This paper describes approaches for stacking large volumes of sample solutions containing a mixture of chlorophenols and chlorophenoxyacetic acids as their anions in capillary zone electrophoresis, and compares results to standard capillary electrophoresis (CE) and normal stacking modes. In order to increase the amount of sample injected beyond the optimal conditions and maintain high resolution, the sample introduction buffer must be removed after the stacking process is completed. This is achieved by pumping the sample buffer out of the column using polarity switching. Large sample volumes are loaded by hydrodynamic injection, then stacked at the injection buffer/run electrolyte interface, followed by the removal of the large plug of low-conductivity sample matrix from the capillary column using polarity switching and finally the separation of the stacked anions in a basic buffer (pH 8.65). Around 10- and 40-fold improvement of sensitivity was achieved by normal stacking and large-volume stacking with polarity switching, respectively, when compared to the standard CE analysis. Sweeping-micellar electrokinetic capillary chromatography (MEKC) was also investigated for the purpose of comparison to the stacking technique. The method should be suitable for the analysis of these chemical compound classes in industrial chlorophenoxyacetic acid manufacture.
In this study, the choice of electrolyte systems for the separation and detection of a range of chlorophenoxyacetic acids and chlorophenols by means of capillary zone electrophoresis (CZE) is discussed. A series of acetate buffers over the buffering capacity pH range 4.03-5.5 were initially chosen for the separation. It was found that chlorophenoxyacetic acids could be separated at pH 4.03 and 4.5 but the most satisfactory separation of chlorophenols was obtained at pH 5.5. The factors affecting separation selectivity, including the addition of organic modifiers, was also studied. The use of 25% 2-butanol, 5% ethylene glycol and 10% acetonitrile as organic solvents resulted in the total separation of both classes of these compounds but poor peak shape of chlorophenols resulted and a number of chlorophenoxyacetic acids were not well separated. A borate-phosphate buffer gave improved peak shape of chlorophenols. Further improved separation of the components of the mixture was obtained by the addition of 2 mM fully methylated-beta-cyclodextrin to the 35 mM borate- 60 mM phosphate buffer at pH 6.5, maintaining good peak shape. In this case, separation of the two compound classes, chlorophenoxyacetic acids and chlorophenols, is achieved, with complete resolution of individual compounds in less than 5 min with high efficiency (of the order of 150,000 plates for the ca. 40 cm column). The method is applied to a commercial 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide mixture.
In this study, the choice of electrolyte systems for the separation and detection of a range of chlorophenoxyacetic acids and chlorophenols by means of capillary zone electrophoresis (CZE) is discussed. A series of acetate buffers over the buffering capacity pH range 4.03-5.5 were initially chosen for the separation. It was found that chlorophenoxyacetic acids could be separated at pH 4.03 and 4.5 but the most satisfactory separation of chlorophenols was obtained at pH 5.5. The factors affecting separation selectivity, including the addition of organic modifiers, was also studied. The use of 25% 2-butanol, 5% ethylene glycol and 10% acetonitrile as organic solvents resulted in the total separation of both classes of these compounds but poor peak shape of chlorophenols resulted and a number of chlorophenoxyacetic acids were not well separated. A borate-phosphate buffer gave improved peak shape of chlorophenols. Further improved separation of the components of the mixture was obtained by the addition of 2 mM fully methylated-beta-cyclodextrin to the 35 mM borate- 60 mM phosphate buffer at pH 6.5, maintaining good peak shape. In this case, separation of the two compound classes, chlorophenoxyacetic acids and chlorophenols, is achieved, with complete resolution of individual compounds in less than 5 min with high efficiency (of the order of 150,000 plates for the ca. 40 cm column). The method is applied to a commercial 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide mixture.
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