Response patterns with indirect UV detection in capillary zone electrophoresis

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Abstract
Capillary zone electrophoresis with indirect UV‐detection was used to separate mixtures containing both positively and negatively charged species. In order to understand the dependence of detector response patterns on the changes in compositions of the background electrolytes and the charge of marker ions (UV‐absorbing ions), the separations were performed in two different systems. In a three‐ion system (analyte ion, coion and counterion) a marker ion was the major ionic component of a buffer solution and in a two‐coion or counterion system the marker ion was used as an additive. In the three‐ion system the response profile of an analyte was in good agreement with the mathematical treatment based on the Kohlrausch regulation function. In the two‐coion or counterion system the response patterns were more complicated; however, the experimental results agree well with data obtained from a computer simulation program. Peak directions of the analytes were not only determined by their relative charge to the marker ion, but were also associated with their relative mobilities to the buffer coion and the marker ion. The analytes with higher effective mobilities compared to the marker ion were detected as positive peaks and the ones with lower effectice mobilities as negative peaks. Similarly to the three‐ion system, the detector response of an analyte was stronger by applying a marker coion compared to a counterion. An interesting result was obtained in the separation of a mixture of quaternary ammonium ions and sugars by using a cationic marker ion. The highest and most symmetrical peak was not a cation, but raffinose anion, which appeared most closely to the system peak. The observation suggests that the electromigration dispersion in its zone was eliminated by migrating close to the electroosmosis. A system peak with the mobility corresponding to the electroosmotic flow was obtained in both systems, and an additional system peak with a mobility close to that the marker ion was present in the systems using marker ions as additives.