Fixed-potential amperometry

While carbohydrates and alcohols can be oxidized at gold electrodes, they cannot be detected by fixed-potential amperometry. Explain why, and suggest an alternative more suitable detection scheme for their measurements in flowing streams. 

Hering J. G., Sunda W. G., Ferguson R. L., and Morel F. M. M. (1987) A field comparison of two methods for the determination of copper complexation bacterial bioassay and fixed-potential amperometry. Mar. Chem. 20, 299-312. 

An interesting approach to static electrochemical immunoassay arrays that employs microcontact printing has been reported [60]. In this work, the authors printed hydrophilic patterns onto a hydrophobic poly(dimethylsiloxane) substrate to create virtual beakers to allow spatial separation of aqueous solutions. A demonstration of the device employed anti-dinitrophenyl-galactose oxidase as a model, and the galactose oxidase product hydrogen peroxide was detected by fixed potential amperometry. 

A sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

Amperometry is the application of voltammetric measurements at a fixed potential to detect changes in currents as a function of concentration of electroactive species. Electrochemical sensors can be designed based on amperometric measurement. An important example is the oxygen electrode. 

Amperometry is when voltammetry is used at a fixed potential and the current alone is followed. This current is proportional to the concentration of a species in a stirred or flowing solution. It is, therefore, a very suitable detection method for use in flow injection analysis systems and in chromatographic separations its use as a detector for separation techniques is discussed in that section. The current is the result of the electrochemical oxidation or reduction of the analyte after application of a potential pulse across the working and auxiliary electrodes. 

Amperometry is the measurement of current at a fixed potential. An analyte undergoes oxidation or reduction at an electrode with a known, applied potential. The amount of analyte is calculated from Faraday s law. Amperometry is used to detect titration endpoints, as a detector for liquid chromatography and forms the basis of many new sensors for biomonitoring and environmental monitoring applications. 

Amperometric sensors In contrast to potentiometric measurements where the monitored parameter, the cell potential, arises from spontaneous reactions, in amperometry the cell reaction is driven by an external fixed potential, and the current is monitored. With these sensors, in the idealized specific sensor case, the signal resulting from a redox reaction is proportional to the concentration of the analyte species. Cells can be either two-electrode (working and reference) or three-electrode (working, counter, and... 

Classically amperometry has been concerned with the maintenance of a fixed potential between two electrodes. More recently, pulsed techniques have come to the fore some details of these are included in Table 8.3. The applied potential at which the current measurement is made is usually selected to correspond to the mass transport limited portion of the corresponding voltammetric scan. Theoretically, in a quiescent solution, this is electrochemical suicide. The current ii obtained at conventional electrodes under such circumstances gradually decays to zero according to the Cottrell equation (4) ... 

In amperometric transduction, the working electrode is maintained at a fixed potential, with respect to a reference, while the current is monitored. Frequently the current is injected through a counter electrode. The applied potential drives the redox reaction of analytes and the oxidation or reduction current provides information about the electron transfer reaction and in turn the concentration of the analytes. Amperometry can be single potential or pulsed potential. In single-potential amperometry, a fixed... 

Amperometry consists in the apphcation of a fixed potential in a determined time and recording the anodic or cathodic current. Since Clark and Lyons proposed the gln-cose biosensor in 1962, amperometric biosensing has been used and can be applied for POC diagnosis. ... 

The largest division of interfacial electrochemical methods is the group of dynamic methods, in which current flows and concentrations change as the result of a redox reaction. Dynamic methods are further subdivided by whether we choose to control the current or the potential. In controlled-current coulometry, which is covered in Section IIC, we completely oxidize or reduce the analyte by passing a fixed current through the analytical solution. Controlled-potential methods are subdivided further into controlled-potential coulometry and amperometry, in which a constant potential is applied during the analysis, and voltammetry, in which the potential is systematically varied. Controlled-potential coulometry is discussed in Section IIC, and amperometry and voltammetry are discussed in Section IID. 

LCEC is a special case of hydrodynamic chronoamperometry (measuring current as a function of time at a fixed electrode potential in a flowing or stirred solution). In order to fully understand the operation of electrochemical detectors, it is necessary to also appreciate hydrodynamic voltammetry. Hydrodynamic voltammetry, from which amperometry is derived, is a steady-state technique in which the electrode potential is scanned while the solution is stirred and the current is plotted as a function of the potential. Idealized hydrodynamic voltammograms (HDVs) for the case of electrolyte solution (mobile phase) alone and with an oxidizable species added are shown in Fig. 9. The HDV of a compound begins at a potential where the compound is not electroactive and therefore no faradaic current occurs, goes through a region... 

Direct-current amperometry (the measurement of electrochemical current in response to a fixed electrode potential) continues to be the most widely used finite-current electrochemical technique. Popular applications include endpoint... 

Owing to the time dependence of the resulting anodic or cathodic current, the reliable analytical utilization of amperometry requires measurement at a fixed time interval. However, the variable nature of the resulting anodic and cathodic currents can be useful in elucidating the electrochemical process(es) on various electrode surfaces. This has indeed led to the development of a variant of this technique, known as chronoamperometry , which involves the application of the desired anodic or cathodic potential and the subsequent measurement of the resulting current versus time. The main requirements in chronoamperometry are that ... 

The advantage of amperometric measurements is that the faradaic currents are observed, at fixed electrode potentials. In these circumstances, capacitative currents no longer contribute to the overall cell current, and much lower detection limits are obtainable compared to linear sweep voltammetry. However, some of the newer variants of amperometry do involve pulsing the electrode potential to the active region measurements in these cases need to be made carefully to produce optimum signal to noise ratios. 

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