Coulometric detector electrode

Figure 3.8 Amperometric detectors (a) measure the current that flows between the working electrode, usually a glassy carbon electrode, and a reference electrode, at a fixed voltage, usually close to the discharge potential for the compound. Coulometric detectors (b) are less common and are designed with a porous carbon flow cell so that all the analyte reacts in the cell, the amount of current consumed during the process being proportional to the amount of the substance.

In coulometric detectors, the eluent flows through a porous graphite electrode such that, in theory, 100% of any electroactive species will undergo electrolytic conversion. As a result, this significantly increases the detection sensitivity, as the surface area is relatively large. 

An assay for NE, E, L-DOPA, DA, 3-nitrotyrosine, m-,o-, and p-tyrosine compared an amperometric detector with a CoulArray detector. A CoulArray detector has the sensitivity of a coulometric detector applied to eight different electrodes to give an array of applied voltages. A C18 column with a mobile phase consisting of an acetate buffer (pH 4.75) and sodium citrate in methanol was used. The assay was... 

All of the fat-soluble vitamins, including provitamin carotenoids, exhibit some form of electrochemical activity. Both amperometry and coulometry have been applied to electrochemical detection. In amperometric detectors, only a small proportion (usually <20%) of the electroactive solute is reduced or oxidized at the surface of a glassy carbon or similar nonporous electrode in coulometric detectors, the solute is completely reduced or oxidized within the pores of a graphite electrode. The operation of an electrochemical detector requires a semiaqueous or alcoholic mobile phase to support the electrolyte needed to conduct a current. This restricts its use to reverse-phase HPLC (but not NARP) unless the electrolyte is added postcolumn. Electrochemical detection is incompatible with NARP chromatography, because the mobile phase is insufficiently polar to dissolve the electrolyte. A stringent requirement for electrochemical detection is that the solvent delivery system be virtually pulse-free. 

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.)...

The coulometric detector is a particular type of amperometric detector in which the percentage of electrolyzed analyte is almost 100%. The area of the peak on a current versus time plot (charge flow during electrolysis) is related to the analyte concentration via Faraday s law. This technique cannot be used with very active compounds because the analyte products would pollute the electrode surface if the reaction proceeded to completion (exhausting all the reactant). 

After separation with a strongly basic anion-exchange resin, dissolved NOJ and NO2 were separated within nine minutes and were detected by a Cd coulometric detector A rotating Cd disk electrode was also used. 

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. 

Johnson and Larochelle developed a coulometric detector based on a platinum surface that was modified by iodine adsorption for the chromatographic determination of Cr(VI). They did not report the stability of the surface. Along with many others, we found that in static solution this modified electrode is stable " however, in a flow system, it lacked long term stability, at least for the oxidation of nitrite and thiocyanate. ... 

At present the ESA Coulochem is the only commercially available coulometric detector. Several different types of cell are now available, most of which contain two analytical electrodes in series. The standard cell (Model 5010) contains equal sized PGEs, but in the high sensitivity cell (Model 5011) the surface area of the second electrode is reduced with the aim of producing a better S/N ratio. Despite the argument that the increased surface area available in PGE systems only increases the noise in proportion to the increased signal," such systems do appear to give an enhanced S/N ratio in certain applications, as compared to glassy carbon electrodes in thin-layer or wall-jet assemblies, and may offer more flexibility in routine use. Even better S/N ratios are claimed for the Model 5014 cell, which has been developed specifically for use with microdialysis samples. 

J. Lankelma and H. Poppe, Design and characterization of a coulometric detector with a glassy carbon electrode for high-performance liquid chromatography, J. Chromatogr. A, 1976, 125, 375-388. 

R. Whelpton, An evaluation of a two-electrode coulometric detector in Drug Determination in Therapeutic and Forensic Contexts, E. Reid and I.D. Wilson (eds). Plenum, London 1984, 189-190. 

Metal meshes (e.g.. Mo) have also been coated. Such electrodes could eventually be useful for electrosynthesis, coulometric detectors, and as supports for electrocatalysts. For many of the envisioned electrochemical applications of diamond, the electrode needs to be in a nonplanar, high surface area form. The low-magnification SEM image in Fig. 4B shows a junction in the mesh between two wires coated with diamond. The film was deposited using a CH4/H2 source gas mixture (—0.5%). The mesh wires are about 200 pm in diameter, and, although not easy to see, the diamond film thickness is several micrometers. 

Coulometric Detectors. Coulometric detectors are based on potentiostatic coulometry [30]. The signal of the constant-potential measurements, as in amperometric detection, is the current resulting from an electron-transfer process (reduction or oxidation of the analyte arriving with the eluent) while the working electrode is held at constant potential. 

To achieve nearly 100% conversion, the ratio of electrode area to cell volume must be very large in coulometric detectors. The working electrodes employed in coulometric detectors mu.st therefore have a much greater surface area (ca. 5 cm ) than those used for amperometric detectors. Large electrode areas, however, lower the signal-to-noise ratio and thus increase the detection limits [62], For this reason, and also because of problems with electrode passivation, coulometric detection has not become an important technique in HPLC, even though the same classes of compounds can be determined as with amperometric detection. 

Figure 24. Coulometric detector with dual porous electrodes and total cell volume < 5 pL (Coulochem 5100 A electrochemical detector)...

The amperometric detection uses less than 10% of the analyte in the flow cell, unlike the coulometric detector, and can be operated in a pulsed mode (cydic voltammetry, with a gold working electrode) in addition to the constant potential mode. The pulsed mode helps cleaning the working electrode. 

The coulometric detector measures current x time (coulomb), usually with a three-electrode system. It not only utilizes a larger surface area in the working electrode, consuming all the analyte, but also generates more noise. The detection limits are, therefore, not necessarily much better with the coulometric detector, but the stability... 

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