Electrochemical detection reduction

Chromatographic methods including thin-layer, hplc, and gc methods have been developed. In addition to developments ia the types of columns and eluents for hplc appHcations, a significant amount of work has been done ia the kiads of detectioa methods for the vitamin. These detectioa methods iaclude direct detectioa by uv, fluoresceace after post-column reduction of the quiaone to the hydroquinone, and electrochemical detection. Quantitative gc methods have been developed for the vitamin but have found limited appHcations. However, gc methods coupled with highly sensitive detection methods such as gc/ms do represent a powerful analytical tool (20). 

The on-line measurement of reducing capacity can be performed with either a single or a series of electrochemical detectors, and linear correlations have been demonstrated between total antioxidative activities determined by the electrochemical detection and those determined by DPPH- reduction or by the ORAC assay (Guo et al, 1997 Peyrat-Maillard et al, 2000). The reducing capacity must also be quantified by post-column reactions, either with DPPH- or by the reduction of phosphomolybdenum complexes followed by UV-VIS-detection (Bandoniene and Murkovic, 2002 Cardenosa et al, 2002). A combination of HPLC and semi-automatic ORAC analysis has also been described (Caldwell, 2001). 

A modification of the RDC design, based on the ring-disk arrangement of the RDE [36], incorporated an arc electrode [37,38] deposited on the surface of the membrane around the untreated area. This facilitated the electrochemical detection of species reacting at the interface at short times following the reaction. This method was used to study the solvent extraction of cupric ions, which were detected by reduction to copper metal at the arc electrode. The resulting current flow was related to the interfacial flux at the membrane. 

In this connection, cyclic voltammetric measurements on the electrochemical reduction of C02 at n-Ti02 and platinized Ti02 film electrodes were reported a little later by Augustynski and co-workers.87 The existence of two electrochemically detectable species resulting from C02 reduction was suggested by anodic peaks on the cyclic voltammograms (Fig. 4). Unfortunately, however, no... 

Electrochemical oxidation-reduction of eluting mixture components is the basis for amperometric electrochemical detectors. The three electrodes needed for the detection, the working (indicator) electrode, reference electrode, and auxiliary electrode, are either inserted into the flow stream or imbedded in the wall of the flow stream. See Figure 13.13. The indicator electrode is typically glassy carbon, platinum, or gold, the reference electrode a silver-silver chloride electrode, and the auxiliary a stainless steel electrode. Most often, the indicator electrode is polarized to cause oxidation of the mixture components... 

In actual practice, however, it is rather difficult to utilize the functions of electrochemical reduction as a means of detection of HPLC by virtue of the fact that the serious interference (/. e., large background current) generated by reduction of oxygen in the mobile phase. As complete removal of oxygen is almost difficult, therefore, electrochemical detection is normally based upon the oxidation of the solute. 

Nitro-PAH were determined in air particulate matter by RP-HPLC with reductive electrochemical detection sensitivity of 3-0.3 ng injected483. 

Wilhelm M, Battista HJ, Obendorf D. 2001. HPLC with simultaneous UV and reductive electrochemical detection at the hanging mercury drop electrode a highly sensitive and selective tool for the determination of benzodiazepines in forensic samples. J Anal Toxicol 25(4) 250-257. 

The direct electrochemical detection of DNA was initially proposed by Palecek [8,9], who recognized the capabihty of both DNA and RNA to yield reduction and oxidation signals after being adsorbed. The DNA oxidation was shown to be strongly dependent on the DNA adsorption on the substrate it requires meticulous control of the DNA-adsorbed layer. 

Whilst many functionalities may be reduced, reductive HPLC detectors are less commonly used. Another form of electrochemical detection is conductivity, which is useful when an analyte is ionic. Hence, it is often used in conjunction with ion-exchange chromatography (see Section 4.5.1.4). 

Barek et al. have reported on the determination of AT-nitroso compounds, azo compounds, heterocychcs, aromatic nitro compounds, heterocychc amines and even benzyl chloride using electrochemical methods such as voltammetry and polarog-raphy. The nitro and AT-nitroso compounds work particularly well in reductive mode [47, 48]. For appropriate analytes, adsorptive stripping voltammetry and anodic stripping voltammetry can give orders of magnitude lower detection hmits than are available from HPLC with electrochemical detection [48]. 

Bratin, K., P. T. Kissinger, R. Briner, and C. Bruntlett. Determination of nitro aromatic, nitramine, and nitrate ester explosive compounds in explosives mixtures and gunshot residue by liquid-chromatography and reductive electrochemical detection. Anal. Chim. Acta 130, 295-311 (1981). 

Early theoretical calculations established that Cgo has a low energy LUMO orbital that is triply degenerate and hence capable of accepting six electrons upon reduction [53]. However, the electrochemical detection of the Ceo hexaanion was only... 

An FIPLC method using electrochemical detection in the reductive mode for the determination of artemether 28a and its metabolite dihydroartemisinin 29a <1997JCFI(B)145> and for the simultaneous quantification of artesunate 31 and dihydroartemisinin 29a in plasma has been developed <1997JCFI(B)259, 1998JCFI(B)201>. An effective reversed-phase FIPLC method using electrochemical and UV detection has been developed for the simultaneous determination in plant extracts of artemisinin and its bioprecursors such as arteannuin B 32a, and artemisitene 27 <1995JNP798, 2001JIC489>. 

Despite being relatively new technology, aptamers have a tremendous potential and can be envisioned to rival antibodies and other traditional recognition elements for molecular detection and recognition, due to their inherent affinity, selectivity, and ease of synthesis. In addition, the combination of aptasensors with electrochemical detection methods has the added advantage of further cost reduction and miniaturization of such systems. 

Electrochemical detection is based on measurement of the current resulting from oxidation/reduction reactions of analytes at a suitable electrode as a function of the applied voltage. Since the level of the current is directly proportional to the analyte concentration, these detectors are used for both qualitative and quantitative purposes (127). 

Electrochemical detection, in the reductive mode, has been successively applied by Carignan et al. (380) for the determination of underivatized dimetridazole residues in swine muscle. The major difficulty in this detection mode is the strong interference by oxygen, thus deoxygenation of the entire system is required. 

SOLION. A small electrochemical oxidation-reduction cell consisting of a small cylinder containing a solution and divided into sections by platinum gauze, porous ceramics, or other materials. A type of solion for detecting sound waves consists of a potassium iodide-iodine solution m which the iodide ions are oxidized to truodide ions at the anode, and the reverse process occurs at the cathode. The cell is constructed so that the sound waves cause agitation of the solution between die electrodes, and... 

---