Analyte oxidation

Fig. 9. Generalized hydrodynamic voltammo-gram. i, = background current, i, = Faradaic onodic current from analyte oxidation...

It is generally agreed that the CL obtained with Inminol (124) is based on the series of transformations shown in Scheme 3, where the analyte (oxidant) as such or in combination with a catalyst prodnces a free radical (125), which in him captnres a superoxide anion to yield an endoperoxide (126), which on elimination of N2 prodnces an excited intermediate (127), which finally settles down to the 3-aminophthalate ion (128) on emission of a photon. A hnear correlation may be established between the intensity of the CL emission and the concentration of the analyte. ... 

An inductively coupled argon plasma eliminates many common interferences. The plasma is twice as hot as a conventional flame, and the residence time of analyte in the flame is about twice as long. Therefore, atomization is more complete and signal is enhanced. Formation of analyte oxides and hydroxides is negligible. The plasma is remarkably free of background radiation 15-35 mm above the load coil where sample emission is observed. 

Alternatively, air bubbled through water aqueous soln. of the analyte oxidized with periodic acid into formaldehyde the latter analyzed by chromotropic acid colorimetric method (Tucker and Deye, 1981). 

Pulsed amperometric detection is used for the analysis of carbohydrates and other nonchromophoric molecules such as alcohols, aldehydes, and amines. In the DC mode the products of the oxidation reaction of those compounds poison the surface of the working electrode, and further analyte oxidation is inhibited. This results in peaks that decrease rapidly in height. To maintain a stable and active electrode surface, alternatively positive and negative potentials are repeatedly pulsed. 

A nonporous aromatic polyimide membrane that is selectively permeable to H2, H, and H O has also been used for water vapor removal before the sample enters the ICP-MS [32]. Molecular analyte oxide ion signals were reduced approximately two orders of magnitude and O-containing polyatomic ions, such as ArO+ and C10+, were reduced by one to two orders of magnitude. 

Analyte oxidation and reagent generation in flow systems using tubular ultrasonic reactors have so far been unsuccessful [39], This is another under-explored area for analytical chemists. 

Most analytical oxidation/reduction reactions are carried out in solutions that have such high ionic strengths that activity coefficients cannot be obtained via the Debye-Hiickel equation (see Equation 10-1, Section lOB-2). Significant errors may result, however, if concentrations are used in the Nernst equation rather than activities. For example, the standard potential for the half-reaction... 

Moiroux, J., and Elving, P.J. 1979. Optimization of the analytical oxidation of diliydronicotinamide adenine dinucleotide at carbon and platinum electrodes. Analytical Chemistry 51. 346-350. 

Thus, in practice, amperometric detection involving analyte reduction is generally carried out by application of negative potentials, while analyte oxidation requires the use of relatively positive applied potentials. Successful bench-scale CE/EC in the amperometric mode requires the accurate maintenance of EC potentials on the order of roughly 1 V at working electrodes placed in CE fields on the order of 5-30 kV and the measurement of EC currents typically pA in magnitude in the presence of pA-level background CE currents. 

Amperometric detection refers to a detection method in which the current is proportional to the concentration of the species generating the current. It consists of two electrodes, a working electrode and a reference electrode, across which a DC voltage is applied. A redox reaction is induced at the working electrode when analyte solution flows in between two electrodes and the current change is monitored. The current response is directly proportional to the number of moles of analyte oxidized or reduced at the working electrode surface as described by Faraday s law ... 

Selective determinations of bromide and iodide have been performed by implementation of gas diffusion units for halogen permeation after analyte oxidation with permanganate or dichromate, followed by the phenol red reaction or amperometric measurements, and spectrophotometric detection of triiodide, respectively. 

In analytical chemistry, a redox titration is based on an oxidation-reduction reaction between analyte and titrant. Common analytical oxidants include iodine (I2), permanganate (MnOJ), cerium(IV), and dichromate (Cr207 ). Titrations with reducing agents such as Fe " (ferrous ion) and Sn " (stannous ion) are less common because solutions of most reducing agents need protection from air to prevent reaction with O2. 

The coupling of horseradish peroxidase acting as a hydrogen peroxide detector, with enzymes forming hydrogen peroxide as a co-product of analyte oxidation, has been used in designs of potentiometric sensors for determination of glucose (52) and lactate (55). 

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