Electrochemical detection, selectivity

In principle, the added value of MS detection in GC and LC of vitamin E could be threefold. This powerful technique provides superior selectivity and sensitivity as well as additional information on the identity of chromatographic peaks. As demonstrated earlier, owing to the use of fluorescence and electrochemical detection, selectivity, and sensitivity are no longer a real concern in LC of vitamin E, unlike in GC with FID. Consequently, there appears to be rarely a need to use MS for quantitative purposes, except in those cases where the analysis of extremely small complex samples, e.g., tissue biopsies, puts stringent demands on sensitivity and selectivity. However, MS is also useful for the off-line confirmation of presumed tocopherol peaks in LC. 

A liquid chromatographic experiment resulted in the same retention time for the electroactive compounds A and B. Which electrochemical detection scheme would offer a selective detection of the two coeluting analytes Explain your selection. (E = +0.43 V Eg = +0.77 V.)... 

Because of this lack of resolving power, much electroanalytical research is aimed at providing increased selectivity. This can be accomplished in two ways. First, electrochemistry can be combined with another technique which provides the selectivity. Examples of this approach are liquid chromatography with electrochemical detection (LCEC) and electrochemical enzyme immunoassay (EEIA). The other approach is to modify the electrochemical reaction at the electrode to enhance selectivity. This... 

Monitoring enzyme catalyzed reactions by voltammetry and amperometry is an extremely active area of bioelectrochemical interest. Whereas liquid chromatography provides selectivity, the use of enzymes to generate electroactive products provides specificity to electroanalytical techniques. In essence, enzymes are used as a derivatiz-ing agent to convert a nonelectroactive species into an electroactive species. Alternatively, electrochemistry has been used as a sensitive method to follow enzymatic reactions and to determine enzyme activity. Enzyme-linked immunoassays with electrochemical detection have been reported to provide even greater specificity and sensitivity than other enzyme linked electrochemical techniques. 

The demand for monitoring common metabolites of diagnostic utility such as glucose, urea and creatinine continue to provide the impetus for a staggering research effort towards more perfect enzyme electrodes. The inherent specificity of an enzyme for a given substrate, coupled with the ability to electrochemically detect many of the products of enzymatic reactions initiated the search for molecule-selective electrodes. 

Electrochemical detection is extremely selective and is consequently useful for determination of known components in complex mixtures. The nomenclature... 

Figure 1 Electrochemical detection of catechol, acetaminophen, and 4-methyl catechol, demonstrating the selectivity of differential pulse detection vs. constant potential detection. (A) Catechol, (B) acetaminophen, and (C) 4-methylcatechol were separated by reversed phase liquid chromatography and detected by amperometry on a carbon fiber electrode. In the upper trace, a constant potential of +0.6 V was used. In the lower trace, a base potential of +425 mV and a pulse amplitude of +50 mV were used. An Ag/AgCl reference electrode was employed. Note that acetaminophen responds much more strongly than catechol or 4-methylcatechol under the differential pulse conditions, allowing highly selective detection. (Reproduced with permission from St. Claire, III, R. L. and Jorgenson, J. W., J. Chromatogr. Sci. 23, 186, 1985. Preston Publications, A Division of Preston Industries, Inc.)...

Figure 2 Selective electrochemical detection of a mixture on multielectrode amper-ometry. AA = Ascorbic acid, NE = norepinephrine, DOPAC = 3-4-dihydroxy-phenylacetic acid, E = epinephrine bitartrate, 5-HIAA = 5-hydroxyindole-3-acetic acid, HVA = homovanillic acid, TRP = tryptophan, 5-HT = 5-hydroxytryptamine, and 3-MT = 3-methoxytyramine (separated by RPLC). Detection was with a 4-electrode glassy carbon array, with electrode 1 at 500 m V) electrode 2 at 700 mV, electrode 3 at 900 mV, and electrode 4 at 1100 mV. Note that at electrode 1, HVA, TRP, and 3-MT are not seen. At electrode 2, only TRP is not seen. A standard calomel electrode was used as reference. (Reprinted with permission from Hoogvliet, J. C., Reijn, J. M., and van Bennekom, W. P., Anal. Chem., 63, 2418, 1991. 1991 Analytical Chemistry.)...

