Concentration

Another intriguing application of this technique is sample concentration. This approach takes advantage of the ability to modulate the capacity of a stationary phase, for example, by changes in the oxidation state of an electrochemically transformable stationary phase (e.g. eqn [5]). This strategy is similar to that employed in solid-phase extractions except that changes in applied potential rather than changes in solvent can be used to concentrate and then strip the analyte from the column.

An example of this strategy is the concentration of the dansylated amino acid tryptophan using a non-porous carbon stationary phase that is coated with a thin film of polypyrrole. Polypyrrole, which can be polymerized electrochemically onto a wide range of electrode materials, can be switched between its oxidized and reduced forms with the concomitant uptake or expulsion of anions, as represented in Figure 9. Thus, changes in applied potential transform the stationary phase between one devoid of ionexchange sites to one with a large number of exchange sites. This transformation is evident in the cyclic voltammetric curve shown in Figure 10. This curve demonstrates the change of the coating from its neutrally charged (reduced) form at the more negative

Figure 8 Separations of hexobarbital enantiomers (d and l), using a mobile phase of 20% acetonitrile and 80% water (0.1 M LiCIO4, 20 mM phosphate buffer (pH&2)), a flow rate 0.9 mL min~1, a detection wavelength of 220 nm, and an Ag/AgCI (saturated NaCI) reference electrode. (A) Separations in mobile phase devoid of ^-cyclodextrin at — 1.0 V, — 0.5 V, 0.0 V and # 0.5 V. (B) Separations with 15 mM ^-cyclodextrin as a mobile phase additive at -1.0 V, -0.5 V, 0.0 V and # 0.5 V. The concentration of hexobarbital is &500 ppm, and the injection volume was 0.5 ^L. (Reproduced with permission from Ho MK, Wang S and Porter MD (1998) Analytical Chemistry 70: 4314-4319. Copyright ACS Publications.)

Figure 10 Cyclic voltammetric curve for a nonporous glassy carbon packing that has been coated with a thin film of polypyrrole in 0.10 M sodium benzoate. A silver wire is used as a quasi-reference electrode, and the scan rate was 5mVs~1. (Reproduced with permission from Deinhamer RS, Shimazu K and Porter MD (1995) Journal of Electroanalytical Chemistry 387: 35-46. Copyright Elsevier SA.)

Figure 8 Separations of hexobarbital enantiomers (d and l), using a mobile phase of 20% acetonitrile and 80% water (0.1 M LiCIO4, 20 mM phosphate buffer (pH&2)), a flow rate 0.9 mL min~1, a detection wavelength of 220 nm, and an Ag/AgCI (saturated NaCI) reference electrode. (A) Separations in mobile phase devoid of ^-cyclodextrin at — 1.0 V, — 0.5 V, 0.0 V and # 0.5 V. (B) Separations with 15 mM ^-cyclodextrin as a mobile phase additive at -1.0 V, -0.5 V, 0.0 V and # 0.5 V. The concentration of hexobarbital is &500 ppm, and the injection volume was 0.5 ^L. (Reproduced with permission from Ho MK, Wang S and Porter MD (1998) Analytical Chemistry 70: 4314-4319. Copyright ACS Publications.)

values of applied potential to its cationic (oxidized) form as the applied potential becomes increasingly positive.

Figure 10 Cyclic voltammetric curve for a nonporous glassy carbon packing that has been coated with a thin film of polypyrrole in 0.10 M sodium benzoate. A silver wire is used as a quasi-reference electrode, and the scan rate was 5mVs~1. (Reproduced with permission from Deinhamer RS, Shimazu K and Porter MD (1995) Journal of Electroanalytical Chemistry 387: 35-46. Copyright Elsevier SA.)

Figure 11 presents the details of the experiment by showing the breakthrough curve for the concentration process. The experiment begins by passing a mobile phase containing dansylated tryptophan through the column continuously while the applied potential is held at -1.00 V. The applied potential is then stepped to and held at 0.00 V to oxidize the coating to its cationic form. Upon stepping potential to 0.00 V, the absorbance of the eluent rapidly decreases to the value of the acetate mobile phase, reflecting the exhaustive uptake of the amino acid by the coating. After &67min, the absorbance increases slowly to the value initially observed at -1.00 V.

This increase signals the saturation of the coating by the uptake of the amino acid from the highly dilute solution; the coating is fully saturated in 72.5 min. After saturation, the applied potential is stepped back to -1.00 V to reduce the coating to its neutral form, and strip the amino acid from the column in a narrow elution band. An analysis of the data gives a concentration factor of &33 and a recovery of &98%. Thus, this strategy offers an alternative approach to the solvent elution processes used in most other concentration techniques in that the stripping of an analyte from the column can be facilitated by a change in applied potential, opening the possibility of large reductions in the volumes of generated wastes.

Figure 9 Electrochemical switching of polypyrrole between its oxidized and reduced forms showing the respective uptake or expulsion of an anion A".

Figure 9 Electrochemical switching of polypyrrole between its oxidized and reduced forms showing the respective uptake or expulsion of an anion A".

66.7 min

Figure 11 Breakthrough curve showing the concentration of a 1.6 ppm solution of dansylated tryptophan at a polypyrrole stationary phase. The applied potential was initially held at - 1.00 V, stepped to 0.00 V for 72.5 min, and then stepped back to - 1.00 V. The mobile phase consisted of 1.6 ppm dansylated amino acid tryptophan in 10 mM pH 6 acetate buffer. (Reproduced with permission from Deinhamer RS, Shimazu K and Porter MD (1995) Journal of Electroanalytical Chemistry 387: 35-46. Copyright Elsevier Science SA.)

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