Jan Plaček

Improvement of analytical accuracy of EPR spectrometry by taking into account variations in the shapes of samples

SummaryA method is proposed for the elimination of the systematic errors of quantitative electron paramagnetic resonance (EPR) spectrometric analysis resulting from the nonuniformity of microwave and modulation fields in the resonator. The errors are especially significant by a non-destructive analysis of solids when the samples are large and the sample to be analyzed differs from the reference one in shape and size. The method involves preliminary experimental characterization of the real configuration of the fields in a specific resonator on the basis of the set of signals recorded by approximately 500 different positions of a tiny sample in the resonator. The dependence of intensity of the ‘point’ sample signal on the sample coordinates can be expressed by an analytical function, and the correction factor can be obtained by means of integration of this function over the shapes of the two samples. In most cases the method enables an analyst to reduce the values of the systematic errors mentioned down to the level typical for random errors. The analytical description of the configuration of intraresonator fields is also beneficial for the solution of various methodological problems.

Free Radicals in Rabbit Spinal Cord Ischemia: Electron Spin Resonance Spectroscopy and Correlation with SOD Activity

Abstract1. In nonanesthetized rabbits temporal occlusion of the abdominal aorta was used to induce oxidative stress in the lower part of the body including distal segments of the spinal cord.2. Spinal cord samples were taken from the animals exposed to 25-min aortic occlusion (AO ) or to occlusion followed by 1- or 2-hr reperfusion (AO/R1 or AO/R2, respectively) or from sham-operated animals (C). The presence of free radicals (FR) in the spinal cord samples frozen in liquid N2 was assessed by ESR spectroscopy without spin trapping. Moreover, superoxide dismutase (SOD) activity and conjugated diene (CD) levels were measured in the samples.3. In the AO group FR were detected in the spinal cord regions close to the occlusion (lower thoracic and distal segments) along with a decrease in SOD activity. The calculated g value (g = 2.0291) indicated that the paramagnetic signal recorded might be attributed to superoxide radicals. FR were absent in the AO/R1 group. Concurrently, the SOD activity revealed a significant tendency to return to the control level. FR appeared again in the AO/R2 group, mostly in the upper and middle lumbar regions, along with a decrease in SOD activity. No sample from the C group revealed FR. A significant increase in CD levels was observed in the thoracolumbar region only in the AO/R2 group. The temporary absence of FR in the AO/R1 group suggests activation of defense antioxidant mechanisms (e.g., specific enzymatic systems such as SOD), which might have been exhausted later.4. Changes in SOD activity similar to those observed in the thoracolumbar region, though less noticeable, occurred in the obviously noncompromised tissue (upper cervical region). This points to a kind of generalized reponse of the animal to aortic occlusion.5. Direct ESR spectroscopy revealed the presence of FR as well as their time course in the spinal cord during the early phase of ischemia/reperfusion injury and the inverse relationship between FR and SOD activity.

Pressure dependence of free radical decay in polyamide VI: An ESR study

Abstract The ESR spectrum of polyamide VI has been analysed and the rate constant of free radical decay was determined over a wide range of pressure (1–16,000 atm) and temperature (80–120°). It appeared that the rate constant of decay decreased with increasing pressure. The ESR spectrum consisted of three component spectra, the proportions of which determined the shape of the overall spectrum for various conditions.

E.S.R. study of alkaline, oxidative degradation of saccharides. Identification of 2,5-dihydroxy-p-benzosemiquinone

Abstract The formation of radical intermediates during alkaline, oxidative degradation of saccharides and α-hydroxycarbonyl compounds has been studied by e.s.r. spectroscopy. Quantum chemical calculation and experiments in alkaline D 2 O solution showed that the dominant component of the overall spectrum corresponds to 2,5-dihydro- p -benzosemiquinone. Formation of this radical was also observed in the alkaline-degradation products of cellulose, starch, and (4- O -methylglucurono)xylan in the presence of air.

Free radicals in mechanically destructed polyamide 6 as studied by the ESR method

Abstract ESR spectra of polyamide 6 mechanically destructed were investigated at various temperatures. On the basis of theoretical analysis of the spectra, the free radicals present in the system, the rate constants for their decay, and the corresponding activation energies were determined.

Radicals in radiolyzed poly(methyl methacrylate)

Abstract The ESR method was used for studying radicals generated in poly(methyl methacrylate) at low temperature by γ-irradiation. On removing monomer from the sample, several types of radicals were observed; according to their thermal stability, they were examined in three regions of temperature.

The Stability of Free Radicals in Densely Crosslinked Acrylate Polymer Networks

The four types of crosslinked polymer samples based on pentaerythritol tetraacrylate (PETA) and propylene glycol-425-diacetoacetate (PGDAA) were prepared in two steps. At first the Michael dark addition of vinyl groups of PETA to PGDAA results in polymer network (cf. Scheme 1). The higher conversion of residual vinyl monomer and pendant double bonds built up in the Michael network was achieved by a photo-initiated free radical polymerization in the second step of polymer network synthesis. The lifetime of trapped radicals was estimated by following the decay of radicals in the dark period of intermittent illumination directly in the resonator cavity of ESR spectrometer. The determined values of bimolecular termination rate constants k t are of order 10 -1 to 10 2 kg/mol . s in dependence on composition, structure of network, and environment. It was observed that oxygen facilitates the decay of free radicals. The k t values are of about 1 to 2 decimal orders higher than that determined in nitrogen. Moreover, as in each subsequent light-dark cycle the k t was increased, we believe that it is a consequence of enhancement the diffusion of free radicals by assisted oxygen chain reaction with formation of hydroperoxides. The exception is the sample B with the highest final crosslinks density and conversion of monomers to network. More tightly trapped free radicals were present in polymer networks photo-cured in nitrogen. The k t values in all four types of networks decreased in each consecutive light-dark cycle.

ESR study of mechanically destructed methyl methacrylate-maleic anhydride copolymer

Abstract Methyl methacrylate-maleic anhydride copolymers were destructed mechanically; their ESR spectra were studied over the range −50° to +50° using three copolymers of different compositions. From theoretical spectra, radicals present in the system were identified at various temperatures; their decay was investigated. Radicals in the mechanically destructed copolymer were compared with those trapped in gamma-irradiated copolymer. At −50° in the copolymer destructed, some radicals are stabilized which do not occur in the gamma-irradiated copolymer; at room temperature, the differences between the spectra are not substantial.

Influence of pressure of free radical decay in irradiated polyethylene

Abstract Information is given on the effect of high pressure on the decay of free radicals in irradiated polyethylene. The dependence of the rate constant for decay on pressure (1–13,000 atm) was determined at 80, 90, 100 and 110°. The mechanism of the effect of pressure on the rate constant and activation energy for free radical decay is discussed.

ESR study of the generation and decay of peroxy radicals in mechanically destructed polyamide 6

Abstract Polyamide 6 has been mechanically destructed in vacuo. At -70°, the ESR spectrum corresponds to the sum of the component spectra of three radicals NHĊHCH2, ·CH2NHCO, and ·CH2CONH. After introducing air into the ampoule, the spectrum changes even at -70°; the changes have been studied up to 0°. The spectrum of the peroxy radical ROO· (with line width 1.57 mT, g1 = 2.0089 and g| = 2.0301) predominates.