Amperometric principle

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 described voltammetric and amperometric sensors can be used very effectively to carry out qualitative and quantitative analyses of chemical and biochemical species. The fundamentals of this sensing technique are well established, and the critical issue is the applicability of the technique to a complex, practical environment, such as in whole blood or other biologic fluids. This is also the exciting challenge of designing a biosensor using voltammetric and amperometric principles. 

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. 

One of the attractive features of SECM is that the UME tip response is based on well-established electrochemical principles, making the technique quantitative. This aspect of SECM can be illustrated by considering the case of simple diffusion-limited electrolysis at an amperometric disk-shaped tip. When the tip is positioned a long way from the target... 

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]. 

J. Zeravik, T. Ruzgas, and M. Franek, A highly sensitive flow-through amperometric immunosensor based on die peroxidase chip and enzyme-channeling principle. Biosens. Bioelectron. 18,1321—1327 (2003). 

FIGURE 16.9 Principle of reagentless amperometric immunosensor based on immobilized antigen, competitive immunological reaction, and direct electrochemistry of HRP label (adapted from [138]). 

Besides these potentiometric sensors there are also amperometric sensors using the principle of ion conductive solid electrolytes. In addition to the heating voltage those sensors are also equipped with a second voltage supply, inducing a current, which varies depending on the concentration of the test gas. Fig. 3.19 shows a schematic view of these so-called saturating current probe. 

Fig. 3.20 Schematic view of an 02-probe with combined potentiometric and amperometric functional principle [4].

There are also some types of sensors that use the amperometric as well as the potentiometric principle. These seem to have an even higher potential for the application in surveillance and control of small performance combustion systems. With the knowledge of the strength and weakness of either principle, these sensors have been developed specifically for use in small systems. One example of these is given in Fig. 3.20. 

Different types of sensor based on solid electrolytes have been developed following a report by Kiukkola and Wagner (1957). These sensors are based on one of two principles (a) the chemical potential difference across the solid electrolyte (potentiometric sensor), or (b) the charge passed through the electrolyte (amperometric sensor). In the following galvanic cell,... 

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. 

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... 

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... 

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. 

Trichosporon cutaneum [89]. The principle of this method is based on the measurement of the rapid alteration (or acceleration) of respiration after addition of ammonium ions in the presence of glucose. The physiological background of this principle is probably the uptake of ammonium ions in connection with the respiration process. Finally, the combination of nitrifying bacteria with urease on a hybrid sensor also allows an amperometric determination of urea [93]. 

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). 

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