Electron Paramagnetic Resonance Cavity Research Articles

Uniaxial stress system for electron paramagnetic resonance studies

A standard X-band electron paramagnetic resonance cavity has been adapted to measurements under uniaxial stress parallel to the external magnetic field direction. The system is applied to study single crystals under stress up to 100 MPa in the temperature range 100–400 K.

Characterization of photo-degradation of a polyurethane coating system by electron spin resonance

Photo-degradation studied by electron spin resonance (ESR) is described. Selected polyurethane film samples were exposed to different accelerated aging environments. These included narrow-band irradiation from 340 and 313 nm UV sources, broadband irradiation from a xenon arc lamp, and elevated temperature without irradiation. The effect of oxygen was investigated. In addition, the influence of titania on free radical formation was examined. Early stage radicals were detected by generating them in the ESR cavity at −196°C. A correlation between ESR and positron annihilation spectroscopy (PAS) data is shown, which indicates that free-radical-type chemical defects may be the cause of sub-nanometer physical defects.

In situ EPR and UV-vis spectroelectrochemistry of hole-transporting organic substrates

A newly developed in situ electron paramagnetic resonance (EPR)/ultraviolet-visible (UV-vis) spectroelectrochemical cell equipped with a laminated indium-tin oxide (ITO) working electrode was used in the investigation of various organic substrates which are potential hole-transporting materials. The experiment demonstrated the possibility of using such a technique for examining redox behavior of conducting polymers (polypyrrole, PPy), oligomers (thiophene dimmer and quarterthiophene) and bis-anilines (N,N,N′,N′-tetraphenylbenzidine, TPB). All investigated structures formed stable paramagnetic intermediates in the first oxidation step characterised with UV-vis spectra in the region 400–600 nm. In the second oxidation step EPR-silent di-cationic structures are formed with broad vis bands in the region 600–1000 nm. The measurement of the reference UV-vis spectra direct in the EPR cavity was possible using a specially-constructed non-contacted ITO plate in the spectroelectrochemical cell in the case of polypyrrole.

Determination of Hydrogen Radicals in Analytical Flames Using Electron Spin Resonance Spectroscopy Applied to Direct Investigations of Flame-Based Atomization Units for Hydride Generation Atomic Absorption Spectrometry

The atomization mechanism for the low-temperature flame used in hydride generation AAS has been studied by means of electron spin resonance (ESR) spectroscopy. By employing a miniaturized oxygen/hydrogen flame torch, especially constructed to fit in the center of the ESR cavity, it was possible to directly monitor the production of hydrogen radicals in the flame, as well as their consumption upon introduction of arsine into the flame. In this way, using arsenic as a model analyte, it could for the first time be shown in direct experiments that hydride-forming elements are atomized by radical recombination in the flame. The spatial distribution of hydrogen radicals in the quartz tube, whose dimensions are similar to those used for atomizing hydride-forming elements in AAS, was also studied. The principle of directly measuring radicals in miniature flames should be applicable also to other important analytical flame processes, such as native and modified flame ionization and flame photometric detectors for ...

Time-Resolved EPR: A Novel Method For Studying Living Chains

Time-Resolved EPR: A Novel Method For Studying Living Chains

Electronic Spin Resonance Detection of Superoxide and Hydroxyl Radicals During the Reductive Metabolism of Drugs by Rat Brain Preparations and Isolated Cerebral Microvessels

A spin trapping technique was used to analyze by electron spin resonance (ESR) the formation of oxygen-derived free radicals during the cerebral reductive metabolism of xenobiotics able to undergo a single electron reduction, i.e. quinones, pyridinium compounds and nitroheterocyclics. Paraquat, menadione and nitrofurazone were used as model compounds of these three classes of molecules. ESR spectra indicative of superoxide and hydroxyl radical formation were obtained by incubation of brain homogenates directly within the ESR cavity at 37°C for each of the three molecules tested. These signals were dependent on nucleotide cofactors, and increased in a time-dependent manner. The NADPH and NADH dependent free radical production was further characterized in brain microsomal and mitochondrial fractions, respectively. By using various combinations of reactive species inactivating enzymes (superoxide dismutase, catalase), a metal chelator (deferoxamine), and an hydroxyl trapping agent (dimethylsulfoxide), it was shown that (1) the primary radical generated was the superoxide anion; and (2) a significant production of the hydroxyl radical also occurred, that was secondary to the superoxide anion production. Consistent signals indicative of the production of both oxygen-derived free radicals were obtained when isolated cerebral microvessels which constitute the blood–brain barrier were incubated with the model molecules. This is of particular toxicological relevance, because this barrier represents a key element in the protection of the brain, and is in close contact with blood-born exogenous molecules.

