Electrochemical detection systems

When electrochemical detection systems are used, it is essential that the mobile phases used are electrically conductive but only result in a low background current at the voltage selected. Cell volumes are of the order of 1 /d and the control of flow rate, pH and temperature is critical for reliable detection. 

In any chromatographic analysis the method of detection is determined by the nature of the analyte and the mobile phase used must not interfere with this system. The use of ultraviolet absorption detection systems is very common but the solvents used must not absorb significantly at the wavelength used. For instance, absorption at 280 nm is frequently used to detect protein but some solvents, e.g. acetone, absorb at this wavelength. Similarly the use of concentration gradients in the mobile phase may present problems with refractive index and electrochemical detection systems. 

The most common detectors used for the determination of SPA are the detectors that operate at the UV wavelength of 233 or 280 nm (104,111,113,115-119,126,144,71). Fluorometric detectors are used mainly for the determination of ethoxyquin (133-135), and it is also possible to use electrochemical detectors (123,137). A very progressive electrochemical detection system is based on measuring the array potential of SPA, with the previous oxidative removal of impurities (128). 

P. Ertl, C.A. Emrich, P. Singhal and R.A. Mathies, Capillary electrophoresis chips with a sheath-flow supported electrochemical detection system, Anal. Chem., 76 (2004) 3749-3755. 

Immusoft is a software that has been developed to perform computer-driven assays in our microchips. This software has a user-friendly graphical user interface, and it enables control of the pump, the valves and the electrochemical detection system, as well as the development of specific assay protocols, the running of simultaneous or sequential experiments in eight parallel microchannels, the automatic read-out of the results and the processing of the obtained data. These different functions are managed by way of three main menus, named Method, Analysis and Results, and the software also comprises two additional items dedicated to the setting of the computing parameters and to the maintenance of the instrumentation. 

The reduction of sample size calls for an improvement of detection sensitivity. Optical detection methods have been most commonly applied in most cases. However, fluorescence detection will still gain more importance due to the higher sensitivity of this technique. An interesting approach is the combination of reaction vessels and testing cells. One solution is the incorporation of microlenses below the wells which allow a detection on-site . Highly sensitive methods may be also obtained by the use of miniaturized electrochemical detection systems. 

It was reported that in-channel amperometric detection was possible without using a decoupler. This has been achieved using an electrically isolated (non-grounded, floating) potentiostat [750]. On the other hand, when the CE power supply was battery operated (12-V battery to power 3-kV HV module) and therefore it was electrically isolated from the electrochemical detection system [765], In another report, a portable HV power supply (using 6 V battery for the HV module and 9 V battery for the electrochemical detection circuit) was constructed for CE-EC detection on a glass chip [751],... 

The simple and inexpensive electrochemical detection systems are easier to miniaturize and included within the biosensor chip than their optical counterparts. They will thus lead to truly miniaturized systems in the near future. However, improvements in the optoelectronic technology will allow... 

The electrochemical detection system requires reducible or oxidable substances. The eluted substance is reduced or oxidized by an electrode. This not very common detection system is a useful tool for determination of amino-compounds and other organic nitrogen-compounds, phenols and quinones [8],... 

Tang, T. C. Deng, A. P. Huang, H. J., Immunoassay with a microtiter plate incorporated multichannel electrochemical detection system, Anal. Chem. 2002, 74, 2617-2621... 

Unfortunately no truly universal detector which satisfies these criteria has as yet been developed for HPLC although mass spectrometry and electrochemical detection systems arguably approach this ideal. 

In the author s subjective and rather speculative opinion on the basis of the present performance of electrochemical detection systems, there is sufficient reason to promote further research in this field. Topics of special interest could be as follows ... 

Microfluidic devices that use electrical fields for fluidic propulsion perform successfully for a variety of electrically driven separations, and thus, present unique opportunities to the analytical and biomedical community. The availability of a broad range of microfluidic functional elements, materials, and processes facilitated the miniaturization of most types of electrically driven separations that use optical and electrochemical detection systems capillary electrochromatography... 

Figure 2.7. The electrochemical detection system for 0X5d eracycline [OTC] using aptamer-immoblllzed interdigitated array (IDA) gold electrode chip. Left An IDA gold electrode chip and the IDA gold electrode. Right A typical electrochemical reaction occurring after aptamer binds to its target...

