Selectivity amperometry

Selecting the Voltammetric Technique The choice of which voltammetric technique to use depends on the sample s characteristics, including the analyte s expected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of a single analyte. The portability of amperometric sensors, which are similar to po-tentiometric sensors, make them ideal for field studies. 

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. 

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.)...

Z. Samec, E. Samcova, and H.H. Girault, Ion amperometry at the interface between two immiscible electrolyte solutions in view of realizing the amperometric ion-selective electrode. Talcmta 63, 21—32 (2004). 

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). ... 

It is used in IC systems when the amperometric process confers selectivity to the determination of the analytes. The operative modes employed in the amperometric techniques for detection in flow systems include those at (1) constant potential, where the current is measured in continuous mode, (2) at pulsed potential with sampling of the current at dehned periods of time (pulsed amperometry, PAD), or (3) at pulsed potential with integration of the current at defined periods of time (integrated pulsed amperometry, IPAD). Amperometric techniques are successfully employed for the determination of carbohydrates, catecholamines, phenols, cyanide, iodide, amines, etc., even if, for optimal detection, it is often required to change the mobile-phase conditions. This is the case of the detection of biogenic amines separated by cation-exchange in acidic eluent and detected by IPAD at the Au electrode after the post-column addition of a pH modiher (NaOH) [262]. 

In amperometry, we measure the electric current between a pair of electrodes that are driving an electrolysis reaction. One reactant is the intended analyte and the measured current is proportional to the concentration of analyte. The measurement of dissolved 02 with the Clark electrode in Box 17-1 is based on amperometry. Numerous biosensors also employ amperometry. Biosensors8-11 use biological components such as enzymes, antibodies, or DNA for highly selective response to one analyte. Biosensors can be based on any kind of analytical signal, but electrical and optical signals are most common. A different kind of sensor based on conductivity—the electronic nose —is described in Box 17-2 (page 360). 

The resolution of the column provides much of the selectivity in LCEC therefore, the practical limitations of amperometry are circumvented to a large extent. Nevertheless, amperometry is more often than not used to improve the selectivity of an LC method. Compounds that oxidize or reduce at low potentials can be detected with great selectivity. 

NaOH H20 Anion-exchange resin Ion exchange Pulsed amperometry High selectivity for mono-, di-, and trisaccharides high sensitivity Need for specific instrumentation instability of some products at basic pHs... 

Amperometry 10 18-10 19 10 1°—10-11 Sensitive Selective but useful only for electroactive analytes Requires special electronics and capillary modification... 

For monitoring catalytic (enzymatic) products, various techniques, such as spectrophotometry [32], potentiometry [33,34], coulometry [35,36] and amperometry [37,38], have been proposed. An advantage of these sensors is their high selectivity. However, time and thermal instability of the enzyme, the need of a substrate use and indirect determination of urea (logarithmic dependence of a signal upon concentration while measuring pH) cause difficulties in the use and storage of sensors. 

Sinusoidal voltammetry (SV) is an EC detection technique that is very similar to fast-scan cyclic voltammetry, differing only in the use of a large-amplitude sine wave as the excitation waveform and analysis performed in the frequency domain. Selectivity is then improved by using not only the applied potential window but also the frequency spectrum generated [28]. Brazill s group has performed a comparison between both constant potential amperometry and sinusoidal voltammetry [98]. 

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). 

Amperometry applies a constant potential to the microelectrode that will oxidize the analyte at the electrode surface (Fig. 8a). The current is limited solely by the mass transport rate to the electrode. Measurements can be made with extremely high temporal resolution, typically 500 Hz, because time resolution is not limited by the electrochemistry at the electrode. However, little analyte selectivity occurs with amperometry because a change in the concentration of any molecule electroactive at the applied potential will alter the measured signal. 

More than brief discussion of the numerous ways in which end points can be taken other than by visual methods is beyond our scope. For example, end-point techniques may involve photometry, potentiometry, amperometry, conductometry, and thermal methods. In principle, many physical properties can be used to follow the course of a titration in acid-base titrations, use of the pH meter is common. In terms of speed and cost, visual indicators are usually preferred to instrumental methods when they give adequate precision and accuracy for the purposes at hand. Selected instrumental methods may be used when a suitable indicator is not available, when higher accuracy under unfavorable equilibrium conditions is required, or for the routine analysis of large numbers of samples. 

Any type of detector with a flow-through cell can be used for FIA. Photometric detectors are most often used in FIA (15-18, 25). However, many other analyses using fluorimeters (28, 29), refractometers (24), atomic absorption (30, 31), and inductively coupled plasma emission spectrometers (32) have been described. Electrochemical detectors based on potentiometry with ion-selective electrodes (15, 33), anodic stripping voltammetry (15, 34), potentiometric stripping (35), and amperometry (36) have also been used. 

Various detection systems can be used in ion-exclusion chromatography, among them ultraviolet (UV)/vis spectrophotometry, conductivity, electrochemistry, fiuorometry, refractive index (RI) measurement, are the most common techniques. Additionally, combined detection systems (e.g., UV/amperometry, UV/RI) may be used, leading to enhanced selectivity. 

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