Amperometric analyzers

The continuous colorimetric instrumental method is described in detail by the Intersociety Committee and the American Society for Testing and Materials/ and colorimetric and amperometric analyzers are discussed extensively in the papers both of Tokiwa et al. and of Hodgeson/ ... 

Amperometric analyzers are often referred to as coulomb metric analyzers. Coulombmetry is a mode of andysis in which the quantity of electrons (charge) necessary to oxidize or reduce a desired substance... 

FIGURE 6-9 Amperometric analyzer with Brewer electrolytic cell. Reprinted with permission from Intersociety Committee. 

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... 

Cotton, effect of oxidants on, 462,687 Cotton fiber, ozone damage to, 665 Coulombmetry. See Amperometric analyzers <>i ee mechanism, for liquid-phase ozone-olefin reaction, 72-74, 76 Cydic olefins, 4,60,76 aerosols from, 70-72,83,88 importance of, 104... 

For APHA Methods 2350-C and 4500-CL02-E, amperometric analyzers are used to measure chlorine dioxide in water. Amperometric analyzers measure the current that is necessary to maintain a constant concentration of titrant as chlorine dioxide reduces the titrant (e.g., phenylarsine oxide). This method is limited by interference from compounds that might react with the titrant (e.g., chlorine and chloroamine) (APHA 1998). 

Avdikos, E.M., Prodromidis, M.I., Efetathiou, CJi., 2005. Construction and analytical applications of a palm-sized microcontroller-based amperometric analyzer. Sens. Actuators B 107, 372—378. 

Scale of Operation Voltammetry is routinely used to analyze samples at the parts-per-million level and, in some cases, can be used to detect analytes at the parts-per-billion or parts-per-trillion level. Most analyses are carried out in conventional electrochemical cells using macro samples however, microcells are available that require as little as 50 pL of sample. Microelectrodes, with diameters as small as 2 pm, allow voltammetric measurements to be made on even smaller samples. For example, the concentration of glucose in 200-pm pond snail neurons has been successfully monitored using a 2-pm amperometric glucose electrode. ... 

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... 

The use of non-inert and chemically modified electrodes and other strategies for the detection of species that are difficult to analyze with the normal electrode materials have been reviewed.55 Photosensitization prior to amperometric detection is another tactic that has proved useful for the analysis of substances that are normally considered to be electrochemically inert.56 The use of pulsed amperometry has recently been reviewed.57... 

The above system of directly sensing a process stream without more is often not sufficiently accurate for process control so, robot titration is preferred in that case by means of for instance the microcomputerized (64K) Titro-Analyzer ADI 2015 (see Fig. 5.28) or its more flexible type ADI 2020 (handling even four sample streams) recently developed by Applikon Dependable Instruments20. These analyzers take a sample directly from process line(s), size it, run the complete analysis and transmit the calculated result(s) to process operation (or control) they allow for a wide range of analyses (potentiometric, amperometric and colorimetric) by means of titrations to a fixed end-point or to a full curve with either single or multiple equivalent points direct measurements with or without (standard) addition of auxiliary reagents can be presented in any units (pH, mV, temperature, etc.) required. 

Numerous assays are also available in the literature for analysis of biogenic amines and their acid metabolites in brain tissue. For example, Chi and colleagues (1999) developed a rapid and sensitive assay for analyzing NE, DA, 5-HT, 5-hydroxyindole-3-acetic acid (5-HIAA), and homovanilHc acid (HVA) in rat brain. The assay used a C18 column (150 x 4.6 mm) coupled to an amperometric electrochemical detector. The mobile phase consisted of a phosphate buffer (pH 4.75) and octane sulphonic acid as an ion-pair reagent in acetonitrile. The sensitivity of the analytes reported was 3-8 pg on column. 

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. 

Both O2 and H2O2 can be analyzed in a FIA system consisting of a chemical reactor where the process takes place, a device for sample withdrawal that avoids contact with the atmosphere, injection into a CZE unit, where the analytes become separated in a short time, and an ELD unit for amperometric end analysis. The method was applied for determination of glucose and photochemical deterioration of ketoprofen (70), by measuring the H2O2 generated according to equations 16 and 21, respectively . 

HPLC methods were developed for the detection and isolation of the free and Fe(III) complexed forms of the siderophores, pseudobactins, containing a pyrimido[l,2-a]quinoline chromophore (93JPR157 94MI1, 94MI2). A microbial siderophore and its Fe(III) complex were analyzed by means of HPLC with amperometric detection (93MI4). 

Electrochemical ion-selective sensors (ISSs), including potentiometric ion-selective electrodes (ISEs) and potentiometric or amperometric gas-selective sensors (GSSs), attracted the interest of clinical chemistry because they offer fast, reliable, inexpensive analytical results in service-free automated analyzers. In this way, the electrochemical sensors satisfy the present demands of central hospital laboratories and peripheral point-of-care medical service points, such as bedside, emergency or first-contact healthcare centers. 

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