Dual coulometric detection

Dual coulometric detection was used with online SPE with LiChrolut EN [63] for the determination of polar priority phenols at ng/L levels. The first electrode was intended for sample cleanup (normally set at a low potential), and the detection of the phenols was made at the second electrode. 

Jorg, E. and Sontag, G. (1992). Determination of phenolic acids in honey by HPLC using coulometric dual electrode detection. Dtsch. Lebensm. Rundsh. 88,179-183. 

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.)...

MeOH/50 mM acetate buffer pH 3.0, 97 3 containing 0.1 mM EDTA Dual-electrode coulometric detection (redox mode), porous graphite electrodes,... 

Dual-electrode coulometric detection (redox mode), porous graphite electrodes ... 

The selectivity of electrochemical detection can be improved by the use of two electrodes (dual-mode detection). Basically, two different combinations are used two amperometric cells and the combination of a coulometric cell with an amperometric cell. The difference between these two cell types is that in amperometric cells only a fraction of the eluting analytes react, whereas in coulometric cells analytes (and all other eluting compounds ) may be quantitatively converted depending on the working potential. Using these two combinations a variety of different experimental setups are possible [74,271]. 

Johansson IM, Schubert B (1990) Separation of hordenine and N-methyl derivatives from germinating barley by liquid chromatography with dual-electrode coulometric detection. J Chromatogr 498 241-247... 

Several modifications, including the use of dual electrode coulometric detection, a high speed reciprocating pump and the A,A-dimethyl analogue of physostigmine as the internal standard, decreased the LoD to 25ngL (RSD=19.6%, 4mL... 

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. 

Figure 4.9 Coulometric titration cell with generator [II (generator anode, 0.7 x 0.7 cm)] and isolated auxiliary [I (generator cathode, 0.7 x 0.7 cm)] electrodes on the left side and a pair of identical platinum electrodes [III, IV (1.4 x 1.8 cm and 2.5 X 1.8 cm)] on the right for dual-polarized electrode amperometric endpoint detection.

Because the generator electrodes must have a significant voltage applied across them to produce a constant current, the placement of the indicator electrodes (especially if a potentiometric detection system is to be used) is critical to avoid induced responses from the generator electrodes. Their placement should be adjusted such that both the indicator electrode and the reference electrode occupy positions on an equal potential contour. When dual-polarized amperometric electrodes are used, similar care is desirable in their placement to avoid interference from the electrolysis electrodes. These two considerations have prompted the use of visual or spectrophotometric endpoint detection in some applications of coulometric titrations. 

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. 

Hordenine was detected with a dual-electrode coulometric cell. The potential of the first electrode was set to -I-0.50V [vs. solid palladium reference electrode (Pd)], which was at the base of hordenine s hydro-dynamic voltammogram. This electrode cleaned the sample by removing easily oxidizable impurities. Hordenine and its precursors were subsequently detected by a second electrode at a potential of -I-0.75V. This method of detection is called the screen mode . In addition, the mobile phase was purified by connecting a coulometric cell between the pump and injector as a guard cell. This additional cell operated at +0.80 V. The detection limit of hordenine was at 1.1 ng, which was 25 times better than detection by UV at 275 nm. 

Based on these electrochemical studies we developed a method for the quantitation of ajmalicine and catharanthine in cell cultures. These alkaloids were extracted from freeze-dried cells and purified by the solid-phase procedure described by Morris et al. (1985), except that ethanol was used as the extracting solvent instead of methanol. A dual-electrode coulometric cell was used in the screen mode. The potential of the first electode was set at +0.2 V (vs. Pd), which was at the base of catharanthine s voltammogram. The alkaloids were detected by the second electrode at +0.8 V, as this offered the best S/N ratio. Higher potentials led to lower S/N ratio, since the background current and noise started to increase exponentially above +0.85 V, due to the oxidation of water. The mobile phase was purified by a guard cell between the pump and injector. The guard cell operated at +0.8V. 

One such technique that has been shown to both improve the selectivity and sensitivity is that of the dual electrode electrochemical detection (DED). There are two different types of dual electrode detector systems in series, or in parallel. Further modification of these terms is gained by the application of either amperometric or coulometric electrodes. 

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