Introduction

Liquid chromatographic separations rely on subtle differences in the interactions of analytes between a mobile phase and a stationary phase. Examples include the separation of charged species using ionexchange chromatography, enantiomeric compounds via chiral selective chromatography, and polymeric materials by size-exclusion chromatography. Each of these separations, however, requires a different stationary phase. Thus, a large number of such phases and mobile phase elution schemes are required to tackle the diversity of the separations routinely encountered in the analytical laboratory.

Recently, a technique has been developed that begins to address the need for a large number of stationary phases. The approach, which is termed 'electrochemically modulated liquid chromatography' (EMLC), is based on the transformation of a conductive stationary phase in a conventional liquid chromatographic system into the working electrode of a three-electrode electrochemical cell. With this arrangement, the column packing serves both as a stationary phase and as a working electrode. This dual-purpose function provides the ability to induce changes in the interfacial properties (e.g. donor-acceptor strength, solvophobicity, and oxidation state) of conductive stationary phases through alterations in applied potential which subsequently translate into changes in analyte retention. The column can therefore be viewed as a compositionally tunable stationary phase with retention characteristics that can be manipulated both prior to as well as during elution by changes in applied potential. The latter capability presents an approach whereby improvements in a separation can be realized through a dynamic alteration in the composition of a stationary phase that is conceptually, but not mechanistically, analogous to conventional solvent gradient elution techniques in liquid chromatography or to temperature-programming strategies in gas chromatography. With electrochemically modulated liquid chromatography, however, the changes in retention reflect a temporal gradient in the composition of a stationary phase.

The following discusses the emergence of this new separation strategy as a tool in the arsenal of liquid chromatography techniques. After examining issues concerning the design and construction of the column, processes to control retention, illustrative applications demonstrating the versatility of the technique and issues related to improvements in column performance are presented.

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Solar Panel Basics

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