Detector electrode systems application

The electrical conductivity detector measures the conductivity of the mobile phase. Conductivity detectors are universal and nondestructive and can be used in either direct or indirect modes. The conductivity sensor is the simplest of all the detectors, consisting of only two electrodes situated in a suitable flow cell. The basis of conductivity is the forcing of ions in solution to move toward the electrode of opposite charge on the application of a potential. To prevent polarisation of the sensing electrodes, AC voltages must be used and so it is the impedance (not the resistance) of the electrode system that is actually... 

The basic requirements of detectors for liquid chromatography and especially HPLC are fast response, low distortion of the concentration profile of the sample (low washing out of the concentration peaks) and high ratio of analytical to background signal. These properties are achieved both by proper geometry of the cell and electrode system and application of a suitable measuring technique. 

Electrodes modified only with porphyrins or porphyrins associated with other species were explored as potentiometric detectors for different applications. A manganese(lll) porphyrin was utilized as the main component of recovered glassy carbon electrodes used as potentiometric detector of thiocyanate in urine. This sensor was coupled to a FIA system to optimize parameters such as membrane composition, pH, and flow injection parameters. Under the best conditions, a Nemstian response with 58.0 mV per decade of thiocyanate concentration was recorded. The new sensor presented a linear response in the 4.2 x to... 

The electrical conductivity detector is probably the second most commonly used in LC. By its nature, it can only detect those substances that ionize and, consequently, is used frequently in the analysis of inorganic acids, bases and salts. It has also found particular use in the detection of those ionic materials that are frequently required in environmental studies and in biotechnology applications. The detection system is the simplest of all the detectors and consists only of two electrodes situated in a suitable detector cell. An example of an electrical conductivity detector sensing cell is shown in figure 13. 

The development of hydrodynamic techniques which allow the direct measurement of interfacial fluxes and interfacial concentrations is likely to be a key trend of future work in this area. Suitable detectors for local interfacial or near-interfacial measurements include spectroscopic probes, such as total internal reflection fluorometry [88-90], surface second-harmonic generation [91], probe beam deflection [92], and spatially resolved UV-visible absorption spectroscopy [93]. Additionally, building on the ideas in MEMED, submicrometer or nanometer scale electrodes may prove to be relatively noninvasive probes of interfacial concentrations in other hydrodynamic systems. The construction and application of electrodes of this size is now becoming more widespread and general [94-96]. 

The carrier stream is merged with a reagent stream to obtain a chemical reaction between the sample and the reagent. The total stream then flows through a detector (Fig. 1.1 (b)). Although spectrophotometry is the commonly used detector system in this application, other types of detectors have been used, namely fluorometric, atomic absorption emission spectroscopy and electrochemical, e.g. ion selective electrodes. 

Trojanowicz, M., Electrochemical Detectors in Automated Analytical Systems , in Modem Techniques in Electroanalysis, Vanysek P. (Ed.), Wiley-Interscience, New York, 1996, pp. 187-239. This chapter contains a fairly thorough discussion of the possible arrangements of electrodes within flow systems, and for a variety of applications. 

The method of complete electrolysis is also important in elucidating the mechanism of an electrode reaction. Usually, the substance under study is completely electrolyzed at a controlled potential and the products are identified and determined by appropriate methods, such as gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis. In the GC method, the products are often identified and determined by the standard addition method. If the standard addition method is not applicable, however, other identification/determination techniques such as GC-MS should be used. The HPLC method is convenient when the product is thermally unstable or difficult to vaporize. HPLC instruments equipped with a high-sensitivity UV detector are the most popular, but a more sophisticated system like LC-MS may also be employed. In some cases, the products are separated from the solvent-supporting electrolyte system by such processes as vaporization, extraction and precipitation. If the products need to be collected separately, a preparative chromatographic method is use-... 

Arrays can be divided into three groups depending on their design (Fig. 32.2). In the first design, each electrode is independently addressed (Fig. 32.2a). This type of array allows several practical applications. On one hand, these devices can be fabricated for obtaining the same information at different sites of a medium. For example, studies about the responses of neuronal cell cultures under different stimulus [85-87]. On the other hand, these devices can be designed to obtain different information at reduced spaces, i.e. multianalyte determination. Those arrays can be employed directly like detectors in flow systems [88] (where each electrode is maintained at a different potential) or they can be individually modified for specific analytes [89],... 

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