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 (> or = 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.