Electrochemical detectors electrode material

Antioxidant levels (ubiquinone-10, ubiquinol-10, a- and y-tocopherol) in neonate and infant plasma were determined using a Cjg column (electrochemical detector, electrode 1 = — 150my electrode 2 = -1-600 mV) and an 800/180/65 metha-nol/ethanol/IPA with 31.7mM ammonium formate mobile phase [337]. Elution was complete in 30 min. A standard solution containing 1-35 pmol of each material gave readily detected peaks. 

It would appear certain that the most important need in LCEC is the development of improved electrode materials. It may be possible in the near future to design an electrode that will give superior performance for certain classes of compounds. Modifying electrode surfaces by covalent attachment of various ligands or electron-transfer catalysts (including enzymes) can provide the key to better amperometric devices for all sorts of analytical purposes. Research in the area of chemically modified electrodes (CMEs) has been reviewed (see Chap. 13) [6,11]. Those interested in improving the performance of electrochemical detectors would do well to study these developments in detail. 

Electrochemical detectors for liquid chromatography have reached a level of maturity in that thousands of these devices are used routinely for a variety of mundane purposes. Nevertheless, the technology is advancing rapidly in several respects. Multiple electrode and voltammetric detectors have been developed for more specialized applications. Small-volume transducers based on carbon fiber electrodes are being explored for capillary and micropacked columns. Recently, electrochemical detection has also been coupled to capillary electrophoresis [47]. Finally, new electrode materials with unique properties are likely to afford improved sensitivity and selectivity for important applications. 

Besides cell designs that employ metals or other electrode materials in disc shape for external reflection spectroscopy, Robinson and McCreery have successfully employed cylindrical carbon fibers of 12 pm diameter [58, 59]. The carbon fiber was illuminated with a tunable dye laser. Scattered light was collected with fiber optics and guided to a photomultiplier detector. Because no thin layer arrangement and consequently poor electrochemical cell response were involved, fast experiments on a microsecond time scale were possible. Studies of polyaniline films deposited on platinum discs have been described [60]. 

The electron-transfer rates on the surface of the electrode affects the sensitivity of electrochemical detectors, the resolution of the analytes, the reproducibility of the electrode, etc. It has been demonstrated that the performance of CE-AD is strongly influenced by the electrode materials. Usually, the electrode materials should provide high signal-to-noise and reproducible... 

Recently, electrochemical detection methods, namely, conductimetry, amperometry, and potentio-metry, have also become accessible. All three variants of electrochemical detection are intrinsically simpler than the optical methods, and their success depends highly on the electrode materials and designs used. Conductivity detection relies on measurement of the differences between the conductivities of the analyte and the separation electrolyte this provides a direct relationship between migration times and response factor, and makes this detector universal. On the contrary, amperometric detection is restricted to electroactive species and potentiometric detection is not possible for certain small ions with multiple charges. Conductimetric detection works better for inorganic compounds since the higher mobility of... 

Since then, carbon paste-based electrodes, sensors, and detectors became soon quite common in both electrochemical and electroanalytical measurements, representing apparently the most popular electrode material that could be made in the laboratory without special equipment. This is well documented by nearly 3000 scientific articles, which is an estimate coming from the real literature search made recently and concerning the reports on carbon paste in all its forms and imaginable appfications published within the periods 1958—2010 [3—5] and 2011-2013 [6]. 

Graphite electrodes used as liquid chromatography detectors are relatively free from filming effects. The amount of material normally electrolyzed is very small and the continuous washing of the surface by the flow stream appears to keep film formation at a minimum. Nevertheless, if electrochemical detectors are used in liquid chromatography, the quantities injected (especially of serotonin compounds) should not exceed several hundred nanograms. Indeed, if higher amounts are to be detected, one can use less sensitive, nonelectrochemical detector systems. 

Hepler, B. R., Weber, S. G., and Purdy, W. G., 1978, The behavior of an electrochemical detector used in liquid chromatography and continuous flow voltammetry. 2. Evaluation of low-temperature isotropic carbon for use as an electrode material, Anal. Chim. Acta 102 41-59. 

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