Dual electrodes detection systems

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. 

This improvement can be readily seen when compared to single electrode detection shown in Fig. 2. Here the determination of p-nitrophenol (PNP) has been undertaken in human urine extracts by both dual electrode detection and single oxidative mode detection. The oxidative mode detection trace is characterised by large number of co-eluting peaks which obscure the retention time expected for PNP. However, hy DED a relative clean trace is recorded. The presence of PNP in the fortified urine extract can be readily recorded. 

However, when using amperometric cells such as the TLC in the series mode only a small percentage of the compounds passing through the upstream generator cell will be electrochemically oxidised or reduced. The same is true for the second downstream amperometric electrode which will in turn convert only a fraction of the products generated by the first electrode, which represents only fraction of the total analyte entering the cell. 

When two separate cells are used, the collection efficiency is simply equal to the conversion efficiency of the second electrode. Therefore, with separate cells, the first cell should have a high conversion efficiency. For the downstream cell the same arguments on sensitivity are valid as for single electrode detection. 

A different situation arises when the two working electrodes are placed in the same thin-layer cell compartment together. The collection efficiency in such will be higher than separate TLCs, since the products of the first upstream electrode are concentrated on the electrode side of the TLC compartment, and diffusion takes place over a shorter distance normal to the electrodes. 

---

Fig. 1 Parallel and series configurations for dual electrode detection systems. Wi = working electrode 1 W2=working electrode 2. (a) parallel (b) series and (c) parallel adjacent, Arrow indicates direction of flow.

This review describes the design and application of liquid chromatography dual electrode detection. The first section discusses the design of this detector system, highlighting some of the different detector systems and configurations, such as parallel and series detectors. In the second section, a number of different example applications are described for environmental, pharmacology and industrial areas. 

A dual-electrode liquid chromatography-electrochemistry (LCEC) system used in the detection and identification of flavanols and procyanidins in wines and grape seeds is a valuable tool (30). Voltammetric behavior of phenolic compounds by LCEC could provide information that cannot be obtained using HPLC with UV detection, for which the identification is usually based on a comparison of the retention time with that of standard compounds, especially for the identification of catechins and procyanidins with a small amount of sample available (30). Figure 10 shows the procyanidins commonly found in wines. 

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 . 

This principle has been expanded to a dual electrode arrangement in which pH differences in the samples could be compensated (Durand et al., 1984). With a BuChE loading of 7.5 U/cm and under substrate saturation conditions the system was sensitive to micromolar inhibitor concentrations. The inhibition was markedly different with different pesticides. Such sensors are superior to physicochemical assays in that they detect the effectiveness of the inhibition. 

Okabayashi et al.[29] rep)orted on a dual preconcentration column system for the determination of low concentrations of fluoride using a fluoride electrode. 1 ml of sample was sorbed selectively on an anion-exchanger loaded with alizarin fluorine blue sulphonate-lanthanum complex. The columns were loaded alternately, and successively eluted with IM sodium chloride - O.S M sodium chloride. A detection limit of 1 /xg r was achieved at a sampling frequency of 24 h ... 

Isaksson and Kissinger (1987) detected physostigmine in plasma with a different system. Physostigmine was eluted on a C-8 column using an acidic eluent with an ion-pairing reagent. This detection system was based on the redox mode, where a dual-electrode thin-layer cell was used in a series configuration, i.e., the reduction current from the second electrode was measured. 

These pyrolyzed photoresist three-dimensional electrodes have applications in batteries, but they are also applicative to microchip bioanalysis systems. Fischer et al. have used pyrolyzed photoresist to make three-dimensional electrodes for dual amperometric detection for microchip electrophoresis [8]. However, it is important to note that many electrodes for microchip electrophoresis are three-dimensional electrodes and can be fabricated from a number of sources including carbon-based screen printing inks, precious metal-based screen printing inks, carbon fibers, electroplating of metallic electrodes, vapor deposition of metallic electrodes, etc. The choice of material is usually a function of... 

A simple and sensitive liquid chromatographic method coupled with ECL was developed for the separation and quantification of naproxen (a nonsteroidal antiinflammatory drug) in human urine. The method was based on the ECL of naproxen in basic NaN03 solution with a dual-electrode system. The detection limit was 1.6 x 10 g mL (S/N = 3) [58]. Furthermore, MEKC chromatography was used with ECL of Ru(bpy)3 as a fast and sensitive approach to detect an antipsychotic and antihypertensive drug, i.e., reserpine in urine. Field-amplified injection was used to minimize the effect of ionic strength in the sample and to achieve high sensitivity. In this way, the sample was analyzed directly without any pre-treatment with LOD (S/N = 3) to be 7.0 x 10 mol L [59]. 

Haroon et al. (38) developed a dual electrode system for the detection of vitamin K compounds. The device utilized two sequential generator/detector electrodes. Vitamin K was electrolyzed at the first electrode and the reaction products were then detected electrochemically at a second electrode. The minimum mass of vitamin that they claim could be detected was 100 pg. They applied their system to the analysis of rat liver extracts and claimed it was superior in both sensitivity and specificity to the UV detector. 

Cardwell and Christophersen reported a dual channel FI system with amperometric detection for the determination of ascorbic acid and sulfur dioxide in wines and fruit juices (Cardwell and Christophersen, 2000). Here, the ascorbic acid was detected at a glassy carbon electrode polarized at 0.42 V (vs. Ag/AgCl), whereas the sulfur dioxide was detected at a Pt electrode polarized at 0.90 V (vs. Ag/AgCl) after separation of the analytes by a gas diffusion unit. The determination of ascorbic acid showed a linear range between 3 and 50 mg L with an FOD of 1.5 mg L for sulfur dioxide the linear range was between 0.25 and 15 mgF i and an FOD of 0.05 mgF" was obtained. The sample frequency achieved with the system was 30 h b The proposed method showed a good agreement with a reference method in the results obtained for white wines and juice samples, while for red wines and sweet wines an extraction procedure of the analytes by solid-phase extraction was required. 

---