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Some factors affecting enantiomeric impurity determination by capillary electrophoresis using ultraviolet and laser‐induced fluorescence detection

The key factors influencing enantiomer trace determination were investigated; these include resolution capillary diameter, limit of detection, linear range and type of detection. Chiral reagents, (+)‒ and (−)‒1‒(9‒fluorenyl)ethyl chloroformate (FLEC), were employed as probes to demonstrate the influence of the variables. In order to find the best resolution, separation variables were optimized in both capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MEKC) modes by the application of factorial design experiments. A highly efficient chiral separation of the (±)‒FLEC, derivatized with nonchiral amino acids, was achieved when using γ‒cyclodextrin as the chiral selector. The benefits of using a small diameter capillary for direct determination of both (+) and (−)‒FLEC impurity (0.05—0.1% area/area) were demonstrated using UV detection and applying a sample stacking condition. A frequency‒doubled argon ion laser (244 nm) was used as light source for laser‒induced fluorescence (LIF) detection. Excitation light was provided by means of an optical fiber directed into the Hewlett Packard 3D capillary cartridge. The signals from UV and LIF were monitored simultaneously. The application of LIF detection greatly improved sensitivity and linear range. Further, as a consequence of the increased sensitivity, sample loading could be decreased, which led to an improvement of separation efficiency. Direct determination of 0.005% impurity could be achieved within the linear range.