High Spatial Resolution Multi-Site EPR Oximetry: The Use of a Convolution-Based Fitting Method

We describe a new method to enhance the spatial resolution of multi-site electron paramagnetic resonance (EPR) oximetry. The method is suitable for any shape (density distribution function) of a solid paramagnetic material implanted in tissue. It corrects distortions of lineshapes caused by the gradient and thus overcomes limitations of previous multi-site EPR oximetry methods that restricted the ratio of the particle size to the distance between sites. The new method is based on consecutive applications of magnetic field gradients with the same direction but with a different magnitude and uses a convolution-based fitting algorithm to derive Lorentzian EPR linewidths of each individual peak of the EPR spectrum. The method is applicable for any particulate EPR oxygen sensitive materials whose EPR spectra can be approximated by a Lorentzian function or a superposition of Lorentzian functions. By incorporating this model of the lineshape in the data processing, we are able to decrease significantly the number of parameters needed for the calculations and to recover the oxygen concentration, even from quite noisy spectra. We (i) describe our method and the data-processing algorithm, (ii) demonstrate our approach in model and in vivo experiments, and (iii) discuss the limitations.     

Using of Digital Demodulation of Multiharmonic Overmodulated EPR Signals to Improve EPR Oximetry Reliability

Overmodulation of electron paramagnetic resonance (EPR) lines is routinely used in EPR oximetry in order to increase the signal-to-noise ratio and thus to improve the accuracy with which the line width of a spin probe can be measured. For a known probe type, the line width is easily translated into the oxygen partial pressure. A standard EPR spectrometer uses the analog phase-sensitive detection (PSD) to demodulate the EPR signal. PSD imposes the restriction that only one spectrum is measured at a time, which is normally the first-harmonic EPR line. Information about EPR signals centered at the other harmonics of the modulation frequency is irreversibly destroyed by PSD. The question is raised whether this information can be utilized for EPR oximetry, for overmodulation enhances the second- and the other harmonic spectra, so that they approach the first-harmonic spectrum in intensity. To find an answer, numerical simulation and experimental measurements have been conducted. The experiment required modification of the detection scheme, so that all EPR-related information in the overmodulated signal is preserved. This permits measuring of the multiharmonic EPR spectrum, which when fitted to a set of the corresponding theoretical lines produces more accurate results in comparison with the standard overmodulation method.     

High spatial resolution multi-site EPR oximetry. The use of convolution-based fitting method

We describe a new method to enhance the spatial resolution of multi-site electron paramagnetic resonance (EPR) oximetry. The method is suitable for any shape (density distribution function) of a solid paramagnetic material implanted in tissue. It corrects distortions of lineshapes caused by the gradient and thus overcomes limitations of previous multi-site EPR oximetry methods that restricted the ratio of the particle size to the distance between sites. The new method is based on consecutive applications of magnetic field gradients with the same direction but with a different magnitude and uses a convolution-based fitting algorithm to derive Lorentzian EPR linewidths of each individual peak of the EPR spectrum. The method is applicable for any particulate EPR oxygen sensitive materials whose EPR spectra can be approximated by a Lorentzian function or a superposition of Lorentzian functions. By incorporating this model of the lineshape in the data processing, we are able to decrease significantly the number of parameters needed for the calculations and to recover the oxygen concentration, even from quite noisy spectra. We (i) describe our method and the data-processing algorithm, (ii) demonstrate our approach in model and in vivo experiments, and (iii) discuss the limitations.     

Effects of resveratrol and its analogs on scavenging hydroxyl radicals: Evaluation by EPR spin trapping method

Resveratrol (3,4′,5-trihydroxy-trans-stilbene) and six analogs, polyhydroxystilbenes, were synthesized. Their effects on scavenging hydroxyl radicals were studied by electron paramagnetic resonance (EPR) spin trapping method. The EPR signal intensity of the spin adduct of hydroxyl radical to 5,5-dimethyl-1-pyrroline N-oxide was detected and used as a standard for the evaluation of the effect of the seven compounds on scavenging hydroxyl radicals. While all seven compounds exhibited hydroxyl radical-scavenging activity, some of them proved to be more effective than resveratrol in this model. Another stable but low-intensity spin adduct was also observed by EPR. A possible assignment is proposed.     

