Dual-electrode detector

FIGURE 3-26 Dual-electrode thin-layer detector configurations for operation in the series (a) and parallel (b) amperometric modes. 

Draw clearly schematic diagrams of a thin-layer flow detector utilizing (1) single working electrode, and (2) dual electrode. Explain how the latter improves the power and information content. 

Dual-electrode LCEC is very useful for the selective detection of chemically reversible redox couples. In this case, two electrodes are placed in series (Fig. 27.1 OB). The first electrode acts as a generator to produce an electroactive species that is detected more selectively downstream at the second electrode, which is set at a more analytically useful potential. One excellent example of the use of a dual-electrode detector for electrochemical derivatization is the detection of disulfides [34]. In this case, the first electrode is used to reduce the disulfide to the corresponding thiol. The thiol is then detected by the catalytic oxidation of mercury, described earlier. Because of the favorable potential employed at the second electrode, the selectivity and sensitivity of this method are extremely high. In addition, thiols can be distinguished from disulfides by simply turning off the generator electrode. 

The use of dual-electrode amperometric detectors provides advantages in sensitivity and detection limits. Series configuration and parallel configuration are both possible. Ion-selective electrodes allow the selective quantification of selected analytes even in complex matrices. 

The first combined HPLC-electrochemical measurements of vitamin K used the reductive mode, but this technique suffered from interference from the reduction of oxygen. A redox method was later developed that eliminated this interference, and provided a 10-fold increase in sensitivity over photometric detection and an improved selectivity. The coulometric detector employed in the redox mode is equipped with a dual-electrode cell in which phylloquinone is first reduced upstream at the generator electrode and the hydroquinone is reoxidized downstream at the detector electrode. 

Fig. 14 Analytical HPLC of the phylloquinone fraction from an extracted sample of brown rice isolated by semipreparative HPLC. Column, Spherisorb C8 (octyl) mobile phase, methanol/50 mM acetate buffer pH 3.0 (97 3) containing 0.1 mM EDTA, dual-electrode coulometric detection (redox mode), porous graphite electrodes, — 1.5 V (generator electrode), +0.05 V (detector electrode). The arrows signify the fraction containing tritiated phylloquinone 2,3-epoxide (internal standard) and phylloquinone (analyte) that is collected for quantitation by radioisotopic dilution. (Courtesy of M. J. Shearer.)...

A dual-electrode detector, based on a film inside the channel and a wire at the end of the channel, can be used in order to evaluate the accurate alignment of the end-channel electrode. Thus, when the end-channel wire is properly aligned, analytical signals are quite similar for the in- and end-channel detector [159]. 

S. M. Lunte, Carbon paste-based electrochemical detectors for microchip capillary electrophoresis/electrochemistry, Analyst, 126 (2001) 277-280. A.J. Gawron, R.S. Martin and S.M. Lunte, Fabrication and evaluation of a carbon-based dual-electrode detector for poly(dimethylsiloxane) electrophoresis chips, Electrophoresis, 22 (2001) 242-248. 

L. A. Allison and R. E. Shoup, Dual-electrode liquid chromatography detector for thiols and disulfides, Anal. Chem. 55, 8-12 (1983)... 

A unique voltammetric detector has been developed using a series dual-electrode cell. In the detector, the upstream electrode s potential is scanned while the downstream electrode is used to monitor the redox reaction occurring at the upstream electrode without the charging current contributions. In essence, the upstream electrode is operated voltammetrically and the downstream electrode operated amperometrically so that the detector has been named a voltammetric-amperometiic detector. Detection limits of 10 M have been reported using this detection scheme. 

Roston, D.A. Kissinger, P.T. Series dual-electrode detector for liquid chromatography/electrochemistry. Anal. Chem. 1982, 54, 429 34. 

Lunte, C.E. Kissinger, P.T. Determination of pterins in biological samples by liquid chromatography/electro-chemistry with a dual-electrode detector. Anal. Chem. 1983, 55, 1458-1462. 

