Amperometric Ion Detection in Capillary Zone Electrophoresis by Ion Transfer across a Liquid−Liquid Microinterface

Abstract

Ion transfer across the electrically charged interface of two immiscible electrolytes has been employed for the amperometric detection of ions in capillary zone electrophoresis (CE). The method is inherently selective to hydrophobic ions, which are characterized by a low standard Gibbs energy of partition between the aqueous CE buffer solution and 2-nitrophenyl n -octyl ether employed as organic phase. The applicability of this detection scheme for nonredox ions is demonstrated by means of the cations choline and acetylcholine and by some alkyl and aryl sulfate and sulfonate anions. The response of the detector is sufficiently fast; no additional peak broadening has been observed. Plate numbers of 100 000 (170 000 m - 1 ) were determined for the quickly migrating choline cation. It is shown that the use of buffer modifiers such as organic solvent additives, cyclodextrins, and polyelectrolytes is compatible with the detector within certain boundaries. The limit of detection was ∼1 μM for the ions tested.