Electrochemical methods amperometric

Biosensors evanescence SPR, absorbance), Electrochemical methods (amperometric, conductimetric, potentiometric), piezoelectric acoustic, colorimetric, mechanical, thermal methods et al., 2002 Baeumner, 2003 Nakamura and Karube, 2003... 

Coulometry can be regarded as an analog of titration where the substance being examined is quantitatively converted to a reaction product not by the addition of titrant, but by a certain amount of electric charge Q. As in titration, the endpoint must be determined. To determine the endpoint during current flow, one combines coulometry with another of the electrochemical methods described, and accordingly is concerned with conductometric, potentiometric, or amperometric coulometry. 

Other important alternate electrochemical methods under study for pCO rely on measuring current associated with the direct reduction of CO. The electrochemistry of COj in both aqueous and non-aqueous media has been documented for some time 27-29) interferences from more easily reduced species such as O2 as well as many commonly used inhalation anesthetics have made the direct amperometric approach difficult to implement. One recently described attempt to circumvent some of these interference problems employs a two cathode configuration in which one electrode is used to scrub the sample of O by exhaustive reduction prior to COj amperometry at the second electrode. The response time and sensitivity of the approach may prove to be adequate for blood ps applications, but the issue of interfering anesthetics must be addressed more thorou ly in order to make the technique a truly viable alternative to the presently used indirect potentiometric electrode. 

Part—III exclusively treats Electrochemical Methods invariably and extensively used in the analysis of pharmaceutical substances in the Official Compendia. Two important methods, namely potentiometric methods (Chapter 16) deal with various types of reference electrodes and indicator electrodes, automatic titrator besides typical examples of nitrazepam, allopurinol and clonidine hydrochloride. Amperometric methods (Chapter 17) comprise of titrations involving dropping-mercury electrode, rotating—platinum electrode and twin-polarized microelectrodes (i.e., dead-stop-end-point method). 

Laser-induced fluorescence (LIF) has also been utilized as a highly sensitive detection principle for CE [48-51]. However, while the LIF detector is now able to achieve zeptomole (10 21) detection limits, conventional derivatization techniques are inefficient at these exceptional levels [52]. Also, CE has successfully been coupled with mass spectrometry (MS) [53], nuclear magnetic resonance (NMR) [54, 55], near-infrared fluorescence (NIRF) [56, 57], radiometric [58], flame photometric [59], absorption imaging [60], and electrochemical (conductivity, amperometric, and potentiometry) [61-63] detectors. A general overview of the main detection methods is shown is Table 1 [64]. 

Electrochemical methods of detection affinity interactions at the surfaces are rather effective due to their relative simplicity and low cost. Amperometric aptasensor based on sandwich assay was proposed by Ikebukuro et al. [42]. They used two aptamers selective to thrombin. 

Another voltammetric method, sinusoidal voltammetry (SV), was also employed. This was a frequency-based electrochemical method, which was found to be more sensitive than the usual constant potential (DC) amperometric detection method. SV has been achieved to detect on-chip separated catecholamines. This method is very similar to fast-scan cyclic voltammetry (CV), except that a... 

Two developmental difficulties existed in the application of electrochemical methods to CE detection. The first involved the cross-talk or interference that resulted from the high voltages used in CE separations. These dc voltages may be as high as 50 kV, and the resulting electroosmotic currents can be up to six orders of magnitude greater than the faradaic currents measured at an amperometric detector poised... 

In the detection modes applied to ion chromatography one distinguishes between electrochemical and spectroscopic methods. Conductometric and amperometric detection are electrochemical methods, while the spectroscopic methods embrace UV/Vis, fluorescence, and refractive index detection. Added to this are the various applications of these detection methods, described in detail below. 

GOase activity is conventionally assayed in a Warburg apparatus to measure the uptake of oxygen (Guilbault, 1976). Electrochemical methods (potentiometric or amperometric) are less sensitive than the fluorimetric methods, but are well-suited for automation (Pardue et al., 1964 Blaedel and Olson, 1964). 

In the case of the A -nitrosamines, the easy electrochemical reduction of the nitro group has allowed the development of direct electrochemical methods for their determination. The reductive amperometric determination has been described using dropping mercury electrodes or hanging mercury drop electrodes. Likewise the oxidative amperometric determination using... 

Photo-acoustic spectroscopy has been used for ultratrace levels of Hg in air and snow (de Mora etal. 1993). X-ray fluorescence is nondestructive, rapid, requires minimal sample preparation, and was, for example, used successfully to determine the maximal level of mercury in maternal hair to assess fetal exposure (Toribora et al. 1982). However, the procedure is less sensitive compared to AAS and INAA if no pre-concentration is used. Electrochemical methods have been replaced as detectors in chromatography by other instrumental techniques because of poorer detection limits. High-performance liquid chromatography (HPLC) with reductive amperometric electrochemical reduction, however, was shown to be capable of speciating Hg(II), methyl- ethyl- and phenylmercury, with detection limits <2pgL (Evans and McKee 1987). 

