Capillary zone electrophoresis in nonaqueous solvents in the presence of ionic additives

Capillary zone electrophoresis (CZE) in nonaqueous media and in the presence of ionic additives has been successfully applied to the determination of compounds that differ only slightly in their electrophoretic mobilities. Triazine herbicides of environmental interest were chosen as test compounds because they behave as very weak bases. CZE separation of these analytes (especially chlorotriazines) in aqueous solution is difficult due to the low pH required for their conversion into protonated cationic form (HA(+)). However, in mixed nonaqueous solvents, 50% (v/v) acetonitrile-methanol, the acid-base characteristics of these compounds are modified, yielding the protonated ionic species that is susceptible to migration when subjected to an electric field. A noteworthy increase in separation selectivity and resolution can be achieved by using ionic additives. Thus, in this mode of capillary zone electrophoresis, separation is based on ionic interactions between the charged analytes and the ionic additive present in the separation medium. These interactions contribute to enhancing mobility differences and to improving analyte separation. For the separation of chloro- and methylthiotriazines, 10 mM perchloric acid in 50% (v/v) acetonitrile-methanol and 20 mM SDS proved to be satisfactory, providing high resolution in short analysis times. The selectivity achieved was found to depend on the degree of association of the analyte with the ionic additive in the nonaqueous medium. This permits manipulation of the selectivity of the electrophoretic separations as a function of the type and concentration of the ionic additive and of the nature of the nonaqueous medium employed.