High field conductivity

Studies conducted by Barenghi eta.1. (1990) and Lodge etal. (1993) independently have demonstrated the facile, multicomponent analysis of a wide range of PUFA-derived peroxidation products (e.g. conjugated dienes, epoxides and oxysterols) in samples of oxidized LDL by high-field H-NMR spectroscopy. Figure 1.9 shows the applications of this technique to the detection of cholesterol oxidation products (7-ketocholesterol and the 5a, 6a and 5/3,60-epoxides) in isolated samples of plasma LDL pretreated with added coppcr(Il) or an admixture of this metal ion with H2O2, an experiment conducted in the authors laboratories. 

There is a modest increase in the electrical conductance with an increase in the electric-field gradient, an effect that operates with both strong and weak electrolytes (the first Wien effect). More important in the present context is the marked increase in electrical conductance of weak electrolytes when a high-intensity electric field is applied (second Wien effect). The high field promotes an increase in the concentration of ion pairs and free ions in the equilibrium... 

Relaxation dispersion data for water on Cab-O-Sil, which is a monodis-perse silica fine particulate, are shown in Fig. 2 (45). The data are analyzed in terms of the model summarized schematically in Fig. 3. The y process characterizes the high frequency local motions of the liquid in the surface phase and defines the high field relaxation dispersion. There is little field dependence because the local motions are rapid. The p process defines the power-law region of the relaxation dispersion in this model and characterizes the molecular reorientations mediated by translational displacements on the length scale of the order of the monomer size, or the particle size. The a process represents averaging of molecular orientations by translational displacements on the order of the particle cluster size, which is limited to the long time or low frequency end by exchange with bulk or free water. This model has been discussed in a number of contexts and extended studies have been conducted (34,41,43). 

To characterize the temperature dependence of this high field conductivity, the ordinate intercept current values of Schottky plots like those of Figure 3 are plotted against invene Absolute temperature in Figure 4. The data, taken on PIQ samples at temperatures between 100°C and 200° C, is described by a thermal activation energy of about 0.7 eV. 

At high field strengths a conductance Increase Is observed both In solution of strong and weak electrolytes. The phenomena were discovered by M. Wien (6- ) and are known as the first and the second Wien effect, respectively. The first Wien effect Is completely explained as an Increase In Ionic mobility which Is a consequency of the Inability of the fast moving Ions to build up an Ionic atmosphere (8). This mobility Increase may also be observed In solution of weak electrolytes but since the second Wien effect Is a much more pronounced effect we must Invoke another explanation, l.e. an Increase In free charge-carriers. The second Wien effect Is therefore a shift in Ionic equilibrium towards free ions upon the application of an electric field and is therefore also known as the Field Dissociation Effect (FDE). Only the smallness of the field dissociation effect safeguards the use of conductance techniques for the study of Ionization equilibria. 

From the foregoing discussion of electric field effects In Ionic equlibria It Is clear that a solution of a weak electrolyte shows a non-linear behavior In conductance (or resistance) at high field strengths. With an Interdisciplinary look at the field of electronics we note that such nonlinearities are at the heart of all modern electronic circuits and devices. We therefore can use a solution of a weak electrolyte subjects to high electric fields as an electronic device, which Is the basic Idea of the Field Modulation Tecnnlque, the general principles we will discuss now. 

The diffusion coefficients of the ion are usually estimated from their mobilities (or conductances), which can be measured independently. Diffusion coefficients (or mobilities) vary over very large ranges for the solvated electron in different solvents (neopentane, D 2 x 10 4 m2 s 1 and n 7 x 10 3 m2 V-1 s 1 hexane, D 2 x 10 7 m2 s 1 and ju 8x 10 6 m2 V-1 s 1) [320]. There is considerable evidence that the mobility of electrons is not constant, but on the contrary, the mobility depends on the applied electric field, increasing approximately proportionately with electric field at high fields in solvents where n is small and decreasing with electric field in solvents where n is large. If the solvated electron mobility depends on the electric field, then the diffusion coefficient may also depend on the electric field. The implications of these complications are discussed in Sect. 2.2 and in Chap. 8, Sect. 2.7. 

The present performance of high field NMR instruments allows the recording of a spectrum in quantities of micrograms. Under these conditions, it is possible to install a flow cell of only a few microlitres into the magnet of the instrument which allows the spectra of the analytes to be recorded. The experiment is conducted with a very small flow rate of the mobile phase (D20 or CD3CN) or in the stop-flow mode. In this mode, the mobile phase is momentarily stopped in order to record the spectrum. This technique, which requires very expensive materials, is of limited use. It is used mainly to isolate and identify very unstable compounds that cannot be isolated through classical means. 

Figure 29.21 The upper portion shows the EPR lines recorded prior to exposing polyacetylene film to gaseous AsFs and the transformation of a symmetric EPR line shape to a dysonian line for conductive material. The ratio of the low-field to high-field amplitudes is shown in the inset as a function of the composition of the doped film. The lower portion of the curve shows a single line fit to the theoretical dysonian line shape (circles). 

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