Practical conditions and limitations for high-spatial-resolution multisite EPR oximetry

We have previously reported a high-spatial-resolution multisite electron paramagnetic resonance (EPR) oximetry method that is based on consecutive applications of magnetic field gradients with the same direction but different magnitudes. This method that could be called also two-gradient convolution EPR oximetry has no restrictions for the shape of solid paramagnetic materials implanted in tissue and is applicable for any particulate EPR oxygen-sensitive matieral with a Lorentzian line shape. To enhance the utilization of this method, a previously described algorithm was used to develop user-friendly Windows-based software. Practical conditions of application of the method were established using several different model systems. It has been shown that the spectral overlap from the adjacent sites can be neglected if the splitting between the corresponding lines exceeds the largest line width by at least a factor of 1.3. An additional requirement of the method is that the second field gradient should exceed by at least 30% the value of the first gradient. It was confirmed that the error in line width determination at L-band is proportional to the noise-to-signal ratio, and does not exceed 1% a noise-to-signal ratio of 0.1 in a typical in vivo experiment. We demonstrate that the line widths of up to 10 different sites can be determined.     

EPR diagnostics of laser materials based on ZnSe crystals doped with transition elements

The electron paramagnetic resonance (EPR) spectra observed in laser materials based on zinc selenide (ZnSe) crystals doped with transition elements have been analyzed and identified. It has been shown that, in addition to working impurities (Cr²⁺, Co²⁺, or Fe²⁺), the diffusion layer exhibits EPR spectra of accompanying impurities due to the diffusion of transition elements (chromium, cobalt, or iron) used in the preparation of active materials for quantum electronics (lasers, switches) operating in the mid-infrared range. EPR diagnostics of these impurities can be used in the development of appropriate regimes for minimizing concentrations of accompanying impurities that adversely affect the performance characteristics of laser materials. It has been found that, during the diffusion of transition metals, ions of the accompanying impurity Mn²⁺, which is characterized by extremely informative EPR spectra, are embedded in the crystal lattice. It has been proposed to use these ions as ideal markers to control, on the electronic level, the crystal structure of the active diffusion layer.     

In vivo temporal EPR study using a region-selected intensity determination method to estimate cerebral reducing ability in rats treated with olanzapine

Region-selected intensity determination (RSID) is a method for obtaining the temporal changes in electron paramagnetic resonance (EPR) signal intensity from a target region, without the use of complicated procedures employed in the conventional imaging methods. An in vivo 700-MHz radio frequency EPR spectrometer equipped with a bridged loop-gap resonator was used with the RSID method to estimate intracerebral reducing ability in the rat following acute administration of olanzapine (OZP) or haloperidol (HPD). To this end, temporal changes in EPR signal intensity of target regions (the striatum and the prefrontal cortex) of rats which had received a blood-brain-barrier-permeable nitroxide radical (3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl) via an intravenous route were observed. The half-lives of EPR signal intensity in both regions of OZP- or HPD-treated rats were significantly longer than in control animals. This indicated that reducing abilities of the striatum and cerebral cortex decreased in the rats to which either OZP or HPD had been acutely administered.     

Application of magnetic field over-modulation for improved EPR linewidth measurements using probes with Lorentzian lineshape

