Specific analyte–electrolyte additive interaction in transient isotachophoresis–capillary electrophoresis

While cationic surfactants are usually included in the separation electrolyte to reverse the electroosmotic flow, the presence of the surfactant may also offer a means of capillary electrophoresis (CE) separation selectivity control over the anionic analytes, especially those that are prone to ion-pairing interaction. For one such analyte anion, iodide, the formation of several ion-association/partition products with cetyltrimethylammonium chloride (CTAC) was first discovered when optimizing (decelerating) iodide mobility (in order to achieve effective transient isotachophoretic stacking). At comparatively high concentrations of iodide (≥0.01 mM) and the cationic surfactant well above the critical micelle concentration (25 mM), an additional peak due to interactions with the CTAC micelle was recorded, with a UV absorption spectrum fairly different from those of both interacting partners and also the iodide–monomeric surfactant ion pair. Never observed before in normal CE mode, this phenomenon is believed to have occurred due to the enrichment effect of the initial isotachophoresis state.