Electrochemical detection voltammetry

Key words Nitroaromatic explosives, electrochemical detection, voltammetry, sensors, microchips. 

Electrochemical Detectors Another common group of HPLC detectors are those based on electrochemical measurements such as amperometry, voltammetry, coulometry, and conductivity. Figure 12.29b, for example, shows an amperometric flow cell. Effluent from the column passes over the working electrode, which is held at a potential favorable for oxidizing or reducing the analytes. The potential is held constant relative to a downstream reference electrode, and the current flowing between the working and auxiliary electrodes is measured. Detection limits for amperometric electrochemical detection are 10 pg-1 ng of injected analyte. 

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 main electroanalytical techniques are electrogravimetry, potentiometry (including potentiometric titrations), conductometry, voltammetry/polarography, coulometry and electrochemical detection. Some electroanalytical techniques have become very widely accepted others, such as polarography/voltammetry, less so. Table 8.74 compares the main electroanalytical methods. 

Hardcastle JL, Compton RG (2001) The electroanalytical detection and determination of copper in heavily passivating media ultrasonically enhanced solvent extraction by N-benzoyl-N-phenyl-hydroxylamine in ethyl acetate coupled with electrochemical detection by sono-square wave stripping voltammetry analysis. Analyst 126 2025-2031... 

The electrochemical detection of pH can be carried out by voltammetry (amper-ometry) or potentiometry. Voltammetry is the measurement of the current potential relationship in an electrochemical cell. In voltammetry, the potential is applied to the electrochemical cell to force electrochemical reactions at the electrode-electrolyte interface. In potentiometry, the potential is measured between a pH electrode and a reference electrode of an electrochemical cell in response to the activity of an electrolyte in a solution under the condition of zero current. Since no current passes through the cell while the potential is measured, potentiometry is an equilibrium method. 

Aniline, methyl aniline, 1-naphthylamine, and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin-layer, dual glassy-carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behavior of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nmol/l were achieved using colourimetric and amperometric cells, respectively, when using an in-line preconcentration step. 

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]. 

An electrochemical sensor using an array microelectrode was tested for the detection of allergens such as mite and cedar pollen (Okochi et ah, 1999). Blood was used in the assay and the release of serotonin, a chemical mediator of allergic response, which is electrochemically oxidized at the potential around 300 mV, was monitored for electrochemical detection by cyclic voltammetry. 

Ruan, C., Yang, L., and Li, Y. (2002b). Rapid detection of viable Salmonella typhimurium in a selective medium by monitoring oxygen consumption with electrochemical cyclic voltammetry. /. Electroanal. Chem. 519, 33-38. 

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. 

There is, of course, a limit on the amount of solution resistance that can be overcome even with microelectrodes. Attempts have been made to perform electrochemistry in the gas phase [79] or in supercritical C02 180] with microelectrodes. In each case, however, the conduction path was shown to be not through the bulk phase, but rather across the insulating surface between the microelectrode and the counter electrode. This mechanism enables electrochemical detection in highly unusual media for voltammetry and illustrates that only very small conduction pathways are required to obtain well-defined electrochemical behavior. 

MFEs are also useful in hydrodynamic techniques, such as rotating-disk voltammetry (Chap. 3) and electrochemical detection for liquid chromatogra-... 

C.D. Garcia and C.S. Henry, Coupling capillary electrophoresis and pulsed electrochemical detection, Electroanalysis, 17 (2005) 1125-1131. J. Wang, R. Polsky, B. Tian and M.P. Chatrathi, Voltammetry on microfluidic chip platforms, Anal. Chem., 72 (2000) 5285-5289. 

N. E. Hebert, W.G. Ruhr and S.A. Brazill, A microchip electrophoresis device with integrated electrochemical detection A direct comparison of constant potential amperometry and sinusoidal voltammetry, Anal. Chem., 75 (2003) 3301-3307. 

Immunoassays, electrochemical — A quantitative or qualitative assay based on the highly selective antibody-antigen binding and electrochemical detection. Poten-tiometric, capacitive, and voltammetric methods are used to detect the immunoreaction, either directly without a label or indirectly with a label compound. The majority of electrochemical immunoassays are based on -> voltammetry (-> amperometry) and detection of redox-active or enzyme labels of one of the immunochemical reaction partners. The assay formats are competitive and noncompetitive (see also -> ELISA). 

With respect to chromatography, electrochemical detection means amperometric detection. Amper-ometry is the measurement of electrolysis current versus time at a controlled electrode potential. It has a relationship to voltammetry similar to the relationship of an ultraviolet (UV) detector to spectroscopy. Whereas conductometric detection is used in ion chromatography, potentiometric detection is never used in routine practice. Electrochemical detection has even been used in gas chromatography in a few unusual circumstances. It has even been attempted with thin-layer chromatography (TLC). Its practical success has only been with liquid chromatography (LC) and that will be the focus here. 

This kind of amperometry is the most widely used electrochemical detection method in liquid chromatography. A constant DC potential is continuously applied to the electrodes of the detector cell. The theory of amperometry with constant working potential does not differ from the theory of hydrodynamic voltammetry, even though the applied potential remains constant. 

The various tunable properties of zeolites have inspired a great variety of concepts in electrochemistry with zeolite-modified electrodes. For example, silver ions inside the zeolite pore system arc not electrochemically active in amperometric detection. Flowever, indirect analyte detection can occur when the analyte causes the removal of silver ions into the solution where they are electrochemically detected.[94] This indirect approach was extended to different copper-exchanged zeolites and demonstrated for the detection of several non-elcctroactive ions including alkali metal, ammonium and calcium.[95] A zeolite-modified electrode (ZME) with high selectivity towards Pb over Cd in cyclic voltammetry was prepared via electrophoretic deposition of zeolite Y, coated with Nafion.[96]... 

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