Amperometric detection principle

The analytic principles that have been applied to accumulate air quality data are colorimetry, amperometry, chemiluminescence, and ultraviolet absorption. Calorimetric and amperometric continuous analyzers that use wet chemical techniques (reagent solutions) have been in use as ambient-air monitors for many years. Chemiluminescent analyzers, which measure the amount of chemiluminescence produced when ozone reacts with a gas or solid, were developed to provide a specific and sensitive analysis for ozone and have also been field-tested. Ultraviolet-absorption analyzers are based on a physical detection principle, the absorption of ultraviolet radiation by a substance. They do not use chemical reagents, gases, or solids in their operation and have only recently been field-tested. Ultraviolet-absorption analyzers are ideal as transfer standards, but, as discussed earlier, they have limitations as air monitors, because aerosols, mercury vapor, and some hydrocarbons could, interfere with the accuracy of ozone measurements made in polluted air. 

Photometric. Amperometric. Fluorometric. Electrochemical detection principles. 

A thin film of manganese oxides deposited over a glassy earbon electrode dramatically lowers the overpotential for oxidation of various hydrazines ad hydrogen peroxide, thereby facilitating their amperometric detection in flow systems. Sensors based on this principle are highly sensitive and provide... 

Amperometric detection is a very sensitive technique. In principle, voltammetric detectors can be used for all compounds which have functional groups which are easily reduced or oxidized. Apart from a few cations (Fe , Co ), it is chiefly anions such as cyanide, sulfide and nitrite which can be determined in the ion analysis sector. The most important applications lie however in the analysis of sugars by anion chromatography and in clinical analysis using a form of amperometric detection know as Pulsed Amperometric Detection (PAD). 

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]. 

A dual EC detection employing amperometry/conductivity allows measurements of a wider range of analytes per separation (ionic via CCD mode and electroactive compounds via amperometric detection) has also been reported [73]. This scheme also gave two simultaneous signals for the same component that is both charged and electroactive. This is the only report in which both principles are combined. Most of the CE-EC microchips are based in the amperometric detection mode that is more deeply considered in next section. 

In amperometric detection, a reference electrode was usually employed. However, in one report, a platinized Au electrode was used as a pseudoreference electrode in a three-electrode system for amperometric detection. The operation principle follows that of the hydrogen reference electrode [242]. 

Monitoring of the intracellular redox activity in eukaryotic cells imposes the requirement that the utilized mediator is capable of readily crossing the plasma membrane into the intracellular environment to communicate with the enzyme(s), the activity of which is to be monitored. This strictly requires the utilization of a lipophilic mediator that can diffuse through the plasma membrane. Using chip based amperometric detection on S. cerevisiae, menadione was shown to possess the desired properties [28]. Figure 3 depicts the functional principle of the chip based detection technique to monitor CRE in eukaryotic cells, which aside from the lipophilic menadione,... 

In general, electroanalytical detection principles can be divided into three potentiometry, amperometry, and conductometry (or impedometry). Potentiostats used for electrochemical biosensors are mostly equipped with amperometric and... 

Unlike dissolved amino acids, carbohydrates have not seen the development of sensitive techniques that are capable of routine chromatographic analysis at natural concentrations. Although there are procedures that in principle should allow determinations of dissolved saccharides at natural concentrations, these have so far not been tested in routine analysis. Mopper et al. (1992) used pulsed amperometric detection after separation of underivatised monosaccharides with an anion-exchange column. Thus, techniques which require sample pre-concentration/desalting and post-column derivatisation are still the ones that have been shown to be reliable for determinations of dissolved free monosaccharides in seawater (Dawson and Liebezeit, 1983). 

The enzymatic method described above has two disadvantages (1) trapping of CO2 is a cumbersome procedure, and (2) the use of a radioactive substrate requires special precautions for use and disposal of reagents. Measurement of the primary amine formed by decarboxylation of the amino acid can also be exploited to monitor the PLP-dependent, enzyme-catalyzed reaction. This principle has been applied by Allenmark et al. (106), who used L-3,4-dihydroxyphenyl-alanine (L-DOPA) as substrate for tyrosine decarboxylase the dopamine produced by the decarboxylation reaction was determined by HPLC followed by amperometric detection. Both Hamfelt (107) and Lequeu et al. (108) utilized apo-tyrosine decarboxylase with tyrosine as substrate. The tyramine produced by the decarboxylation reaction was separated from the substrate (tyrosine) by HPLC and quantitated by either amperometric (108) or fluorometric (107) detection. The procedures discussed above are still subject to the main disadvantage of enzymatic methods possible interference by other materials present in the PLP containing extract which could either inhibit reconstitution of the holoenzyme or alter the reaction rate of enzyme catalysis. Moreover, HPLC with amperometric detection can hardly be described as less cumbersome than CO2 trapping difficulties in baseline-stabilization encountered with these detectors are well known. 

