Enhancement Effects

An ortho nitro group exerts a comparable rate enhancing effect m Chloronitrobenzene although much more reactive than chlorobenzene itself is thousands of times less reac tive than either o or p chloronitrobenzene... 

Zinc Borate. Zinc borate is also effective in enhancing the flame-inhibiting powers of chlorine. Although zinc borate increases flame resistance, it is not as effective as antimony oxide, as is illustrated in Figure 3. However, zinc borate can be used in combination with antimony oxide to obtain equivalent and in some instances enhanced effects over what can be obtained using either of the two synergists alone (Table 9). 

However, clinical results with compounds enhancing cholinergic function have not been overly convincing (272). In the case of tacriae, however, the beneficial therapeutic iadex was sufficient to justify regulatory approval ia several countries. Psychostimulants such as pemoline, amphetamine, procaine, and methylphenidate have failed to show cognitive enhancing effects ia patients with dementia, except possibly as iadirect consequences of mood elevation. 

Substituted tetrazoles readily exchange the 5-hydrogen for deuterium in aqueous solution. A major rate-enhancing effect is observed with copper(II) or zinc ions due to complexation with the heterocycle. The rate of base-induced proton-deuterium exchange of 1-methyltetrazole is 10 times faster than 2-methyltetrazole (77AHC(2l)323). 

DETERMINATION OF DNA BY ITS ENHANCEMENT EFFECT ON THE FLUORESCENCE OF Tb -NORFLOXACIN COMPLEX... 

XRF analysis of multiple-layer films is very complex because of the presence of XRF absorption and enhancement effects, not ordy between elements in the same layer but also between all layers in the film. Equations for the calculation of XRF intensities for multiple-layer films are available firom the literature. Proper correc-... 

The Duoplasmatron (Eig. 3.18). In the Duoplasmatron, gas-discharge ion sources are used for bombardment with oxygen or argon. In dynamic SIMS, especially, the use of O2 ions is common because of the chemical enhancement effect. With a duoplasmatron ion beam currents of several microamps can be generated. The diameter of the beam can be focused down to 0.5 pm. 

The element sensitivity is determined by the ionization probability of the sputtered atoms. This probability is influenced by the chemical state of the surface. As mentioned above, Cs" or OJ ions are used for sample bombardment in dynamic SIMS, because they the increase ionization probability. This is the so-called chemical enhancement effect. 

The oxygen ion beam diameter is limited to 0.5 pm by the duoplasmatron source used. For mapping electropositive elements this drawback must be tolerated because of the chemical enhancement effect. 

Recent developments in Raman equipment has led to a considerable increase in sensitivity. This has enabled the monitoring of reactions of organic monolayers on glassy carbon [4.292] and diamond surfaces and analysis of the structure of Lang-muir-Blodgett monolayers without any enhancement effects. Although this unenhanced surface-Raman spectroscopy is expected to be applicable to a variety of technically or scientifically important surfaces and interfaces, it nevertheless requires careful optimization of the apparatus, data treatment, and sample preparation. 

Fig. 25. Room temperature Raman spectra for purified single-wall carbon nanotubes excited at five different laser wavelengths, showing evidence for the resonant enhancement effect. As a consequence of the ID density of states, specific nanotubes (n, m) are resonant at each laser frequency [195].

If the rate enhancement effected by the enzyme is defined as rate enhancement = v /... 

Okamoto et al. found that A-oxidation activates 4-halogeno-quinolines in the reaction with piperidine in aqueous alcohol by kinetic factors of 9 to 25, at 100°. This rate-enhancing effect is accompanied by a fairly large decrease in the enthalpy of activation (up to 10 kcal/mole in the chloro compounds), the effect of which is partly offset by a decrease in the entropy of activation. 

