preseparation c olumn of a 1.0 mm ID and the trapping column of a 0.8 mm ID. Such a configu ration of the CE separation unit offers several working regimes suitable to preparative separations of enantiomers. 2,4-Dinitrophenyl-DL-norleucine (D NP- Norleu) was employed as a model analyte in our experiments with beta-cy clodextrin serving in the electrolyte solutions as a chiral selector. The p reparative separations lasting about 20 min were evaluated by ITP and (more often) by capillary zone electrophoresis (CZE). It was found that one prep arative run provided up to 14 mu g of pure DNP-Norleu enantiomers. This cor responded to a 75 times higher production rate of ITP relative to a maximum value of this parameter as estimated for preparative CZE runs in cylindric al capillaries (0.5 pmol/s). About 75% of the DNP-Norleu enantiomers loaded into the preparative equipment could be recovered in pure enantiomer fract ions. Contiguous natures of the zones in the ITP stack and adsorption losse s of the enantiomers in the isolation step were found to set practical limi ts for a further enhancement of the recovery rates in the isolation of pure enantiomers.