High-Speed Electrochemically Modulated Liquid Chromatography

Abstract

The performance advantages of carrying out electrochemically modulated liquid chromatography (EMLC) at elevated temperatures and mobile-phase flow rates are investigated. EMLC has the unique ability to manipulate analyte retention and enhance separation efficiencies through changes in the potential applied to a conductive stationary phase. Operation of high-performance liquid chromatography systems at elevated column temperatures also provides pathways to improve chromatographic performance by enhancing analyte diffusivity and facilitating the use of higher mobile-phase flow rates than conventionally attainable. The results show that performing EMLC separations at elevated temperatures (e.g., 100 °C) reduces the analysis time of a mixture of aromatic sulfonates in a mixed mobile phase by more than a factor of 20. Moreover, use of higher operating temperatures enables the separation of this mixture with an entirely aqueous mobile phase in less than 2 min. This paper investigates the advantages of performing separations using electrochemically modulated liquid chromatography (EMLC) at elevated column temperatures. EMLC is a unique chromatographic technique that manipulates retention through changes in the potential applied (E app ) to a conductive stationary-phase-like porous graphitic carbon (PGC). This capability is realized by fashioning a high-performance liquid chromatography (HPLC) column into an electrochemical cell and utilizing the packing as both a chromatographic stationary phase and as a working electrode. As a consequence, a change in E app alters the effective surface composition of the stationary phase and, in turn, retention. 1 Several laboratories, 2-5 including our own, 6-10 have demonstrated that EMLC can be utilized for the separation of a wide range of mixtures (e.g., aromatic sulfonates, 10 monosubstituted benzenes, 9 protonated pyridines and anilines, 11 corticosteroids, 7 benzodiazepines, 6 short-chain alkanoic acids, 12 and metal ion complexes 13 ).Recent studies have shown that elevated column temperatures can also be used to manipulate retention in HPLC by significantly reducing analysis times while maintaining an effective separation. 14-16 This observation was theoretically predicted several years ago 17,18 and comes about because of three different temperature-dependent phenomena. First, retention in most reversed-phase separations is an exothermic process. 19-21 An increase in column temperature therefore leads to a decrease in elution time. Second, the viscosity of most mobile phases decreases with increasing temperature. As a consequence, the back pressure of the HPLC system decreases, enabling operation at higher flow rates. Third, higher column temperatures increase analyte diffusivity and desorption kinetics. Enhanced diffusivity increases the efficiency of a separation by lowering the C-term in the van Deemter equation, 22 which counters the loss in efficiency that arises at higher mobile-phase flow rates.

Electroadsorptive Removal of Gaseous Pollutants

“…However, while this definition seems to cover every scenario in which a polarization of a conductive substrate (due to an application of an electrical current) induces an attraction of oppositely charged species to the surface, literature research highlights that this process is mainly used in capacitive and pseudocapacitive deionization for separation [11][12][13][14], and energy storage [15,16]. Other reported applications are for the removal of dangerous contaminants, such as organic ions and heavy metals, or enhanced liquid chromatography sensing techniques [17]. The scope of the electrosorption process is, therefore, closely linked to the presence of a liquid phase and a solid phase.…”

Development and characterisation of switchable polyaniline-functionalised flow-through capillary monoliths

“…The group of Porter have established a viable column-based electrochemical cell for EMLC. 29,30 However, using conducting materials as capillary tubing prohibits the use of C 4 D for characterisation purposes and so was not adopted for this work. We propose that such a format could potentially be employed in EMLC in future work to form the working electrode component of an electrochemical cell that could permit the electrochemical switching of the CP stationary phase via the application of a potential.…”

Carbon nanotubes-A resin for electrochemically modulated liquid chromatography

“…EMLC was first introduced by Fujinaga et al and Kihara and co-workers [2,3]. Ge and Wallace carried out investigations with conducting polymers [4,5], but most of the publications in the last two decades stem from Porter and co-workers, who concentrated their efforts on studying chromatographic separations, mainly on porous graphitic carbon stationary phases and hydrophobic analytes [6][7][8][9][10][11]. Although promising analytical proceedings have been described in the past, EMLC is currently not being applied at an analytical nor preparative scale, probably due to the fact that taking advantage of electrochemical effects requires added electrochemistry know-how as well as a special device.…”