For the ordinary Diels-Alder reaction the dienophile preferentially is of the electron-poor type electron-withdrawing substituents have a rate enhancing effect. Ethylene and simple alkenes are less reactive. Substituent Z in 2 can be e.g. CHO, COR, COOH, COOR, CN, Ar, NO2, halogen, C=C. Good dienophiles are for example maleic anhydride, acrolein, acrylonitrile, dehydrobenzene, tetracya-noethylene (TCNE), acetylene dicarboxylic esters. The diene preferentially is of the electron-rich type thus it should not bear an electron-withdrawing substituent. 

Table 9 indicates that the rate enhancement (kL/ko) is relatively small when Zn2 + ions or a ligand is used separately for both 50 and 52 substrates. A large rate enhancement is obtained only when a ligand and the metal ion are used together as in the previous examples (Table 1, 3, 4, 7). Ligands L-45 and L-46 are relatively inactive as compared to other ligands having the imidazole moiety. The ligand activation by metal ion is the order of Zn2+ > Co2+ > Ni2+ in all the cases, the same as in non-micellar reactions (Table 1). Rate-enhancing effects (kL/ko) of L-47-Zn2 +, L-48-Zn2 +, and L,L-49-Zn2+ ion complexes are remarkably large in view of the consideration...

They found that the rates for NABS increased as the degree of neutralization (a) was lowered and a maximum in the k—a plot appeared near a = 0.6. For DNPA, such an enhancing effect was not detected. 

E Fe-Ni Predominantly enhancement effect Mass absorption coefficients comparable nickel Ka excites iron Ka... 

Fig. 7—2. Spectral data to illustrate absorption and enhancement effects for three transition elements. (To avoid crowding, only part of the cobalt absorption curve is shown.) See Table 7-1. Case B. Substitution of A1 for Fe decreases absorption of incident beam and has little effect on analytical line. Net positive absorption effect. Case C. Substitution of Pb for Fe decreases absorption of primary beam but greatly increases absorption of analytical line. Net negative absorption effect. Case D. Note wavelength relationship indicated in figure. Enhancement impossible. Case E. Note wavelength relationship in figure. Enhancement occurs.

Analysis of Thick Samples. Enhancement Effects Absent... 

Enhancement effects pose more difficult problems. Enhancement results when the analytical line Ae of element E is excited within the sample by the characteristic line Ap of some other element F also present in the sample thus, when enhancement occurs, the intensity of the analytical line exceeds the value given by Equation 7-6 (Table 7-1). Sherman s treatment15 of the enhancement effect problem is the most thorough and successful to date. 

Sherman compares calculated and observed intensities for a number of known samples in some of which the enhancement components predominate over the intensities by direct excitation. The agreement obtained is usually within a few per cent, and this would be satisfactory even for considerably simpler problems. To be sure, the calculations do not give concentrations from measured intensities. But the fact that intensities can be satisfactorily calculated from known concentrations means that absorption and enhancement effects are thoroughly understood, and that x-ray emission spectrography is on a firm foundation. 

From what has been said about absorption and enhancement effects, it is clear that Equation 7-1 is not realized in practice. (Cf. Equation 7-6.) Deviations from Equation 7-1, conveniently called deviations from proportionality, have received a great deal of attention recently, a sure sign that x-ray emission spectrography s coming of age. 

Deviations from proportionality have three principal causes I, absorption and enhancement effects, placed together because they both involve absorption II, effects traceable to heterogeneity in the samples, principally surface effects and segregation III, instability, including drifts and fluctuations, in the spectrograph and in associated equipment. Class III deviations often increase with the complexity of the electronic circuitry. 

The deviations of Class I, here called absorption and enhancement effects, are known in the literature also as matrix effects, as self-absorption, and as interelement effects. The authors consider the most important objection to each of the last three names to be as. follows. To matrix effect the element sought (ncrt included in the matrix) contributes to the absorption effect for the sample in the same kind of way as any element (free or combined) in the matrix. To self-absorption the name makes no provision for enhancement effects. To interelement effects it fails to recognize that an absorption effect occurs even when only one element is present (Equation 7-4). The term matrix is useful but requires precise definition. What is the matrix when an internal standard is added, or when a powdered sample is dissolved ... 

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