Electrochemical detection properties

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). 

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... 

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). ... 

The unique practical properties of adsorption have promoted its extensive use in genetic analysis. The disadvantages of adsorption with respect to covalent immobihzation are mainly that (1) nucleic acids may be readily desorbed from the substrate, and (2) base moieties may be unavailable for hybridization if they are bonded to the substrate in multiple sites [34]. However, the electrochemical detection strategy based on the intrinsic oxidation of DNA requires the DNA to be adsorbed in close contact with the electrochemical substrate by multi-point attachment. This multi-site attachment of DNA can be thus detrimental for its hybridization but is crucial for the detection based on its oxidation signals. 

Moreover, the unique adsorption properties of GEC allowed the very sensitive electrochemical detection of DNA based on its intrinsic oxidation signal that was shown to be strongly dependent of the multi-site attachment of DNA and the proximity of G residues to GEC [100]. The thick layer of DNA adsorbed on GEC was more accessible for hybridization than those in nylon membranes obtained with genosensors based on nylon/GEC with a changeable membrane [99,101,102]. Allhough GEC has a rough surface, it is impermeable, while nylon is more porous and permeable. DNA assays made on an impermeable support are less complex from a theoretical standpoint [7] the kinetics of the interactions are not compUcated by the diffusion of solvent and solutes into and out of pores or by multiple interactions that can occur once the DNA has entered a pore. This explained the lower hybridization time, the low nonspecific adsorplion and the low quantity of DNA adsorbed onto GEC compared to nylon membranes. 

In contrast to the analytical approach where the emphasis is placed on reagents with only sufficient chromatographic diastereoselectivity for baseline resolutions combined with highly sensitive detection properties (e.g., chromophore, fluorophore, electrochemical sensitive groups), the preparative technique requires different and additional criteria to be fulfilled by the reagents and the reaction products. In this case it is mandatory that the separation products can be cleaved under reasonable conditions to obtain the resolved optically pure parent compounds (selectands). Briefly summarized, the requirements are the following ... 

Semiconductor NPs, such as CdS, are commonly used as labels for optical detection of bioanalytes due to their inherent fluorescent properties. Several reviews on semiconductor NPs as fluorescent labels for biosensors are currently available in the literature.53 However, since these fluorescent labels are beyond the scope of this chapter, only semiconductor NPs that involve electrochemical detection methods (stripping voltammetry or photoelectrochemical detection) will be discussed. 

The chemical properties of HVA, 5HIAA and 3-MD make them amenable to reverse-phase high-performance liquid chromatography (HPLC) with electrochemical detection. Furthermore, the composition of CSF means that little, if any, sample preparation is required prior to analysis. However, the susceptibility of these metabolites to oxidation means that careful sample collection and storage is required in order to minimise analyte degradation. 

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. 

Because the vitamins occur in food in trace quantities, detection sensitivity is often an issue. Ultraviolet absorbance is the most common detection method. Fluorescence and electrochemical detection are used in specific cases where physicochemical properties permit and where increased sensitivity and selectivity are desired. Refractive index is seldom used, due to its lack of specificity and sensitivity. 

HPLC-based electrochemical detection (HPLC-ECD) is very sensitive for those compounds that can be oxidized or reduced at low voltage potentials. Spectrophotometric-based HPLC techniques (UV absorption, fluorescence) measure a physical property of the molecule. Electrochemical detection, however, measures a compound by actually changing it chemically. The electrochemical detector (ECD) is becoming increasingly important for the determination of very small amounts of phenolics, for it provides enhanced sensitivity and selectivity. It has been applied in the detection of phenolic compounds in beer (28-30), wine (31), beverages (32), and olive oils (33). This procedure involves the separation of sample constituents by liquid chromatography prior to their oxidation at a glassy carbon electrode in a thin-layer electrochemical cell. 

Figure 3.21 Properties measured during electrochemical detection. Amperometry measures the current or charge transferred between neutral or ionic analytes and the electrode. Conductivity measures the mobility of ions in an electric field. (Reprinted from Ref. 49 with permission.)...