Magnetic field modulation in CW electron paramagnetic resonance (EPR) is used for signal detection. However, it can also distort signal lineshape. In experiments where the linewidth information is of particular importance, small modulation amplitude is usually used to limit the lineshape distortion. The use of small modulation amplitude, however, results in low signal-to-noise ratio and therefore affects the precision of linewidth measurements. Recently, a new spectral simulation model has been developed enabling accurate fitting of modulation-broadened EPR spectra in liquids. Since the use of large modulation amplitude (over-modulation) can significantly enhance the EPR signal, the precision of linewidth measurements is therefore greatly improved. We investigated the over-modulation technique in EPR oximetry experiments using the oxygen-sensing probe lithium octa-n-butoxy-substitued naphthalocyanine (LiNc-BuO). Modulation amplitudes 2-18 times the intrinsic linewidth of the probe were applied to increase the spectral signal-to-noise ratio. The intrinsic linewidth of the probe at different oxygen concentrations was accurately extracted through curve fitting from the enhanced spectra. Thus, we demonstrated that the over-modulation model is also applicable to particulate oxygen-sensing probes such as LiNc-BuO and that the lineshape broadening induced by oxygen is separable from that induced by over-modulation. Therefore, the over-modulation technique can be used to enhance sensitivity and improve linewidth measurements for EPR oximetry with particulate oxygen-sensing probes with Lorentzian lineshape. It should be particularly useful for in vivo oxygen measurements, in which direct linewidth measurements may not be feasible due to inadequate signal-to-noise ratio.     

Influence of the lens effect in a sample with large dielectric constant in a loop-gap resonator on the EPR signal intensity at 700 MHz

The influence of the lens effect on the electron paramagnetic resonance (EPR) signal intensity was investigated in a loop-gap resonator (LGR) with an inner diameter of 41 mm. TheQ- value and EPR signal intensity were measured when the phantoms containing 3-carbamoyl-2,2,5,5-tetramethyl-pyrrolidin-l-yloxy dissolved in sodium chloride aqueous solutions were put in the LGR. TheQ- value and signal intensity reduced with increasing concentrations of sodium chloride in the phantom, indicating that the imaginary part of the dielectric constant is larger in the phantom with the higher concentration of sodium chloride. However, relationships betweenQ-values of the resonator and EPR signal intensities were not proportional and signal intensities were relatively higher compared with theQ-values. These findings suggest that the signal reduction due to lowQ is slightly compensated by the lens effect in the sample with the large real part of the dielectric constant. In the distribution of the signal intensities of a pinpoint sample made of diphenylpicrylhydrazyl in the agar medium containing sodium chloride in the LGR, it was found that the signal intensity decreased according to the distance from the center and the difference in the signal intensity within 10 mm from the center was about 20%, indicating the inhomogeneity of the alternating magnetic field at the center and marginal region in the sample with the large dielectric constant caused by the lens effect.     

Assessment of liver phagocytic activity using EPR spectrometry and imaging

The aim of the present study was to evaluate the usefulness of electron paramagnetic resonance (EPR) spectroscopy and imaging in assessing the phagocytic activity of the liver after administration of India ink. We conducted experiments on livers from control rodents and from rodents in which the Kupffer cell population had been depleted by pretreatment with gadolinium chloride. The EPR signal intensity recorded in liver homogenates was about two times lower in GdCl(3) treated rats than in control rats. EPR imaging carried out on precision-cut liver slices indicated a good correlation between the depletion of Kupffer cells and the EPR signal intensity.     

Special features of the EPR spectroscopy of a system of centers with different relaxation times

We found that the interaction of paramagnetic centers that have different relaxation times differs fundamentally from the interaction of centers having close relaxation times. Simulation showed that in this case there is an anomalous redistribution of the spectral-line intensity from the center to the wings with a virtually preserved distance between extremal points (super-Lorentzian shape of the line), which leads to underestimation of the total intensity recorded. The results obtained make it possible to explain a number of aspects of the radiospectroscopy of carbon materials of practical importance such as the nature of the generally accepted maximum on the curve for the dependence of the total intensity of an EPR signal on the temperature of the heat treatment of organic compounds and the degree of metamorphism of natural coals, the specific features of the effect of oxygen molecules and paramagnetic ions of metals on the EPR spectra of carbon materials, etc. Belarusian State University, 4, F. Skorina Ave., Minsk, 220080 Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 224–229, March–April, 1998.     

Carbon-based standards for electron paramagnetic resonance spectroscopy

In order to meet the need for a good new EPR intensity andg-value standard whose paramagnetic species are carbon-based radicals, several materials were investigated, including coal, fusinite (a coal maceral), and several carbohydrate chars. Of the prototypical standards prepared, a chemically-treated fusinite is recommended as most suitable because of its chemical stability, spin density, EPR signal line shape and line width, microwave power saturation characteristics, availability, and homogeneity. Effects of dilution with KBr, KCl, and polymer are negligible, although the line width is broadened in the presence of paramagnetic gases. Several model standard compounds have been prepared in a polymer matrix to minimize changes in packing density over time.     