Orientation effects of ESR analysis of alanine-polymer dosimeters

Orientation effects during electron spin resonance (ESR) measurement are studied on alanine-polymer dosimeters prepared by different molding procedures (press-molding or extrusion), and also according to different shapes (rods with different lengths and thin-films). The variation in ESR signal amplitude was measured by rotating a dosimeter around the vertical mid-axis and at a right angle to the magnetic field in the ESR cavity, at a temperature of 22°C. Orientation effects for rod dosimeters with both molding procedures (press and extrusion), and with different lengths, were negligible. Thin-film dosimeters molded by extrusion show a pronounced orientation effect when the samples are cut into disks and positioned horizontally (parallel to the magnetic field) during readout in the ESR cavity. Orientation effects on the film dosimeters can be eliminated, however, when they are positioned vertically and parallel to the mid-axis of the ESR cavity.

ESR and magnetic properties of electrically conducting metallophthalocyanine polymers

AbstractPolymeric metallophthalocyanines of Cu, Ni, Co in which benzene rings are shared in common with the macrocyclic phthalocyanine rings containing peripheral carboxylic groups have been synthesized and their electrical conductivities are shown to increase from 5 to 8 orders of magnitude by thermal treatment. The cobalt polymer exhibits greater conductivity than its nickel or copper analogs. The heated polymers show very broad electron spin resonance (ESR) signals and large paramagnetic susceptibilities. The bulk magnetic susceptibility of these polymers shows Curie‐like behavior when the samples are heated from room temperature to 473 K. This is explained on the basis of coexistence of fixed mobile and conduction electron spins in the system and the interaction of the soliton type defects present in these polymers with the polymeric network to give dipolar charge carriers. This is also supported by the decrease in ESR intensity with increasing temperature when the samples are heated in the ESR cavity. The effect of the presence of unpaired electrons on the d shell of the central metal atoms on the line width and the line shape of the ESR spectra is also explained. © 1994 John Wiley & Sons, Inc.

Development of the microwave Hall effect technique using an ESR spectrometer and a network analyser

An electron spin resonance (ESR) system was modified to conduct microwave Hall effect (MHE) measurements, but the ESR capability of the system was fully retained using interchangeable equipment. An X-band microwave frequency, a square-law detector, and phase-sensitive detection of the ESR signal were utilized during the modification. The ESR cavity was replaced with a bimodal MHE cavity and source modulation was used instead of field modulation. A cancelling channel was added to cancel out the non-ideal power at the secondary mode. A vector network analyser was used to tune the cavity and to measure the mobility of high mobility samples. A bimodal TE112 cylindrical cavity and a TE102 rectangular cavity were designed and constructed, and both gave good results. Silicon semiconductor samples, which had previously been well characterized, were used to calibrate the system and calculate the calibration constants.

Free radicals in the gas phase. ESR investigation of flame and microwave plasma of atomized gases. A contribution to flame retardation and polymer erosion mechanisms