Typically, in gradient elution liquid chromatography, electrochemical detection has been difficult due to base-line shifts that result as a consequence of the altered mobile phase composition. However, a unique property of micelles allows for much improved compatibility of gradients (i.e. gradient in terms of micellar concentration or variation of small amount of additive such as pentanol) with electrochemical detectors. This has been demonstrated by the separation and electrochemical detection of phenols using micellar gradient LC (488). A surfactant (apparently non-micellar) gradient elution with electrochemical detection has also been successfully applied for the assay of some thyroid hormones by LC (491). 

To many analysts the major limitation of electrochemical detection for liquid chromatography (LCEC) is its limited applicability to gradient elution techniques. Amperometric electrochemical detectors exhibit both the best and the worst characteristics of solute property and bulk property detectors. While the Faradaic current arises only from the solute, the non-Faradaic current arises from... 

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 and electrochemical enzyme immunoassay. The second approach is to modify the electrochemical reaction at the electrode to enhance selectivity. This approach is exemplified by modified electrode methods where reaction at the electrode surface is limited beyond mere electrochemical considerations to include physical and chemical properties. The following discussion will illustrate in detail how these approaches can provide analytical techniques with both high selectivity and low detection limits. 

While all detectors place some limitations on the mobile phase composition, in electrochemical detection, it is essential to recognize that a complex surface reaction is involved, which depends on both the physical and chemical properties of the medium. To optimize an LCEC determination, it is necessary to consider both chromatographic and electrochemical requirements simultaneously. Fortunately, most commonly applied chromatographic techniques fall into the category of reverse phase separations, the mobile phase requirements of which are consistent with the requirements for electrochemistry. The primary requirement for electrochemical detection is that the... 

N. Saleh, G. Refaat, I. Redox properties of isradipine and its electrochemical detection in the HPLC determination of the compound in human serum. Biomed. Chromatogr. 2000, 14, 453-458. 156. 

In order to be used as an immunoassay label, an elec-trochemically active compound has to possess suitable electrochemical properties. It has to be soluble in aqueous media and should be stable in solution over a wide pH range. To be detectable, it must allow highly selective electrochemical detection or possess chemical properties to allow selective membranes to be used in the measurement electrode. 

The haloacetic acids (HAA) are comprised of mono-, di- and trichloroacetic acid, mono-, di-, and tribromoacetic acid, and bromo-chloroacetic acid, bromo-dichloroacetic acid, and dibromo-chloroacetic acid. Toxicological studies showed that these compounds have carcinogenic properties and may have adverse reproductive consequences. HAA have no strong chromophore for sensitive UV detection electrochemical detection has been described. Analysis by GC-MS requires derivatization. Due to their relatively low molecular mass, the LC-MS analysis can be hindered by low-mass background interferences. 

To overcome the problem of detection in CE, many workers have used inductively coupled plasma-mass spectrometry (ICP-MS) as the method of detection. " Electrochemical detection in CE includes conductivity, amperometry, and potentiometry detection. The detection limit of amperometric detectors has been reported to be up to 10 M. A special design of the conductivity cell has been described by many workers. The pulsed-amperometric and cyclic voltametry waveforms, as well as multi step waveforms, have been used as detection systems for various pollutants. Potentiometric detection in CE was first introduced in 1991 and was further developed by various workers.8-Hydroxyquino-line-5-sulfonic acid and lumogallion exhibit fluorescent properties and, hence, have been used for metal ion detection in CE by fluorescence detectors.Over-... 

A significant amount of literature regarding the antioxidant properties of flavonoids and other plant polyphenols is available. As the essence of redox chemistry involves electron transfer, it seems natural that electrochemical detection rivals spectrophotometric detection techniques for the compounds that are supposed to be antioxidants. With the improvements in electrochemical detector geometries and electronics over the last decade, coupled with a requirement for increased sensitivity, the use of electrochemical detectors offers significant additional advantages when combined with the traditional UV-VIS detection in the analysis of flavonoids and other plant polyphenols. ... 

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