Electrochemical detection applications, oxidative

Electrochemical detection involves the induction of a change in redox state (electrolysis) by application of an electrical potential to an electrode (71). Compounds that can be readily detected by this means are termed electroactive. Under physiological conditions, these compounds tend to be in their reduced state in the nervous system because of the rich level of antioxidants (e.g., ascorbic acid) and, thus, can be oxidized by application of a positive potential to the electrode. The evolved electrons are detected at the electrode in the form of electrical current. This current is proportional to the number of electroactive molecules at the surface of the electrode, and therefore it is proportional to their concentration in the bulk solution. By implanting an electrode in the extracellular space close to the release site and detecting changes in the local (extracellular) concentration of the neurotransmitter, neurotransmitter release can be monitored. The key advantage of this approach is the high temporal resolution that can be in the millisecond domain. Neurotransmitters that can be detected this way include dopamine, norepinephrine, epinephrine, serotonin, and melatonin. 

The potential of electrochemical detection (ECD) techniques for metabolite generation, detection, and quantification [283] should not be underestimated because the technique is especially useful for reactive metabolites [284,285], ECD can be used as standalone or as a parallel or sequential method with LC-MS systems. Online ECD techniques and their utility in metabolite identification have been recently reviewed by Gamache et al. [286], Most of the documented applications involve the use of the electrochemical cell for oxidizing an NCE to reactive metabolites and trapping and detecting them by online LC-MS and LC-MS/MS techniques. 

Hi) Electrochemical detection finds application in reversed-phase chromatography (see page 101) wherein the components of the sample are electroactive. Components of the eluate from the column are subjected to oxidation or reduction on the surface of an electrode whereby a small current is produced. The current produced is proportional to the amount of material oxidized or reduced. 

There are about 20 laboratory based ASTM standard test methods available for the determination of sulfur in various petroleum products and lubricant samples [6]. These utilize diverse analytical techniques and have applicability range sparming from m% to low mg/kg levels. However, at the very low end of sulfur analysis there are only three or four test methods which can adequately determine sulfur in such fuels. These lab-based standard test methods include ASTM D 2622 - wavelength dispersive X-ray fluorescence, D 3120 - oxidative microcoulometry, D 5453 - combustion UV-fluorescence, and D 6920 - oxidative combustion electrochemical detection methods. Without a doubt, the most widely used two methods out of these in oil industry laboratories are D 2622 and D 5453. Studies have shown that at truly ultra-low levels of sulfur only D 5453 can deliver accurate and precise results. This conclusion has... 

The detection and quantification of tocopherols, carotenoids, and chlorophylls in vegetable oil were effectively used for authentication pnrposes. The presence of tocopherols, carotenoids, and chlorophylls influence the oxidative stability of vegetable oils and their potential health benefits. Puspitasari-Nienaber et demonstrated the application of a rapid and reliable analysis method of direct injection of C-30 RP-NPLC with electrochemical detection for the simultaneous analysis of the above mentioned substances. Aliquots of vegetable oils were dissolved in appropriate solvents and injected directly without saponification, thus preventing sample loss or component degradation. Thus the effective separation of tocopherols, carotenoids, and chlorophylls was achieved. 

Electrochemically generated intermediates are usually easier to detect spectroscopically than the substrate material itself. The electrochemical reduction or oxidation of many organic materials yields products, which are often coloured with high absorption coefficients (UV/Vis), or which give a characteristic radical signature in electron spin resonance (ESR) spectroscopy [42], Based on this highly sensitive detection, spectroelectrochemical detectors have been proposed and employed for analytical applications [43],... 

E. Koyama, Y. Kikuchi, H. Echizen, K. Chiba and T. Ishizaki, Simultaneous high-performance liquid chromatography-electrochemical detection determination of imipramine, desipramine, their 2-hydroxylated metabolites, and imipramine V-oxide in human plasma and urine preliminary application to oxidation pharmacogenetics, Ther. Drug Monit., 1993, 15, 224-235. 

Experiments have shown that other types of gas sensors can also use mesoporous silicas as the sensing layer. For example, selective sensing of nitric oxide has been accomplished in the presence of carbon monoxide through the application of quartz crystal microbalance technology employing a cobalt phthalocyanine-modified sol-gel thin film (Palaniappan et al. 2006,2008). Electrochemical detection of nitrite has been accomplished using a metalloporphyrin-modified silicate material... 

More recently, the activity of serum peptidases was investigated by capillary electrophoresis with electrochemical detection [60]. Increased peptidase activity in blood is characteristic of a number of disease states. In this application, leu-enkephalin was used as a model substrate. Leu-enkephalin and its metabolites were separated and detected with CEEC following on-capillary copper complex-ation. By incorporating copper in the run buffer, peptides were complexed directly on-capillary [61]. The copper(ll) complexes could then be detected at +700 mV by oxidation to Cu(lll). The method shows good selectivity for peptides over amino acids. This method was used to monitor the metabohsm of leu-enkephalin by enzymes present in a serum sample (Eigure 10). 

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