Capillary zone electrophoresis with on-line isotachophoretic sample pretreatment : sample clean-up aspects

A theoretical model intended for characterizations of capillary zone electrophoresis (CZE) separations of samples of unknown matrix compositions with on-line isotachophoresis (ITP) sample pretreatment was developed. Assuming the use of the column-coupling capillary electrophoresis (CE) separation system, this model explicitly defines (i) potential sample interferents for a given CZE analyte in both stages of the ITP-CZE combination and (ii) residual sample interferents in the CZE stage, i.e., interfering sample constituents which cannot be removed from the sample by the ITP pretreatment using a particular combination of the ITP and CZE electrolyte systems. Relationships applicable to estimations of sizes of these groups of sample interferents are provided by the model. A calculation procedure based on these relationships was developed to test the model. The results from these tests obtained for a group of CZE analytes of different acid-base properties (p-sulfanilic acid, hippuric acid, 2,6-dinitrophenol, and o-nitrophenol) show that an effective use of the ITP-CZE combination in the column-coupling separation system requires (i) that the ITP and CZE electrolyte systems introduce different separation mechanisms for the analyte and (ii) that the size of the sample fraction in which the analyte is transferred into the CZE column after the ITP pretreatment is kept at a minimum. Numerical data obtained in these tests for p-sulfanilic acid were in reasonable agreement with the ones provided by computer simulations of the CZE and ITP-CZE separations with the sample matrix consisting of 264 anionically migrating constituents of known migration properties.