Various experimental techniques based on electron spin resonance (ESR) are presented for the detection of paramagnetic species directly in the ‘hot’ H 2/O 2 flame and in the ‘cold’ plasma produced by microwave radiation. The concentration of atomic hydrogen in the H 2/O 2 flame was traced by shifting the burner in the ESR cavity or by exhausting the H· atoms from different positions in the flame. In this way the ‘steady-state’ radical concentration during the change from the diffuse to oscillating and finally to a premixed flame is quantitatively determined and the optimal position for the most effective flame retardation with halogenated flame retardants is located. Diatomic gases, H 2, N 2 and O 2 under reduced pressure, and volatilized molecules of H 2O, D 2O, H 2O 2 and NH 3, are effectively dissociated to their atoms when they are exposed to microwave radiation of 2450 MHz. At constant power of microwave radiation (5 to 100 W) the highest concentration of atomized gases, 2–3 × 10 16 spin/cm 3, is reached in the vacuum interval from 0·8 to 1·5 Torr. With increasing pressure the atomized species gradually disappear as a consequence of recombination. The presence of HO· and HOO· radicals and the effect of the applied magnetic field on gas phase radical reactions were ascertained by introducing the spin-trapping technique, transforming the primary unstable radicals of the plasma to stable nitroxy spin-adducts of DMPO (5,5-dimethyl-1-pyrroline- N-oxide). The concentration relationship between the highly reactive HO· and HOO· radicals as products of the reaction ·O· + HOH → 2HO· was measured after freezing the radicals from the flowing plasma on the internal cryostat situated in the ESR cell and cooled with liquid nitrogen. Using a cross-flow experimental method, the kinetics of the rapid reaction between the colliding atomized gases with different molecules, introduced into the ESR cell from the opposite direction to the plasma flow, was studied. The reactivity of atomized oxygen with solid targets was measured directly in the ESR cavity. In this way the one-electron transfer from chelated cobalt(II) with a 3 d 7 unpaired electron to atomized oxygen to form the complex Co(III)O • was proved. Atomized oxygen can initiate surface crosslinking after its addition to a double bond when it is in contact with exposed films of natural rubber or polyisoprene.

ELECTRON PARAMAGNETIC RESONANCE IMAGING OF ELECTROCHEMICALLY GENERATED p-PHENYLENEDIAMIANE RADICALS

Two-dimensional spectral-spatial electron paramagnetic resonance (EPR) imaging was used to measure the spatial distribution of p-phenylenediamine radicals that were generated electrochemically in the EPR cavity. The multi-line EPR spectrum obtained from a spectral slice through the image was in good agreement with the non-gradient spectrum. A spatial slice showed that the radicals were localized in the vicinity of the working electrode.

Electron nuclear double resonance (ENDOR) of the Qc.cntdot.- ubisemiquinone radical in the mitochondrial electron transport chain

We present an electron nuclear double resonance (ENDOR) study of the bound Qc.- ubisemiquinone in the mitochondrial quinol cytochrome c reductase complex. An ENDOR probe specifically modified for insertion into our electron paramagnetic resonance cavity was used for this study. We observed strongly hyperfine-coupled protons whose exchangeable nature indicated they were hydrogen-bonded to the quinone oxygen(s). It is thought that such hydrogen bonds are critical in binding the ubiquinone to protein, in stabilizing its semiquinone form, and in modulating the thermodynamic properties of the bound ubiquinone in the mitochondrial quinol cytochrome c reductase complex. Additional ENDOR features were assigned to protons of the quinone ring itself and to weakly coupled protons that may be associated with nearby amino acids. From very weakly hyperfine-coupled, distant, exchangeable protons there was also ENDOR evidence to suggest proximity and accessibility of the ubiquinone site to the solvent.

Novel method for accurate g measurements in electron-spin resonance

In high-accuracy work, electron-spin-resonance (ESR) g values are generally determined by calibrating against the accurately known proton nuclear magnetic resonance (NMR). For that method—based on leakage of microwave energy out of the ESR cavity—a convenient technique is presented to obtain accurate g values without needing conscientious precalibration procedures or cumbersome constructions. As main advantages, the method allows the easy monitoring of the positioning of the ESR and NMR samples while they are mounted as close as physically realizable at all time during their simultaneous resonances. Relative accuracies on g of ≊2×10−6 are easily achieved for ESR signals of peak-to-peak width ΔBpp≲0.3 G. The method has been applied to calibrate the g value of conduction electrons of small Li particles embedded in LiF—a frequently used g marker—resulting in gLiF:Li=2.002 293±0.000 002.

ESR spin-labeling method of determining membrane fluidity in biological materials--tissue culture cells, cardiac mitochondria, erythrocytes and epidermal cells.