One criticism of electrochemical methods is the possible lack of specificity. There is, however, some specificity inherent in all electrochemical measurements. Every chemical has its own specific potential at which it will react at an electrode (the standard electrode potential). In mixtures the observed potential difference generated by the sensor will be a function of all the components present. When using amperometric measurements, the application of a given potential to the working electrode may cause all components with standard electrode potentials lower... 

As was mentioned earlier, negatively charged CNTs can act as counterions in the electrochemical deposition of ECPs [19]. This straightforward method has been used for preparation of amperometric enz mie electrodes via entrapment of the enz3mie in the resulting ECP-CNT composite [63, 64]. Wang et al. developed amperometric glucose biosensors based on the PPy-MWCNT-GOD composite [63]. The composite was prepared by a simple one-step electrochemical method in which p3U role was electropolymerized at a constant potential of 0.7 V in the presence of c-MWCNTs and GOD. Results from the CV measurements showed that the incorporated c-MWCNTs act as counterions that maintain the electrical neutrality of the film. The influence of different parameters, such as the amount of the used MWCNTs, and the... 

Amperometry is one of a family of electrochemical methods in which the potential applied to a sensing electrode is controlled instrumentally and the current occurring as a consequence of oxidation/reduction at the electrode surface is recorded as the analytical signal. In its simplest form, the applied potential is stepped to and then held at a constant value and the residting current is measured as a function of time. When amperometric detection is used in conjimction with separation techniques such as capillary electrophoresis or Uquid chromatography, the sensing... 

Amperometric methods Amperometric methods are based on the measurement of current produced in an electrochemical cell at an appropriate applied voltage. Figure 3 shows a typical cell employed for the amperometric determination of dissolved chlorine, which consists of a reference electrode, whose potential is fixed, and a working electrode, whose potential can be adjusted. If a reducible substance is present in the cell, and the cell potential is sufficiently high, the diffusion current will be proportional to the concentration of reducible species. 

Any of the methods of detection used in liquid chromatography can be used in IC, though some are more useful than others. If the eluent does not affect the detector the need for a suppressor disappears. Common means of detection in IC are ultraviolet (UV) absorption, including indirect absorption electrochemical, especially amperometric and pulsed amperometric and postcolumn derivatization. Detectors atomic absorption spectrometry, chemiluminescence, fluorescence, atomic spectroscopic, refractive index, electrochemical (besides conductivity) including amperometric, coulometric, potentiometric, polaro-graphic, pulsed amperometric, inductively coupled plasma emission spectrometry, ion-selective electrode, inductively coupled plasma mass spectrometry, bulk acoustic wave sensor, and evaporative light-scattering detection. 

Electrochemical methods for NO determination offer several features that are not available with spectroscopic approaches. Perhaps the most important is the capability of microelectrodes to directly measure NO in single cells in situ, in close proximity to the source of NO generation. Figure 2 shows sensors that have been developed for the electrochemical measurement of NO. One is based on the electrochemical oxidation of NO on a platinum electrode (the classical Clark probe for detection of oxygen) and operates in the amperometric mode [17]. The other is based on the electrochemical oxidation of NO on conductive polymeric porphyrin (porphyrinic sensor) [24]. The Clark probe uses a platinum wire as a working electrode (anode) and a silver wire serves as the counterelectrode (cathode). The electrodes are mounted in a capillary tube filled with a sodium chlo-ride/hydrochloric acid solution separated from the analyte by a gas-permeable membrane. A constant potential of 0.9 V is applied, and direct current (analytical signal) is measured from the electrochemical oxidation of NO on the platinum anode. In the porphyrinic sensor, NO is catalytically oxidized on a polymeric metalloporphyrin... 

Three broad classifications of electrochemical methods are used in this chapter. Po-tentiometric methods include zero-current potentiometry and methods in which current of controlled magnitude is apphed to the working electrode, such as in potentiometric stripping analysis (PSA). Amperometric methods consider all techniques in which current is measured these include constant-potential amper-ometry and amperometric measurements made in response to a variety of applied potential waveforms in voltammetric methods. Impedimetric methods comprise a final classification in these methods, faradaic currents are generally absent, and impedance, conductance, or capacitance is the measured property. 

Impedimetric measurements are based on the nonfaradaic response observed between two electrodes immersed in a sample solution when a high frequency (1 to 300 kHz) alternating potential is applied. The amplitude of the applied waveform is small, and this, combined with the high frequency relative to other electrochemical methods, effectively prevents faradaic reactions from occurring to any significant extent. In contrast to potentiometric and amperometric methods, impedimetric methods are not analyte-selective instead, bulk properties of the solution and the electrode-solution interfaces are monitored. 

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