Chemical sensing is not a well investigated conventional research field. Research started just a few decades ago. However, some chemical sensors are already in widespread commercial use and contribute greatly to making our lives comfortable. Commercial sensors are roughly divided into three types according to the detection principle (I) conductimetric type, (II) potentiometric type, (III) amperometric type. 

A different concept based on amperometry was described by Fasching et al. [45], They have developed a miniaturized amperometric sensor for CO2 in liquids for applications in clinical blood gas analysis. The detection principle is based on the pH-dependent dissociation of copper complexes. Different CO2 concentrations result in corresponding pH shifts of the internal hydrogel electrolyte. These pH variations lead to certain dissociation levels of the copper complexes... 

Enzyme-based biosensors have been used also for the detection of phenolic estrogens. The detection principle was based on the ability of tyrosinase to catalyze the oxidation of phenolic estrogens to o-diphenol and o-quinone. Using this principle tyrosinase-carbon paste electrodes have been used for the detection of phenol, catechol, bisphenol A, genistein, quercetin, nonylphenol, and diethylstilbestrol with detection levels in the micromolar range.Optical and amperometric biosensors based on estrogen receptors have also been developed. 

Detection of Bromine Vapor. Bromine vapor in air can be monitored by using an oxidant monitor instmment that sounds an alarm when a certain level is reached. An oxidant monitor operates on an amperometric principle. The bromine oxidizes potassium iodide in solution, producing an electrical output by depolarizing one sensor electrode. Detector tubes, usefiil for determining the level of respiratory protection required, contain (9-toluidine that produces a yellow-orange stain when reacted with bromine. These tubes and sample pumps are available through safety supply companies (54). The usefiil concentration range is 0.2—30 ppm. 

The continuous methods combine sample collection and the measurement technique in one automated process. The measurement methods used for continuous analyzers include conductometric, colorimetric, coulometric, and amperometric techniques for the determination of SO2 collected in a liquid medium (7). Other continuous methods utilize physicochemical techniques for detection of SO2 in a gas stream. These include flame photometric detection (described earlier) and fluorescence spectroscopy (8). Instruments based on all of these principles are available which meet standard performance specifications. 

Because process mixtures are complex, specialized detectors may substitute for separation efficiency. One specialized detector is the array amperometric detector, which allows selective detection of electrochemically active compounds.23 Electrochemical array detectors are discussed in greater detail in Chapter 5. Many pharmaceutical compounds are chiral, so a detector capable of determining optical purity would be extremely useful in monitoring synthetic reactions. A double-beam circular dichroism detector using a laser as the source was used for the selective detection of chiral cobalt compounds.24 The double-beam, single-source construction reduces the limitations of flicker noise. Chemiluminescence of an ozonized mixture was used as the principle for a sulfur-selective detector used to analyze pesticides, proteins, and blood thiols from rat plasma.25 Chemiluminescence using bis (2,4, 6-trichlorophenyl) oxalate was used for the selective detection of catalytically reduced nitrated polycyclic aromatic hydrocarbons from diesel exhaust.26... 

Nonspectroscopic detection schemes are generally based on ionisation (e.g. FID, PID, ECD, MS) or thermal, chemical and (electro)chemical effects (e.g. CL, FPD, ECD, coulometry, colorimetry). Thermal detectors generally exhibit a poor selectivity. Electrochemical detectors are based on the principles of capacitance (dielectric constant detector), resistance (conductivity detector), voltage (potentiometric detector) and current (coulometric, polarographic and amperometric detectors) [35]. 

Because of the interference problems with both colorimetric and amperometric analyzers, they are being replaced by instruments based on other principles. (Colorimetric analyzers are no longer commercially available.) One such recently improved technique developed for the specific detection and measurement of ozone is the detection of the... 

The amperometric immunosensors reported so far rely on various methodological principles including use of a Clark electrode for detecting oxygen formation or depletion, an electrochemically active product yielded in an enzyme reaction or an antigen labelled with an electroactive species. 

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