Electrochemical Techniques, Fig. 1 Schematic of capillary electrophoresis with electrochemical detection system. (a) High-voltage supply, (h) grounded platinum electrode, (c) reference electrode, (d) auxiliary platinum electrode, (e) detection electrode, (/) running buffer, (g)... 

Such a variety of compounds requires the application of the full panoply of analytical techniques including ELISA, LC, LC-MS, GC-MS (negative and positive ion), GC-MS-MS. Newer methods may include online SPE systems combined with MS or UV or electrochemical detection systems. One important consideration is that analysis of a particular EDC should also include its degradation and/or metabolic products because these too may remain active. So for alkyl phenols the concurrent measurement of the appropriate alkyl phenol ethoxylates and alkyl phenoxycarboxylic acids is environmentally important and a significant analytical challenge. 

W.R. Matson, Electrochemical detection system and method of analysis, US patent 4,404,065, 1983. 

P. O Dea, P. Shearan, S. Dunne and M.R. Smyth, Comparison of mercury- and glassy carbon-based electrochemical detection systems for the determination of cisplatin following high-performance liquid chromatographic separation. Analyst, 1988, 113, 1791-1794. 

Five major tea catechins (e.g., epicatechin, epicatechin gallate, epigallocatechin 3-galIate) were isolated from plasma and baseline resolved on a C g column (coulometric detector at 70 mV) using a complex 44-min 96/4 -> 67/33 water (1.75% acetonitrile, 0.12% THF, and lOOmM sodium phosphate buffer at pH 3.35)/(58.5/29/12.5 acetonitrile/water/THF with 15mM sodimn phosphate buffer at pH 3.45 ) gradient [1157]. The authors noted that bacterial growth in the weak solvent was foxmd at phosphate levels below lOOmM. They also noted that acetic acid was not compatible with the electrochemical detection system setup used here. Standards from 1 to 10,000 ng/mL were used. Detection limits of 5ng/mL (S/N = 3) were reported. 

Electrochemical Techniques, Rgure 1 Schematic of a three-dlmenslonally adjustable electrochemical detection system for capii-lary electrophoresis, (a) Screw for immobilizing the detection electrode (b) electrochemical detection reservoir (c) BEG (d) detection electrode (e) rubber stopper (f) fused silica capiiiary (g) plastic plate (h) supporting block (i) knobs for three-dimensional adjusting Q) three-dimension manipulator... 

An electrochemical detection system with an F-Pb02 modified electrode for FIA to determine COD has been proposed [71]. The measuring principle is based on the current response on the modified electrode, which is proportional to the COD value. The method is characterized by short analysis time, simplicity, low environmental impact, a limited reagent consumption, and easy automation. 

Solea-Tacussel, DELC Electrochemical detection system for HPLC,... 

The aim of this paper is to describe the principal considerations for setting-up an electrochemical detection system for HPLC, including the determination of electroactivity of organic compounds using a novel microscale voltammetric technique, the choice of mobile phase and the selection of instrumental approaches for optimum performance. The application of electrochemical detection to several classes of less easily oxidized compounds, including sterols, nonionic surfactants and organic acids, is illustrated. 

Electrochemical detection in HPLC has become established for some, more specialized, applications such as catecholamine analysis though it can be exploited for a far wider range of compounds. The work in this paper has attempted to investigate some of the basic properties of an electrochemical detection system and some more difficult applications. The detector has a linear dynamic range and precision that are comparable with those of other detectors for HPLC. It is, however, more dependent on temperature than, for example, the UV absorption detector and must be operated in a temperature controlled environment to obtain the lowest detection limits. For many electroactive compounds with moderate oxidation potentials, the electrochemical detector can yield sub-nanogram detection limits. 

An universal stacked electrochemical detection system is described consisting of a noble metal electrode array, a flow chamber, and a miniaturized microprocessor controlled potentiostat. The electrode array and the flow chamber have been fabricated in silicon technology. The electrodes are arranged in four interdigitated electrode arrays (IDA). These IDA electrodes are controlled independently with a multichannel potentiostat (multipotentiostat). They are suited for the detection of redoxmediators. The combination of these components and biologic recognition elements in combination with various voltammetric procedures results in an electrochemical system for immunoanalysis. 

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