Kinetic Interferences

5 Kinetic interferences. Kinetic interferences are caused by varying rates of formation or liberation of hydride from solution. These interferences do not occur when the hydride is collected before being allowed to enter the absorption tube. 

The use of peak area integration instead of the peak height evaluation may eliminate kinetic interferences. 

During analysis dense foam is produced when NaBH4 is added to biological samples which have not been fully decomposed. This foam retains a portion of the hydride and the resulting signal is significantly lower and broader than those of acidified standard solutions. 

The volume of the sample solution also has an influence on the sensitivity. Hydride is liberated more easily from smaller sample volumes than from larger ones. However, in practice this is not a problem since constant sample volumes are generally employed. In addition, differences in sensitivity between small and large volumes are not very noticeable. 

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In the case where the electrode electron transfer kinetics interferes, equation (4.9) replaces Nemst s law, and therefore... 

When mass transport kinetics interferes, the steady-state current-potential curves show a limiting current for the redox process, which brings into play this mass transport. The limiting current is shown on a current-potential curve as a horizontal plateau. It is often also called a limiting diffusion current. It is dependent on the mass transport parameters (migration, diffusion and convection) and is, with a few exceptionsproportional to the concentration of the consumed species, because their mass transport limits the reaction rateand therefore the current flow... 

Chemical compounds which are thermodynamically stable as isolated, pure compounds under the relevant conditions are not necessarily formed in the fuel matrix. There are problems due to kinetic interferences, to the very high UO2 excess, and to the unfavorable mass ratios of some of the fission product elements under consideration for the formation of compounds that are possible in principle. 

In this paper, the properties of the Dll mutant, a long-lived excited state without kinetic interference of charge separated states, are used to obtain a reliable optical characterization of the excited state. The results are discussed in comparison with functional reaction centers. 

The kinetics of reactions in which a new phase is formed may be complicated by the interference of that phase with the ease of access of the reactants to each other. This is the situation in corrosion and tarnishing reactions. Thus in the corrosion of a metal by oxygen the increasingly thick coating of oxide that builds up may offer more and more impedance to the reaction. Typical rate expressions are the logarithmic law,... 

The presence of a time limitation suggests that there must be a kinetically controlled interference, possibly arising from a competing chemical reaction. In this case the interference is the possible precipitation of CaCOs. 

Despite the variety of methods that had been developed, by 1960 kinetic methods were no longer in common use. The principal limitation to a broader acceptance of chemical kinetic methods was their greater susceptibility to errors from uncontrolled or poorly controlled variables, such as temperature and pH, and the presence of interferents that activate or inhibit catalytic reactions. Many of these limitations, however, were overcome during the 1960s, 1970s, and 1980s with the development of improved instrumentation and data analysis methods compensating for these errors. ... 

Miscellaneous Methods At the beginning of this section we noted that kinetic methods are susceptible to significant errors when experimental variables affecting the reaction s rate are difficult to control. Many variables, such as temperature, can be controlled with proper instrumentation. Other variables, such as interferents in the sample matrix, are more difficult to control and may lead to significant errors. Although not discussed in this text, direct-computation and curve-fitting methods have been developed that compensate for these sources of error. ... 

Selectivity The analysis of closely related compounds, as we have seen in earlier chapters, is often complicated by their tendency to interfere with one another. To overcome this problem, the analyte and interferent must first be separated. An advantage of chemical kinetic methods is that conditions can often be adjusted so that the analyte and interferent have different reaction rates. If the difference in rates is large enough, one species may react completely before the other species has a chance to react. For example, many enzymes selectively cat-... 

Selectivity in FIA is often better than that for conventional methods of analysis. In many cases this is due to the kinetic nature of the measurement process, in which potential interferents may react more slowly than the analyte. Contamination from external sources also is less of a problem since reagents are stored in closed reservoirs and are pumped through a system of transport tubing that, except for waste lines, is closed to the environment. 

Pourbaix diagrams are only thermodynamic predictions and yield no information about the kinetics of the reactions involved nor are the influences of other ionic species which may be present in the solution included. Complexing ions, particularly haUdes, can interfere with passivation and can influence... 

Coulometry. If it can be assumed that kinetic nuances in the solution are unimportant and that destmction of the sample is not a problem, then the simplest action may be to apply a potential to a working electrode having a surface area of several cm and wait until the current decays to zero. The potential should be sufficiently removed from the EP of the analyte, ie, about 200 mV, that the electrolysis of an interferent is avoided. The integral under the current vs time curve is a charge equal to nFCl, where n is the number of electrons needed to electrolyze the molecule, C is the concentration of the analyte, 1 is the volume of the solution, and F is the Faraday constant. 

Following this procedure urea can be determined with a linear calibration graph from 0.143 p.g-ml To 1.43 p.g-ml and a detection limit of 0.04 p.g-ml based on 3o criterion. Results show precision, as well as a satisfactory analytical recovery. The selectivity of the kinetic method itself is improved due to the great specificity that urease has for urea. There were no significant interferences in urea determination among the various substances tested. Method was applied for the determination of urea in semm. 

Knowledge of the rate is important to design chemical reactors for industrial production. It is also important for optimizing the production and to define the safety limits of operation. As was mentioned in the introduction, various transfer processes can influence chemical rates. The recognition of such interference is of primary importance during any study of kinetics, especially in those studies that will serve as the basis of design for production reactors. 

Electron spectroscopic techniques require vacuums of the order of 10 Pa for their operation. This requirement arises from the extreme surface-specificity of these techniques, mentioned above. With sampling depths of only a few atomic layers, and elemental sensitivities down to 10 atom layers (i. e., one atom of a particular element in 10 other atoms in an atomic layer), the techniques are clearly very sensitive to surface contamination, most of which comes from the residual gases in the vacuum system. According to gas kinetic theory, to have enough time to make a surface-analytical measurement on a surface that has just been prepared or exposed, before contamination from the gas phase interferes, the base pressure should be 10 Pa or lower, that is, in the region of ultrahigh vacuum (UHV). 

For kinetic investigations and for activity measurements, either photometric assays or - because of the higher complexity of the reactants converted by biocatalysts - HPEC methods can often be used. Here the ionic liquid itself or impurities may interfere with the analytical method. 

Ellipsometry (kinetics) Electrometric reduction (kinetics thickness) Interference colours and spectrophotometry (kinetics thickness)... 

The empirical rule described above for the enantiofacial differentiation in AE of primary allylic alcohols also applies to secondary allylic alcohols. The new aspect that needs to be taken into consideration in this case is the steric hindrance arising from the presence of a substituent (R4) at the carbon bearing the hydroxy group (Figure 6.3). This substituent will interfere in the process of oxygen delivery, making the oxidation of one enantiomer much faster than the reaction of the other one. The phenomenon is so acute that in practice kinetic resolution is often achieved (Figure 6.4) [27]. 

SPV- from the electric field of the polycation, which leads to a first-order back ET kinetics. Since the addition of NaCl interferes with the electrostatic binding of SPV- by QPh-14, SPV- can escape into the bulk phase by diffusion. Therefore, the back ET occurs via a bimolecular process when NaCl is added. 

Before studying esterification kinetics, it must be kept in mind that side reactions can interfere with the main reaction. They must either be avoided by changing experimental conditions or taken into account in kinetic calculations. Three types of phenomena can occur ... 

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