A parallel dual-electrode detector for capillary electrophoresis

Abstract

An approach to on-capillary dual-electrode detection for CE using a parallel electrode configuration has been developed. The parallel configuration provides two operating modes. In the first mode, one working electrode is held at an oxidizing potential and the second working electrode is held at a reducing potential. This results in redox cycling of analytes between the oxidized and reduced forms, enhancing sensitivity compared to single-electrode detection. In the second mode, both working electrodes are held at different oxidizing potentials. This mode provides electrochemical characterization of electrophoretic peaks. In the redox cyclying mode, signal enhancement of up to twofold was observed for the dual-electrode detection of phenolic acid standards compared to single-electrode detection. Variation in response of less than 10% from electrode to electrode was determined (at a concentration of 60 nM) indicating reproducible fabrication. LODs were determined to be as low as 5.0 nM for dual-electrode configuration. Using the dual-potential mode peak identification of targeted phenolic acids in whiskey samples were confirmed based on both migration time and current ratios.

Figure 1

Dual-parallel CF electrodes inserted ~370 μm into the end of an HF etched capillary. Fabrication of dual-electrode: (A) fused silica capillary (40 μm id) with 33 μm CF exposed at both ends; 23 gauge 1 inch length needle with hole bored in side. (B) assembled β single CF microelectrode with colloidal silver paste on the bored hole of the needle. (C) individual single CF microelectrodes UV glued together. (D) fully assembled CF parallel dual-electrode.

Figure 2

Parallel dual-electrode in redox cycling mode. Response from a 1.0 μM mixture of phenolic acids: (A) 3,4-DHBA, (B) 4-HBA, (C) SA, and (D) GA; only WE_{1(+800 mV)} (—); both WE_{1(+800 mV)} and WE_2(−200mV) (—). Separation conditions: −12 kV, 75 mM sodium tetraborate, 0.5 mM TTAB, pH 8.4. Injection:−12 kV,2 s

Figure 3

Electrochemical mechanism for redox cycling mode.

Figure 4

Response using the parallel dual-electrode in dual-potential mode. Standards were 1.0 μM. Applied electrode potential: WE₁ = +800 mV, WE₂ = +500 mV. Separation conditions: −12kV, 75 mM sodium tetraborate, 0.5 mM TTAB, pH 8.4. Injection: −12 kV, 2 s. Peak identification: (A) 3,4-DHBA, (B) 4-HBA, (C) SA, and (D) GA.

Figure 5

Response at the dual-electrode for a whiskey sample diluted 1:1 (v/v) with 1% in (0.1 M) HClO₄ spiked with 250 nM 4-HBA. Electrodes were used in dual-potential mode with WE₁ = +800 mV and WE₂= +500 mV. Peaks were identified based on migration time and current ratios using peak height. Separation conditions: −10 kV, 75 mM sodium tetraborate, 0.5 mM TTAB, pH 8.4. Injection: −12 kV, 2 s. Peak numbers related to Table 3.

Figure 6

Same conditions as mentioned in Fig. 5, except working potential applied to WE₂ = +200 mV. Peak numbers related to Table 3.

Figure 7

Response at the dual-electrode for a whiskey sample. Electrodes were used in redox cycling mode with WE₁ = +800 mV and WE₂ = −250 mV. Separation conditions were the same as for those reported in Fig.5.