Multiple electrodes have been used to obtain selectivity in electrochemical detection. An early example involved the separation of catecholamines from human plasma using a Vydac (The Separation Group Hesperia, CA) SCX cation exchange column eluted with phosphate-EDTA.61 A sensor array using metal oxide-modified surfaces was used with flow injection to analyze multicomponent mixtures of amino acids and sugars.62 An example of the selectivity provided by a multi-electrode system is shown in Figure 2.63... 

Weber, S. G., Tsai, H., and Sandberg, M., Electrochemical detection of dipeptides with selectivity against amino acids, ]. Chromatogr., 638, 1, 1993. 

The methods most commonly used to detect hydrogen sulfide in environmental samples include GC/FPD, gas chromatography with electrochemical detection (GC/ECD), iodometric methods, the methylene blue colorimetric or spectrophotometric method, the spot method using paper or tiles impregnated with lead acetate or mercuric chloride, ion chromatography with conductivity, and potentiometric titration with a sulfide ion-selective electrode. Details of commonly used analytical methods for several types of environmental samples are presented in Table 6-2. 

Electrochemical detection is sensitive and selective, and it gives useful information about polyphenolic compounds in addition to spectra obtained by photodiode array detectors. Differences in electrochemically active substituents on analogous structures can lead to characteristic differences in their voltammetric behavior. Because the response profile across several cell potentials is representative of the voltammetric properties of a compound, useful qualitative information can be obtained using electrochemical detection (Aaby and others 2004). 

Many other selective techniques such as MS, FT-IR, ICP-MS, and electrochemical detection have also been used and several reviews of FIA applications in pharmaceutical analysis appear in the literature.214-216 Several articles are dedicated to pharmaceutical analysis using SIA.217218 FIA and SIA have been applied to high-throughput analysis (up to 200 samples per hour with good... 

TDM was first carried out on drags in biological samples using ultraviolet (UV) light, fluorescence, and electrochemical detection, which measured physicochemical properties of drags. Used alone, these detection methods had low sensitivity and selectivity and were soon obsolete.10... 

Only experiments as described in sections Current-Voltage Relationship, Background Current and Selectivity will supply sufficient information to select the optimum detection potential. Similar experiments should therefore always be performed when developing a method of analysis using HPLC with electrochemical detection, see note in section Background Current. ... 

Anions play key roles in chemical and biological processes. Many anions act as nucleophiles, bases, redox agents or phase transfer catalysts. Most enzymes bind anions as either substrates or cofactors. The chloride ion is of special interest because it is crucial in several phases of human biology and in disease regulation. Moreover, it is of great interest to detect anionic pollutants such as nitrates and phosphates in ground water. Design of selective anion molecular sensors with optical or electrochemical detection is thus of major interest, however it has received much less attention than molecular sensors for cations. 

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. 

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. 

Expansion of Module 3 to include rules for selection of detectors and detector parameters. The rules will handle optical absorbance and fluorescence (including pre- and post-column deri-vatization) and electrochemical detection. 

One of these is electrochemical detection, which can be used with traditional CE as well as with the microchip design. Electrochemical detection generally provides good sensitivity and bulk property response (conductivity, potentiometry), and can be selectively tuned to a certain class of compounds (amperometry). ... 

In contrast with the sensors described elsewhere in this Chapter, the device proposed by the authors group uses no reagent, but photons, to induce a photochemical reaction, and involves electrochemical detection of the photochemical product, which allows one to continuously monitor the formation of the electroactive product. Kinetic monitoring increases the selectivity of determinations by eliminating matrix effects and the contribution of side reactions, whether slower or faster than the main reaction. The electrochemical system chosen for implementation of this special sensor was the Fe(II)/C204 couple, which was used for the kinetic determination of oxalate ion based on the following reaction ... 

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