Capillary zone electrophoresis of complex ionic mixtures with on-line isotachophoretic sample pretreatment

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Separation modes provided by the column-coupling configuration of the separation unit in an on-line combination of capillary isotachophoresis (ITP) with capillary zone electrophoresis (CZE) were studied from the point of view of their potential in the (trace) determination of ions present in complex ionic matrices. Urine was arbitrarily chosen as such a matrix while sulphanilate and 3,5-dinitrosalicylate (currently not present in urine) served as model analytes. In one of these modes, ITP was employed to remove only the most abundant sample constituent (chloride) and concentrate the rest of those migrating between the leading and terminating zones for injection into the ZE stage. In the other mode, ITP was employed for maximum sample clean-up. Here, only the analyte(s) with a minimum of the matrix constituents was transferred for a final separation in the ZE stage. The fraction to be transferred was defined via a pair of discrete spacers added to the sample. Although a highly efficient sample clean-up was typical in this instance, the use of identical migration regimes in both stages (the separations according to ionic mobilities) did not prove the resolution of one of the analytes (sulphanilate) from the matrix constituent(s) in the ZE stage. A considerable improvement in this respect was achieved easily when the ITP clean-up was based on the separation according to p K values while the constituents present in the transferred fraction were finally separated via differences in their ionic mobilities. This two-dimensional approach provided a way to achieve a 150 ppb (10 −6 mol 1 −1) concentration detection limit for sulphanilate in a 1-μl volume of urine taken for the electrophoretic run.