Quenching effect

There is a maximum safe gap measured experimentally which will prevent the transmission of an explosion occurring within a container to a flammable mixture outside the container. Critical and maximum experimental safe gaps for a number of materials in air are listed in Lees (1980, pp. 491-492). These quenching effects are important in the design of flame arresters and flameproof equipment. 

The mechanical flame barriers, which are used for explosion isolation of flammable gas and solvent vapor explosions, are veiy susceptible to the action of dirt and, with one exception, are thus not suitable for dust-canying pipelines. The exception involves the rotaiy valve (see Fig. 26-45), which is based on the flame-quenching effect through narrow gaps and is mainly used at product charging and discharging points. 

Fig. 4 Explanation of the fluorescence-quenching effect [2]. — (A) chromatograms of the same quantities of saccharin and dulcin observed under UV 254 light, (B) schematic representation of fluorescence quenching, (C) spectral reflectance curves of saccharin and dulcin.

As discussed in the previous chapter, the Phen residue in APh-x forms the CT complex with MV2 + in aqueous solution [76]. Interestingly, the CT formation is suppressed in the poly(A/St/Phen)-MV2+ system in spite of the Phen fluorescence being quenched by MV2 + very effectively. This fact indicates that it becomes very less likely for the Phen moiety to come into a face-to-face contact with MV2+, while the fluorescence from the compartmentalized Phen residue can be quenched effectively via a collision-less ET to MV2 +. ... 

There is also a process called asynchronous excitation, in which the autoperiodic oscillation is released by means of the heteroperiodic one. It might seem that these asynchronous effects were the same that in the quenching effect the autoperiodic oscillation is suppressed while in the excitation effect, it is released. In reality this conclusion is more apparent than real, since in the quenching effect the only requirement is that to should be large enough, while in the excitation effect to may have any value and what counts is the character of the function f (x,x) in Eq. (6-148). 

The quenching effects of these esters and the phenolic products were also measured using standard Stern-Volmer quenching procedures. 

Table VIII. Quenching Effects by Oxalates and Product Phenols on CBI- and Dansyl-Labeled Amino Acids...

Spectrofluorometry presents sensitivity and selectivity greater than the absorbance spectroscopy, being more suitable for chlorophyll estimates in the nmol range and for residual amounts of derivatives in food products. Absorbance spectroscopy is satisfactory for concentrations > 1 xMP Spectrofluorometry is also more accurate for a wide range of chlorophyll a-to-chlorophyll b ratios, but it is less accurate when applied to complex sample matrices because of unpredictable quenching effects. 

The separation of uncolored samples is usually done on TLC plates containing a fluorescent dye so as to use the fluorescence-quenching effect for sample location. If such plates are not available or if samples show no quenching effect, two universal reagents can help. If the TLC plate is simply sprayed with water, sample spots are very often not translucent but white. Clearer zones can be obtained sometimes by first saturating... 

State decarbonylation reaction in total synthesis was reported recently in the case of natnral prodnct (+)-herbetenolide, which farther illustrates the exquisite control that the solid state may exert on the chemical behavior of the otherwise highly promiscuous reactive intermediates. As word or caution, it should be mentioned that intramolecular quenching effects known to act in solution can also affect that reaction in the solid state. Recently reported examples include the well-known intramolecular P-phenyl and electron transfer quenching. ... 

In order to determine the multiplicity of the reactive species, the photodimerization was carried out in the presence of the triplet quenchers oxygen and ferrocene. The results of these experiments are shown in Table 10.4.<41) It is obvious that the presence of oxygen exerts a large quenching effect on the production of the tram dimer and a smaller but significant effect on the formation of the cis dimer (the formation of tram dimer is decreased by oxygen by a factor of 25, while the cis dimer is decreased by a factor of 1.2). As with oxygen, the production of the tram dimer was quenched in the... 

The results of this study show a definite quenching of the 418 nm phosphorescence emission of DMT. One would expect that the quenching effect, in a rigid glass, would fit the Perrin model (73). A plot in In 4>0/4> versus concentration of 4,4 -BPDC yielded a straight line, the slope of which was identified with NV. The radius, R, of the active volume of quenching sphere was calculated by the following equation ... 

If neither mode of energy transfer is acceptable, a different explanation of the apparent quenching of the DMT phosphorescence must be put forth. It must be recalled that both DMT and 4,4 -BPDC absorb 298 nm light, which introduces the argument that competitive absorption causes the apparent quenching effect. 

Antioxidants shift the autoaccelerating increase of chemiluminescence intensity to higher time. This is due to reactions 12 and 13 of the Bolland-Gee mechanism (Section 1, Scheme 2) in which peroxyl radicals and hydroperoxides are scavenged until antioxidants InFl and D are consumed. A typical example of such a behavior occurs for samples of PP containing 0.1 %wt. of Irganox 1010 (a sterically hindered phenol) (Figure 18 and Table 4). The presence of antioxidants usually reduces the maximum chemiluminescence intensity [61,62]. This may be explained by the quenching effect of the antioxidant on excited carbonyls, but it may be also related to the mechanism of oxidation of stabilized PP. Stabilizers in... 

In order to know the quenching effect and the number of decays actually observable per decay of Pb-212 for the Insta Gel sample which contains Pb-212 and its decay products in radioequilibrium, the following experiments were performed. 

In Figure 5, the normalized emission spectra of the two solid hybrid materials, GFP/SBA-15 and GFP/Aerosil , are reported. The shape of the emission profile for GFP/SBA-15 follows closely that of the GFP in buffer solution, whereas the photoemission intensity of GFP/Aerosil is one order of magnitude lower and slightly different in its tale shape (spectra at the actual intensities not reported). This reduction in intensity could be explained by a multilayer arrangement of the protein molecules on the amorphous nanoparticles, which would explain both the difference in emission spectra ("self-quenching effect") and the difference in adsorption amount shown above. 

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