Influence of neutron and heavy-ion irradiation on EPR spectra of CuGeO3

The influence of neutron and heavy-ion irradiation on the electron paramagnetic resonance (EPR) spectra of the spin-Peierls compound CuGeO₃ has been investigated in the wide temperature range of 2–300 K. It is found that the neutron irradiation leads to a decrease of the spin-Peierls transition temperature and induces appreciable changes in the EPR signal intensity, resonance line width andg-factor of this material. These changes may be associated with a partial suppression of both the energy gap and dimerization within the Cu chains due to the irradiation-induced changes in the topological and chemical short-range order. In contrast to this, the heavy-ion irradiation induces only an increase in the intensity of the EPR signal and does not produce appreciable changes in the resonance line width,g-factor and spin-Peierls transition temperature. The experimental results show a large increase in the Curie-Weiss component and complete suppression of the spin-Peierls transition for higher irradiation doses.     

Magnetic Properties of Monomer and Dimer Tetrahedral VO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Entities Dispersed on Amorphous Silica-based Materials: Prediction of EPR Parameters from Relativistic DFT Calculations and Broken Symmetry Approach to Exchange Couplings

Molecular structures of the isolated tetrahedral oxovanadium(IV) and bridged μ-oxo-divanadium(IV) complexes hosted by the clusters mimicking surfaces of amorphous silica-based materials were investigated using density functional theory (DFT) calculations. Principal values of the g and A tensors for the monomer vanadyl species were obtained using the coupled-perturbed DFT level of theory and the spin–orbit mean-field approximation (SOMF). Magnetic exchange interaction for the μ-oxo bridged vanadium(IV) dimer was investigated within the broken symmetry approach. An antiferromagnetic coupling of the individual magnetic moments of the vanadium(IV) centers in the [VO–O–VO]²⁺ bridges was revealed and discussed in detail. The coupling explains pronounced decrease of the electron paramagnetic resonance signal (EPR) intensity, observed for the reduced VO _x /SiO₂ samples with the increasing coverage of vanadia, in terms of transformation of the paramagnetic monomer species into the dimers with S = 0 ground state.     

Influence of the movement of cylindrical samples with variable internal diameter and variable length along thex-axis of a double TE104 and a single TE102 rectangular cavity on the EPR signal intensity: A sample shape study

The response of the cavity to the movement of cylindrical samples with internal diameters from 0.7 to 4 mm and lengths from 5 to 50 mm along thex-axis of the Bruker double TE₁₀₄ and single TE₁₀₂ rectangular cavity has been analyzed. Independently of sample internal diameter, the experimentally observed dependences of the electron paramagnetic resonance (EPR) signal intensity versus sample position in the cavity showed the following: (i) a sharp maximum for sample lengths from 5 to 20 mm; (ii) a “plateau”, over which the signal intensity remained constant within experimental errors of 0.47–1.16%, for lengths from 30 to 40 mm; and (iii) a “sloping plateau” region, which could be approximated by the linear function (correlationr = 0.96–0.98) for the 50 mm sample. Theoretical predictions of the experimental dependences of the signal intensity versus sample position in the cavity were calculated with the “modified” and “revised” sine-squared function, and the correlation between observed and theoretically computed dependences is very good. Additionally, the experimental dependence of the signal intensity versus the sample internal diameter and length for cylindrical samples situated at the position in the cavity at which the signal intensity was a maximum was likewise numerically approximated by the surface fitting with the Lorentzian cumulative additive function (correlationr = 0.999). The experimental dependence of the signal intensity versus the sample internal diameter for the given sample length is nonlinear. The samples with internal diameters of 0.7 and 1 mm gave the total maximum of signal intensity for the 40 mm sample, however, the samples with internal diameters of 2, 3 and 4 mm gave the total maximal value of signal intensity, which was identical for both the 30 and 40 mm samples. The experimental dependence of the EPR signal intensity versus the sample volume clearly showed that the samples with identical volumes, however, with different shapes, can give significantly different signal intensities (with differences ca. 200–400%). Then, the comparison of cylindrical samples with identical volumes but different shapes may be a serious source of significant errors in quantitative EPR spectroscopy. Cylindrical samples to be compared should be of identical shape. Accurate and precise positioning of each sample in the microwave cavity is essential.     

