Dual-electrode coulometric cell

Fig. 3. Dual-electrode coulometric cell black dots represent both reference and auxiliary (counter) electrodes...

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. 

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. 

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. 

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]. 

TL, thin-layer cell equipped with a glassy carbon electrode WJ, wall-jet cell equipped with a glassy carbon electrode CO-screen coulometric cell with dual-electrodes of porous graphite used in screen mode PAD, pulsed amperometric detector equipped with a gold 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.

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

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