On-line concentration of acidic compounds by anion-selective exhaustive injection-sweeping-micellar electrokinetic chromatography

An easy, simple, and highly efficient on-line preconcentration method for acidic compounds in capillary electrophoresis was investigated. It combined two on-line concentration techniques, field-amplified sample injection (FASI) and sweeping. A low-pH (2.5) background electrolyte was used to suppress the electroosmotic flow (EOF), obviating the need of a coated capillary, as well as to neutralize the weakly acidic analytes. After injection of a plug of water inside the separation capillary, negative voltage was applied to initialize FASI for a much longer time than usual. The anions experienced a high electric field and moved quickly to the boundary of the water and the low-pH nonmicellar electrolyte. When the anions encountered the low-pH electrolyte, they were neutralized and a focused sample zone was formed. Then both inlet and outlet vials were changed to those containing the low-pH micellar background electrolyte. As negative voltage was applied, the anionic micelles moved into the capillary, and sweeping and separation began. The novelty in the present procedure is that a low-pH buffer is used to suppress the EOF and also the ionization of the analytes, without need of any other additives or use of a coated capillary. Ibis method afforded 100 000-fold improvement in peak heights for some phenoxy acidic herbicides. The detection limits for these compounds could be low as 100 pg/mL.