Retention Concepts

Conventional liquid chromatographic separations rely on a partitioning of an analyte between a mobile phase and stationary phase. This equilibrium is generally expressed as a distribution coefficient (Kd) where:

and Cs and Cm are the concentrations of an analyte in the stationary and mobile phases, respectively. Elec-trochemically modulated liquid chromatography therefore relies on the impact of the applied potential on the retention of an analyte through alterations in

Four principal formats have been used. The first format deals with an electrochemically induced reduction of metal ions (Mn +) onto a conductive stationary phase as M0. The generic form of this reaction can be written as:

where n represents the electron (e~) stoichiometry of the reaction. The concentrations of the two species are then defined by the difference in the standard reduction potential for the reaction (E0) and the applied potential (Eapp) through the Nernst equation:

Substituting M0 and M"+ into eqn [1] for Cs and Cm, respectively, and then combining eqns [1] and [3] yields:

Eqn [4] shows that Kd changes by a factor of ten for every 0.059/" V alteration of the applied potential. However, while providing a means for the quantitative deposition of metal ions, the process suffers from poor selectivity. That is, metal ions with standard reduction potentials more positive than the applied potential are exhaustively deposited onto the column, whereas those with standard reduction potentials more negative than the applied potential pass through the column without being retained. Strategies that have been devised to address this limitation have been studied in a few specific applications.

The second format takes a notably different tactic in that the potential applied to a graphitic stationary phase is used to manipulate the chemical composition of a mobile phase. This strategy relies on the oxidation or reduction of water either to increase or decrease, respectively, the acidity of the mobile phase. This general tactic, though explored only briefly to manipulate the pH-dependent binding of metal ions to chelates immobilized onto the column packing, will undoubtedly be more extensively exploited as interest in this new separation technique increases.

The third format, which has been the most used of the four, utilizes uncoated graphitic stationary phases. The basis of this format derives largely from the effect of the applied potential on the donor-acceptor properties (e.g. electrostatic and dipolar interactions) of the packing and is illustrated in Figure 2. Thus, as the applied potential becomes more positive, the carbonaceous packing becomes a stronger acceptor and analytes with a predominant donor character are more strongly retained. The retention of analytes with a predominant acceptor character, in contrast, increases as the applied potential becomes more negative. This approach has been employed, as discussed in the following section, to separate a wide ranges of samples, including, charged and neutral aromatic compounds, important pharmaceutical agents, and enantiomeric pairs.

The fourth format takes advantage of a different form of electrochemically modulated separation in which the composition of a column packed with an electroactive material is transformed by its oxidation/reduction. Eqn [5] depicts this process whereby a coating (C) is switched between its reduced and oxidized forms with the concomitant expulsion or uptake of anions (A~). The analogous reaction:

can be written for coatings that involve cations as counterions. Thus, the charge density of the ionexchange sites on a stationary phase can be tuned

Stationary phase

Stationary phase

Figure 2 Idealized representations of the interactions of donor (D) and acceptor (A) analytes with a conductive stationary phase as a function of applied potential (£app). Collectively, changes in applied potential alterthe extent of the interaction of such analytes through a change in the surface charge of the stationary phase. At applied potential greater than the potential of zero charge (pzc), the stationary phase has a net positive surface charge and therefore a greater acceptor strength. This increase results in an increase in the attractive interaction between the stationary phase and a donor analyte. The converse applies as the applied potential becomes more negative than the pzc. In this case, the donor strength of the stationary phase increases, and the attractive interaction between the stationary phase and an acceptor analyte increases.

Figure 2 Idealized representations of the interactions of donor (D) and acceptor (A) analytes with a conductive stationary phase as a function of applied potential (£app). Collectively, changes in applied potential alterthe extent of the interaction of such analytes through a change in the surface charge of the stationary phase. At applied potential greater than the potential of zero charge (pzc), the stationary phase has a net positive surface charge and therefore a greater acceptor strength. This increase results in an increase in the attractive interaction between the stationary phase and a donor analyte. The converse applies as the applied potential becomes more negative than the pzc. In this case, the donor strength of the stationary phase increases, and the attractive interaction between the stationary phase and an acceptor analyte increases.

through changes in applied potential. Several intriguing features of this strategy have been described.

Electrochemically modulated liquid chromatogra-phy, therefore, presents a broad range of tactics that can, as demonstrated in the next section, be exploited in tackling the many separation challenges faced in the analytical laboratory.

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