Amperometric detector electrode

For selective estimation of phenols pollution of environment such chromatographic methods as gas chromatography with flame-ionization detector (ISO method 8165) and high performance liquid chromatography with UV-detector (EPA method 625) is recommended. For determination of phenol, cresols, chlorophenols in environmental samples application of HPLC with amperometric detector is perspective. Phenols and chlorophenols can be easy oxidized and determined with high sensitivity on carbon-glass electrode. 

The comparison of analytical characteristics HPLC methods of determination of phenols with application amperometric and photometric detectors was caiiy out in this work. Experiment was executed with use liquid chromatograph Zvet-Yauza and 100 mm-3mm 150mm-3mm column with Silasorb C18 (5 10 p.m). With amperometric detector phenols were detected in oxidizing regime on glass-cai bon electrodes. With photometric detector phenols were detected at 254 nm. 

The amperometric detector is currently the most widely used electrochemical detector, having the advantages of high sensitivity and very small internal cell volume. Three electrodes are used ... 

The most widely used amperometric detectors are based on the thin-layer and wall-jet configurations (Figure 3-22). The thin-layer cell relies on a thin layer of solution that flows parallel to the planar electrode surface, which is imbedded in a... 

FIGURE 3-23 Schematic of a carbon-fiber amperometric detector for capillary electrophoresis A, fused silica capillary B, eluent drop C, stainless steel plate RE, reference electrode WE, working electrode, AE, auxiliary electrode. (Reproduced with permission from reference 58.)... 

Enzyme linked electrochemical techniques can be carried out in two basic manners. In the first approach the enzyme is immobilized at the electrode. A second approach is to use a hydrodynamic technique, such as flow injection analysis (FIAEC) or liquid chromatography (LCEC), with the enzyme reaction being either off-line or on-line in a reactor prior to the amperometric detector. Hydrodynamic techniques provide a convenient and efficient method for transporting and mixing the substrate and enzyme, subsequent transport of product to the electrode, and rapid sample turnaround. The kinetics of the enzyme system can also be readily studied using hydrodynamic techniques. Immobilizing the enzyme at the electrode provides a simple system which is amenable to in vivo analysis. 

To ensure that the detector electrode used in MEMED is a noninvasive probe of the concentration boundary layer that develops adjacent to the droplet, it is usually necessary to employ a small-sized UME (less than 2 /rm diameter). This is essential for amperometric detection protocols, although larger electrodes, up to 50/rm across, can be employed in potentiometric detection mode [73]. A key strength of the technique is that the electrode measures directly the concentration profile of a target species involved in the reaction at the interface, i.e., the spatial distribution of a product or reactant, on the receptor phase side. The shape of this concentration profile is sensitive to the mass transport characteristics for the growing drop, and to the interfacial reaction kinetics. A schematic of the apparatus for MEMED is shown in Fig. 14. 

Fig. 2.4j is a simplified diagram of an amperometric detector. Three electrodes are used, called working, auxiliary and reference electrodes (we ae and re). The we is the electrode at which the electroactivity is monitored, and the re, usually a silver-silver chloride electrode, provides a stable and reproducible voltage to which the potential of the we can be referenced. The ae, usually stainless steel, is a current-carrying 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.

The diffusion-limited electrochemical oxidation of V-nitrosamines in an aqueous pH 1.5 buffer was demonstrated at a GCE coated with a film of mixed valence ruthenium oxides, stabilized by cyano crosslinks. This electrode was used in a potentiostatic amperometric detector for FIA and HPLC, to allow the determination of representative N-nitrosamines (278a, 278c and 278d) for 278c, LOD was 10 nM and RSD 2% at 0.80 pM... 

In amperometric detectors, the eluent flows by the surface of the glassy carbon electrode in which only 5-15% of the electroactive species is present and this undergoes electrolytic conversion (oxidation or reduction) as the surface area of the electrode is relatively small. 

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... 

Solvent extraction offers unique advantages among separation techniques. A system based on extraction into a polymer [poly(vinyl chloride)] as solvent was examined here because of possible advantages in speedy simplicity, sample size, solvent handlingy etc.f especially when coupled with flow injection and an amperometric detector. Solutes examined included salicylic acid and 8-hydroxy quinoline. The apparatus typically consisted of 0.8-mm i.d. X 170-cm coiled tubing that could be connected directly to the injection loop of a flow-injection amperometric detector system containing a nickel oxide electrode. 

Amperometric detectors can operate over a range of conversion efficiencies from nearly 0% to nearly 100%. From a mathematical point of view, a classical amperometric determination (conversion of analyte is negligible) is one where the current output is dependent on the cube root of the linear velocity across the electrode surface as described by Levich s hydrodynamic equations for laminar flow. Conversely, the current response for a cell with 100% conversion is directly proportional to the velocity of the flowing solution. While the mathematics describing intermediate cases is quite interesting, it is beyond the scope of this chapter. 

Packed-bed electrodes need not be cylindrical. Takata and Muto reported on a rectangular design some years ago in which a bed of carbon fibers was used for the electrode material [15]. While a number of innovative applications were reported, for the reasons described earlier, cells of this type do not provide detection limits competitive with those that can be achieved using more conventional amperometric detectors. 

Electrochemical detectors can be broadly classified as either amperometric or voltammetric. An amperometric detector is one in which the potential applied to the detecting electrode is held constant and the resulting current is measured as a function of time. A voltammetric detector is one in which the applied potential is varied with time and the current response is measured as a function... 

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. 

Fig. 23 Cell construction of an amperometric detector for capillary LC. 1 column 2 and 5 fluoroplastic body 3 working electrode 4 reference electrode. (Reproduced with permission from Elsevier.)...

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 electronic tongue system based on flow injection analysis (FIA) with two amperometric detectors was set up. The FLA apparatus consisted of a Jasco (Tokyo, Japan) model 880 PU pump and two EG G Princeton Applied Research (Princeton, NJ, USA) Model 400 thin-layer electrochemical detector connected in series. Each detector was equipped with a working electrode (a dual glassy carbon electrode and a gold... 

Separation and detection of p-aminophenol and ascorbic acid has also been evaluated in Topas microchips using different end-channel amperometric detectors. Thus, platinum- and gold-wire, screen-printed carbon electrode and gold film have been used as working electrodes [77]. 

A) Design, integration, and alignment of an amperometric detector based on a metal-wire working electrode in a capillary electrophoresis microchip. (B) Evaluation of the amperometric detector and separation/ injection performance. (C) Determination of different parameters for the separation of L-ascorbic acid and hydrogen peroxide using PMMA and Topas CE-microchips. 

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