Principle of Selection

Plasticizers are generally selected on the basis of the following criteria (8,9)  

Compatibility of a plasticizer with a given polymer. Processing characteristics. 

Desired thermal, electrical and mechanical properties of the end product, 

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It should be apparent that the principles of selective ion transport are independent of the specific models being treated here and that many of these principles are at variance with what were traditional views on the basis of selective membrane permeation by inorganic ions. Thus, the concept of selectivity among monovalent cations being based on values of hydrated radii is replaced by the... 

Figure 10.17. Principle of selective catalytic reduction using, for example, urea or a solution of ammonia urea as the reducing agent for application of the SCR reaction on mobile diesel units such as ferries or trucks. (Courtesy of HaldorTopsoe A/S.)...

General principles of selection of semiconductor adsorbents used as a operational sensor elements... 

Gouax, E. and MacKinnon, R. (2005) Principles of selective ion transport in channels and pumps, Science, 310, 1461-1465. 

The lambda sensor, which is found in cars with catalytic converters, is an example of an oxygen probe based on the principle of selective electrodes. This sensor, which looks like a spark plug, has a zirconium sleeve (Zr02) that behaves as a solid electrolyte. The external wall is in contact with emitted gas while the internal wall (the reference) is in contact with air. Two electrodes measure the potential difference between the two walls, which is indicative of the difference in concentration of oxygen. 

Figure 22-21 Principle of selected reaction monitoring, also called tandem mass spectrometry, mass spectrometry/mass spectrometry, or MS/MS.

C. Paper Chromatography(Papyrography or Partography). In this method, filter paper is used as an adsorbent in lieu of materials used in chromatographic columns. Although the principle of selective adsorption by paper (such as papyrus) was known for centuries and... 

In earlier publications we have shown that the principle of selection can be deduced from the premises of a self-replicating system as an extremum principle. It states that inherent linear autocatalysis causes the relative population numbers to take on values that correspond to the highest reproductive efficiency of the system as a whole. The distribution of relative concentrations in the stationary population is, after a short induction period, independent of changes of the system as a whole. The population consists of a uniquely defined wild-type (or several equivalent,... 

The principles of selective adsorption by zeolites are discussed in D. W. Breck, Zeolite Molecular Sieves, Wiley, New York 1974. 

The synthetic methods described were selected under the aspects of applicability, simplicity, and didactic value. Applicability implies that the method has been used in complex syntheses. Simplicity means that the method is not too time-consuming. Finally, didactic value is judged by some principles of selectivity control which we wanted to demonstrate. A list of commercially available starting materials tells the synthetic chemist what industry can do for him, and a selection of complex synthetic procedures shows what he may aim for. 

Three different principles of selectivity are required to achieve this result. First, the difference in rate of epoxidation by the catalyst of a disubstituted versus a monosubstituted olefin must be such that the propenyl group is epoxidized in complete preference to the vinyl group. The effect of this selectivity is to reduce the choice of olefinic faces to four of the two propenyl groups. Second, the inherent enantiofacial selectivity of the catalyst as represented in Figure 6A.1 will narrow the choice of propenyl faces from four to two. Finally, the steric factor responsible for kinetic resolution of 1-substituted allylic alcohols (Fig. 6A.2) will determine the final choice between the propenyl groups in the enantiomers of 80. The net result is the formation of epoxy alcohol 81 and enrichment of the unreacted allylic alcohol in the (35)-enantiomer. 

Principles of Selective Chemical Weed Control in Trees, Vineyards, and Fruit... 

Grasselli, R. K., Fundamental principles of selective heterogeneous oxidation in catalysis, Top. Catal., 79, 2002... 

One especially interesting kinetic resolution/asymmetric epoxidation substrate is (/ .5)-2,4-hexadien-3-ol (80). The racemic diene has eight different alkene faces at which epoxidation can occur and thereby presents an interesting challenge to the selectivity of the epoxidation catalyst. The selectivity can be tested by using slightly less than 0.5 equiv. of oxidant (because the substrate is a racemate, the maximum yield of any one product is 50%). When the reaction was run under these conditions, the only product that was formed was the (l/ ,2/ ,3/ )-epoxy alcohol (81). Three different principles of selectivity are required to achieve this result. First, the difference in rate of epoxidation by the catalyst of a disubstituted... 

Thus the effect on clotting might be seen as an unwanted side effect by someone who suffers stomach bleeding after repeat doses of aspirin, but in some circumstances it is desirable. This again illustrates a principle in the use of chemicals that is illustrated by the use of anti-cancer drugs (see below), the principle of selective toxicity, which we shall revisit in Chapter 4 what might be considered an adverse or toxic effect in some circumstances or in some organisms may be beneficial in others. The assessment and appreciation of toxicity sometimes needs to take this into account. 

Flucytosine is an antimetaboUte of the fluoropyrimidme type. The principle of selectivity of flucytosine for the fungal cell is dual it depends on an enzyme in order to penetrate the cell (fungal cytosine permease) and a fungal enzyme that deaminates flucytosine to the active antimetabolite 5-fluorouracil, which is metabolized to 5-fluor-ouridine. Replacement of 5-fluorouracil in RNA results in the disruption of protein sjmthesis in the fungus. Flucytosine has selective activity against pathogenic... 

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