Kinetic methods, advantages selectivity

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-... 

These have also been applied to the analysis of drug substances because they have advantages over equilibrium techniques, especially when mixtures of closely related compounds, compounds that react slowly, or catalytically acting compounds are to be analyzed. The selectivity and sensitivity of kinetic methods of analysis combined with the selectivity and sensitivity of ion-selective electrodes provide a versatile combination that may lead to new analytical schemes. ... 

One salient advantage of fixed-time kinetic methods is significantly increased selectivity. Tolerated concentrations of interfering metal ions are often reported to be one to two orders of magnitude higher than in equilibrium methods. The increased tolerance to interferences of kinetic methods may be the result of the following facts ... 

The reactions used in kinetic methods fall into two categories catalyzed or uncatalyzed. As noted earlier, catalyzed reactions are the most widely used because of their superior sensitivity and selectivity. Uncatalyzed reactions are used to advantage when high-speed, automated measurements are required or when the sensitivity of the detection method is great. ... 

Below is a list of practical aspects that must be considered before selecting a kinetic method for a given circumstance and system. The relative advantages and disadvantages of each method are discussed with respect to each aspect. 

In order to minimize the influence of the so-called effect of delay in the response function - [03]g=f(x) upon calculating the values of k such sections of the kinetic curves are selected, which appear to be practically parallel or only slightly inclined with respect to the abscissa kj a[03]g d[03]g/dT. The advantages and limitations of this method have been dis-... 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

The method of evaluation of the rate constants for this reaction scheme will depend upon the type of analytical information available. This depends in part upon the nature of the reaction, but it also depends upon the contemporary state of analytical chemistry. Up to the middle of the 20th century, titrimetry was a widely applied means of studying reaction kinetics. Titrimetric analysis is not highly sensitive, nor is it very selective, but it is accurate and has the considerable advantage of providing absolute concentrations. When used to study the A —> B — C system in which the same substance is either produced or consumed in each step (e.g., the hydrolysis of a diamide or a diester), titration results yield a quantity F = Cb + 2cc- Swain devised a technique, called the time-ratio method, to evaluate the rate... 

Selected entries from Methods in Enzymology [vol, page(s)] Theory, 63, 159-162 activation effect, 63, 174, 175 analysis, 63, 140, 159-183 burst, 64, 20, 203, 215 enzyme concentration, 63, 175-177 hysteresis, 64, 197, 200-204 limitations, 63, 181-183 plotting, 63, 177-180 practical methods, 63, 175-177 reversible inhibitor action, 63, 163-175 reversible reaction, 63, 171-175 simulation of, 63, 180 advantages and disadvantages, 249, 61-62 analysis, in kinetic models of inhibition, 249, 168-169 concave-down, 249, 156 concave-up, 249, 156 with enzyme-product complex instability, 249, 88 with enzyme-substrate instabil-... 

It is also possible to screen catalysts according to their selectivities (regio- and stereoselectivity), their stabilities, and rates, since kinetic investigations can be carried out as well [23]. In addition, the application of especially designed NMR pulse sequences further extends the possibilities of the PHIP method [19]. For all of these applications, the main advantage of PHIP is the remarkable signal enhancement which is combined with a spectroscopic in situ method of observation. 

The latter method has the advantage of low alkali-atom densities, thus avoiding radiation trapping and chemical reactions and allows selection of the initial kinetic energy of the A atom—subject, however, to some discussion about the velocity distribution and its relaxation before quenching. The excited atoms will loose their excitation energy be either spontaneous emission... 

Unlike three-, four- or six-membered rings, five-membered rings are often made by standard carbonyl chemistry. This is because five-membered rings are the easiest to make by carbonyl condensations as they have kinetic and thermodynamic advantages over open chain compounds (chapter 29). This chapter gives a selection of such methods and the next chapter looks at some special methods for making five-membered rings. 

The crosslinking kinetics and the final state of cure are commonly studied with the aid of rheometers. NMR relaxation experiments can offer several advantages for the characterisation of the crosslinking kinetics in complex materials because of high method selectivity with respect to the rubbery chains/phases in polymer blends, filled and oil extended rubbers. 

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