Column Development

The development of electrochemically modulated liquid chromatography can be traced back to the early 1950s and 1960s when concepts for the desalination of water and for the separation of metal ions explored a union of liquid chromatographic and electrochemical techniques. These and many subsequent investigations suffered from poor chromatographic efficiency (10-200 plates m-1). As a consequence, interest in this technique has been limited until a few years ago. This limitation reflected the complexity in merging the design criteria necessary for both effective liquid chromatographic and electrochemical performance. In the case of the latter, a column must incorporate a reference electrode as a means to control precisely the potential applied to the stationary phase, and an auxiliary electrode as a means to complete the electrical circuit for current flow. In addition, the components of the column must be electro-chemically as well as chemically inert and the mobile phase may need to be modified by the addition of an inert electrolyte to achieve the necessary solution conductivity; this modification is required to minimize the solution resistance of the mobile phase which, if not effectively manipulated, may degrade the ability to control the potential applied to the stationary phase.

Maintaining control over the applied potential is challenged, however, by the need for conductive stationary phases that have the characteristic size, shape, and surface area (e.g., 200m2g_1) required for high separation efficiencies with packed columns. A critical design attribute for high performance chromatography is a large surface area to solution volume ratio for the stationary phase with respect to the mobile phase. As a consequence, the small voids between the particles and within the pores of the particles that often make up a stationary phase appear as highly resistive pathways in an equivalent electrical circuit model for a column under potential control. These high-resistance networks define the time required to charge the electrical double layer at the surface of the stationary phase, and therefore the time needed for the column to equilibrate between changes in applied potential. This process is analogous to the time for a conventional liquid chromato-graphic column to equilibrate after a change in the composition of the mobile phase. Overall, a column must be designed to merge the demands of two methodologies that are in direct opposition in terms of their design specifications for effective performance.

A schematic diagram for a column that addresses the many design features requisite for the effective coupling of liquid chromatography and electrochemistry is shown in Figure 1. The design uses a ionexchange membrane (e.g. Nafion®) as a tubular insert into a porous stainless-steel cylinder that forms the main body of the column. The porous stainless-steel cylinder functions both as a large surface area auxiliary electrode that can carry the large current

Figure 1 A schematic diagram of a column for electrochemically modulated liquid chromatography. A packing of porous graphitic carbon particles is shown only for illustrative purposes.

flow generated at the high surface area working electrode and as an inflexible container that defines the length and diameter of the column. The electrical circuit is completed by placing a reference electrode (e.g. Ag/AgCl(saturated NaCl)) inside a glass reservoir that surrounds the porous stainless-steel tubing and is filled with an aqueous electrolyte.

The ion-exchange membrane has three important functions in this design. It serves as:

• an insulating container that separates the stationary phase from the stainless-steel auxiliary electrode;

• an ionic bridge that completes the electrical connection between the stationary phase and auxiliary electrode; and

• a barrier that minimizes the loss of analyte through the porous stainless-steel cylinder as well as the possible contamination of the column by any electrolysis products generated at the auxiliary electrode.

With this design, the column equilibrates to alterations in applied potential on a time scale similar to that required for changes in mobile phase (&20 min), and performs at high chromatographic efficiencies (e.g., 15 000 plates m"1).

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