Hou, W. Wang, E. Determination of water-soluble vitamins by hquid chromatography with a parallel dual-electrode electrochemical detector. Talanta 1990, 37, 841-844. 

Some phenols are readily oxidized or reduced, which allows detection by electrochemical detectors (ECD). However, when using single-electrode ECD, the peak identification is only based on retention times (RT). To enhance the selectivity of this technique, it is essential to use dual electrode or electrochemical array detectors, both of which would give more information based on the shape of the voltammograms of the eluting compounds. 

McMurrough, I. and Baert, T. 1994. Identification of proanthocyanidins in beer and their direct measurement with a dual electrode electrochemical detector. J. Inst. Brew. 100 409 116. 

Phenol and the three dihydroxybenzenes (20, 42, 66) in water were determined by LLE with a hydrophilic solvent followed by amperometric titration. LOD was in the ppm range . A dual electrode in a FIA system has been used as detector for total phenols in wastewater. The upstream coulometric electrode has a large surface area and is used to eliminate compounds that cause interference and the second one is an amperometric electrode for oxidative detection of all phenols. Optimal results were found working with a phosphate buffer at pH 6.8, at potentials of +0.35 V and +0.78 V for the coulometric and amperometric electrodes, respectively. A high sample throughput of 60 per hour can be attained with RSD of 0.1-4%. This method is more reliable than the colorimetric method . The concentration of fenobucarb (142) in drinking water was determined after a short alkaline hydrolysis, and oxidation of the resulting 2-s-butylphenol with a GCE at 750 mV, pH 3.5 LOD was 3.6 x 1Q- M, RSD 3.74% for 1 x IQ- M (n = 11, p = 0.05)37 . 

Eluted peaks were detected by electrochemical oxidation using the ESA 5100A coulometric detector equipped with an ESA 5010 dual electrode detector cell and a guard cell (ESA, Bedford, MA). The guard cell was placed between the pump and injector (19) and set at a potential of 0.75V. The first electrode of the analytical cell was set at a potential of 0.5V and the second electrode at which OA and N-acetyl OA are oxidized was set at 0.7V. 

Detector E, ESA 5100A Coulochem, guard cell +1 V, dual electrode analytical cell +0.7 and +0.8 V... 

Detector UV 254 E, ESA Coulochem Model 5100A, Model 5020 guard cell -0.8 V (placed before the injector), Model 5010 dual-electrode analytical cell with glassy-carbon electrodes (-0.8 V first electrode, -1-0.8 V second electrode)... 

Detector E, ESA Coulochem Model 5100A, Model 5020 guard cell before injector 4-1.0 V, model 5011 dual electrode analytical cell, screen electrode (detector 1) 4-0.65 V, sample electrode (detector 2) 4-0.85 V, ESA carbon filters before guard and analytical cells... 

Detector E, ESA 5100A detector. Model 5020 guard cell between pump and autosampler +0.7 V, Model 5011 dual electrode analytical cell, upstream (screening) electrode +0.2 V, downstream electrode +0.45 V against Ag/AgCl. 

Derivatization can be employed to advantage with LCD detection as well. Both precolumn or post-column reactions may be employed to produce electroactive compounds analogous to the formation of fluorescent derivatives. The UV-induced reaction chamber for ECDs described in Section 15.4 and the dual electrode detector oxidation-reduction... 

Pharmaceuticals containing thiols are readily measured using this sdieme. For examine, the Squibb orally active antihypertensive agent captopiil and its disulfide metabolite are rea y measured in urine (Fig. 65) at low parts-per-million levels without cleanup steps. The very dean appearance of the complex urine matrix is due to the hi degree of selectivity inherent in the detector. If present, mixed disulfides such as captopril-cysteine would also be detectable. Before this approach, the disulfi was quantitated by injection of sample extract before and after exhaustive electrochemical reduction (off-line) over a dassical Hg pool electrode. Throughput was thus vastly improved uang dual electrodes. 

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