Amperometric detection instrumentation

Figure 19.5—Amperometric detection in HPLC and CE. a) Two models of amperometric detection are shown. The working electrode, made of porous graphite with a large surface area, operates under coulometric conditions. The flow of the mobile phase at the working electrode ensures renewal of electroactive species b) expanded diagram of the end of the capillary in CE. Ions exiting the capillary impinge on the working electrode, which is placed on the cathodic side of the instrument. The complete detection cell is not shown.

Kappes et al. evaluated the potentiometric detection of acetylcholine and other neurotransmitters through capillary electrophoresis [209]. Experiments were performed on an in-house capillary electrophoresis instrument that made use of detection at a platinum wire, dip-coated in 3.4% potassium tetrakis (4-chlorophenyl) borate/64.4% o-nitrohenyl octyl ether/32.2% PVC in THF. The results were compared to those obtained using capillary electrophoresis with amperometric detection at a graphite electrode. Samples prepared in the capillary electrophoresis buffer were electrokinetically injected (7 s at 5 kV) into an untreated fused silica capillary (88 cm x 25 pm i.d.) and separated with 20mM tartaric acid adjusted to pH 3 with MgO as the running buffer. The system used an applied potential of 30 kV, and detection versus the capillary electrophoresis ground electrode. 

The use of enzyme labels in ELIS A-type immunosensors and simple amperometric detection schemes resulted in simple and cost-effective alternatives to fluorescence immunosensors. In particular, the use of alkaline phosphatase as enzyme label allowed for the fabrication of advanced immunosensors with signal amphfi-cation by means of redox cycling, which has been a success story of its own. This detection scheme has been used in immunosensors and other biosensors and has stimulated significant developments in electrode fabrication. Instrumental electroanalysis, namely capacitance measurements and EIS allow for label-free detection of immunoreactions. 

Electrochemical sensors based on amperometric detection are popular small instruments that can be used to directly probe samples. The electrodes in these sensors do not always have to be made of metal, e.g. platinum or mercury, and do not always have to be bare. A commonly employed working electrode is the rotating disk electrode, which is preferred over the DME for easily reduced species and anodic reactions. It works by convection mixing of the solution so that fresh sample is constantly passed over the surface. 

Electrochemistry involves the study of the relationship between electrical signals and chemical systems that are incorporated into an electrochemical cell. It plays a very important role in many areas of chemistry, including analysis, thermodynamic studies, synthesis, kinetic measurements, energy conversion, and biological electron transport [1]. Electroanalytical techniques such as conductivity, potentiometry, voltammetry, amperometric detection, co-ulometry, measurements of impedance, and chronopotentiometry have been developed for chemical analysis [2], Nowadays, most of the electroanalytical methods are computerized, not only in their instrumental and experimental aspects, but also in the use of powerful methods for data analysis. Chemo-metrics has become a routine method for data analysis in many fields of analytical chemistry that include electroanalytical chemistry [3,4]. 

Amperometry is one of a family of electrochemical methods in which the potential applied to a sensing electrode is controlled instrumentally and the current occurring as a consequence of oxidation/reduction at the electrode surface is recorded as the analytical signal. In its simplest form, the applied potential is stepped to and then held at a constant value and the residting current is measured as a function of time. When amperometric detection is used in conjimction with separation techniques such as capillary electrophoresis or Uquid chromatography, the sensing... 

Amperometric detection Controlled-potential instrumentation Electrochemical detection Oxidation/reduction... 

While most water analysis for phosphate is laboratory based, it is predicted that the emergence of robust, sensitive, and commercially available portable and online instruments for analysis of phosphate and TP will replace a major part of this analytical load. Such a move is likely to be enhanced by the development of sensitive phosphate selective enzyme electrodes using amperometric detection, which would provide a viable and selective alternative to PMB spectrophotometry. Further advances toward miniaturized flow systems are also expected. 

Figure 16.12 Representation of the (a) scheme for instrumental controlling. Reproduced with permission from [190]. Copyright 2010 WILEY-VCH Verlag CmbH Co. KCaA, Weinheim and [187] 2012 WILEY-VCH Verlag CmbH Co. (bj the main unit of a portable chip-based system coupled with amperometric detection. Reproduced with permission from Ref [191]. Copyright 2010 WILEY-VCH Verlag CmbH Co. KCaA, Weinheim...

Amperometric detection has also been used in a flow injection method for the determination of PG, OG, and BHA using a poly(vinyl chloride) graphite composite electrode [60]. Although the detection limits obtained are relatively high (>0.2 mg/L), the method is characterized for its simplicity and low cost of instrumentation. The applicability of the method was checked by determining the analytes in soup and oil samples. A polypyrrole electrode modified with a tetrasulfonated Ni(II) phthalocyanine complex has been described as an amperometric detector in a flow injection system for the determination of PG, BHA, and TBHQ, which had been previously separated using LC [61]. The detection limit obtained for PG was 0.19 J,g/mL. 

Before considering the special requirements for automated on-line determination of metals from industrial effluents, it is worthwhile examining the features of standard laboratory procedures associated with the off-line determination of copper as a dithiocarbamate complex by liquid chromatography with electrochemical detection. The off-line determination of copper as its diethyldithiocarbamate complex in aqueous samples, zinc plant electrol3d e, and urine have been described [3, 7, 10] using reverse phase liquid chromatography with amperometric detection. A standard instrumental configuration for the conventional laboratory off-line method as used in these studies is depicted in Fig. 7.2. 

The continuous methods combine sample collection and the measurement technique in one automated process. The measurement methods used for continuous analyzers include conductometric, colorimetric, coulometric, and amperometric techniques for the determination of SO2 collected in a liquid medium (7). Other continuous methods utilize physicochemical techniques for detection of SO2 in a gas stream. These include flame photometric detection (described earlier) and fluorescence spectroscopy (8). Instruments based on all of these principles are available which meet standard performance specifications. 

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