The spin-labeling technique with electron spin resonance (ESR) was introduced to determine membrane fluidity in plasma membranes. Three kinds of doxylstearic acid were used as spin labeling agents. The experimental conditions of ESR measurement, such as spin probe concentration, number of cells, decay of ESR specturum, incubation time, ESR cavity temperature, and microwave power, were discussed with the aim of obtaining reproducible ESR spectra. As biological material, B-16 melanoma cells, Ehrlich ascites tumor cells, epidermal cells, erythrocytes and cardiac mitochondria were used.

High pressure high temperature electron spin resonance study of oil shale retorting

Previous studies of high pressure retorting under hydrogen have reported large increases in the Fischer assay oil yield for carbonaceous shales. A high temperature, high pressure, electron spin resonance cavity has been constructed and used to measure spin concentrations of the radicals during pyrolysis. The results are independent of the gas and do not show the expected reduction in spin concentration for hydrogen due to hydrogen capping; this can be explained by the failure of steady state electron spin resonance measurements to detect rapid radical formation and decay processes.

Improved temperature control for samples in electron-paramagnetic-resonance spectroscopy

An improved method of temperature control for electron-paramagnetic-resonance (EPR) spectroscopy is described that reduces the large (1°–2 °C/cm) temperature gradients along the sample associated with gas flow EPR temperature controllers. Circulation of dimethylpolysiloxane through an EPR cavity Dewar using a temperature programmable bath reduces gradients to typically 0.01 °C/cm. Temperature changes within the cavity Dewar can be programmed at rates up to 5 °C/min. This technique provides more precise control of sample temperature and allows the sample to be reset to a specified temperature with an uncertainty of less than 0.1 °C.

Use of in-situ electron paramagnetic resonance to assess formation of free radicals and their role in the hydroliquefaction of coal

The depolymerization of coal is generally presumed to proceed via a free radical mechanism. A unique electron spin resonance cavity that allows the observation of free radical formation and behaviour under hydroliquefaction conditions (500 °C and 12.4 MPa) was designed to facilitate the study of the effect of process variables on the formation of free radicals. Parallel experiments were carried out in autoclaves, and the degree of conversion and quality of coal-derived liquids produced was assessed. Correlative statistical models were established that allow the quantitative determination of the effect of process variables, and their interactions, on the production and behaviour of free radicals and of coal-derived liquids produced. Strong evidence was obtained that free radical chemistry plays a key role in determining the degree of coal conversion and quality of products. The kinetics and mechanism of hydroliquefaction are addressed with the explicit inclusion of experimental free radical concentration in the kinetic expressions. Progressive and regressive reactions are discussed. Optimum combinations (‘processing windows’) of process variables can be identified.

Balanced cavity scheme for saturation transfer dispersion electron paramagnetic resonance

Dispersion electron paramagnetic resonance (EPR) suffers from high AM noise levels due to discrimination of FM noise from the klystron by the high Q cavity. If the EPR cavity is placed in one arm of a magic tee, and a similar cavity in the other arm, then this noise can be removed by arranging the two (correlated) noise components to cancel. Signal-to-noise ratio improvements of a factor of 40 are easily attainable. Only minor modifications to commercial instruments are required.

Tube electrode and electron spin resonance transient signals

A tube electrode is positioned so that the electrode is just above an electron spin resonance cavity; radicals generated on the electrode are transported by laminar flow into the cavity. By solving the time-dependent convective-diffusion equation for the transport, the variation of the e.s.r. signal with time as the current is stepped on the electrode can be calculated. Theory and experiment are shown to be in good agreement.

Formation and behaviour of coal free radicals in pyrolysis and liquefaction conditions

In the past several years there has been a renewed interest in fundamental studies of coals and coal conversion. This interest is due to the expectation that an understanding of the structure of coal and of the fundamental processes during coal conversion would aid the development of technology for the utilization of coal on cost-effective and environmentally acceptable terms. In this context, electron spin resonance (ESR) is a promising technique, for it can be used to study paramagnetic centres in coal, including reactive free radicals which are generally assumed to have a key role in conversion processes. The ESR measurements of coals reported to date have all been measurements of pre-existing free radicals or free radicals remaining after some treatment. We report here the fabrication and utilization of a uniquely designed high-pressure–high-temperature ESR cavity and the first in situ observation of free radicals in coals.