Clinical applications of in vivo EPR: Rationale and initial results

In vivo electron paramagnetic resonance (EPR) has been very useful for studies in animals, and these results suggest that there are some very attractive potential applications in human subjects. In this article, we describe our rationale for the clinical application of in vivo EPR, some of the principal technical challenges, the initial results in human subjects, and our evaluation of the areas where in vivo EPR is likely to play an important clinical role in the near future. The most obvious area of very high potential for clinical applications is tissue oximetry, where in vivo EPR can provide repeated and accurate measurements of tissue pO₂, a type of measurement that cannot be obtained by other techniques. Oximetry is capable of providing clinicians with information that can impact directly on diagnosis and therapy, especially for peripheral vascular disease, oncology, and wound healing. The other area of great immediate importance is the ability of in vivo EPR to measure clinically significant exposures to ionizing radiation after the fact, which may occur due to accidents, terrorist activity, or nuclear war. The results obtained already from human subjects demonstrate the feasibility of the use of in vivo EPR for measurements in human subjects. We anticipate that in vivo EPR will play a vital role in the clinical management of various pathologies in the years to come.     

Lithium naphthalocyanine as a new molecular radical probe for electron paramagnetic resonance oximetry

A new lithium naphthalocyanine dye aggregate [Li₂Nc][LiNc] is reported as a potential electron paramagnetic resonance (EPR) oximetry probe for accurate measurement of oxygen concentration in biological systems. The Li₂Nc is diamagnetic; however, the LiNc molecule has an unpaired electron and hence is paramagnetic. The aggregate shows a strong and single line EPR signal that is non-saturating at normal EPR power levels. An oxygen-dependent peak-to-peak EPR spectral width ranging from 0.51 G (at pO₂: 0 mmHg) to 26.2 G (at pO₂: 760 mmHg) has been observed. The application of this probe has been demonstrated in the measurement of arterial and venous oxygen tensions in a rat.     

Paramagnetic centres in exinite, vitrinite and inertinite

Exinite, vitrinite and inertinite from durain, vitrain and clarain of Polish medium-rank coal with 85.6% C were investigated by X-band (9.3 GHz) electron paramagnetic resonance spectroscopy. Multicomponent structure of the EPR spectra of these macérais was analysed. The number of component lines, their lineshapes and parameters: linewidths andg factors, were determined. Total concentrations and concentrations of paramagnetic centres responsible for the component lines were measured. The broad Gaussian, broad Lorentzian and narrow Lorentzian lines were observed in the experimental spectra of exinite and vitrinite. The EPR spectra of inertinite are superposition of two narrow Lorentzian lines with different linewidths. The evolution of paramagnetic centres during heating of the macerais at 300–650°C was studied. Paramagnetic centres with broad Lorentzian lines are the most active ones in the thermal decomposition. The EPR results indicate reactions between individual macerais during thermal decomposition of coal. Thermally excited multiplet states were found in exinite and vitrinite.     

In Vivo Temporal EPR Measurements in the Lung of Mice by a Selected-Region Intensity Determination Method

Selected-region intensity determination (SRID) is a method for obtaining the temporal changes in electron paramagnetic resonance (EPR) signal intensity from a selected region without complicated procedures used in the previous imaging method. We used an in vivo 700 MHz radiofrequency EPR spectrometer equipped with a bridged loop-gap resonator and the SRID method to perform temporal EPR measurements of the lung area of mice which had received a nitroxide radical (3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl) administration via the intravenous route. The half-life and initial level of nitroxide radical in the lung or the mediastinum were calculated from temporal changes in the signal intensity. A mathematical model was devised to determine the nitroxide radical concentration in the lung, which is connected to other organs via the circulatory system. Using this model and the results of the EPR measurements, the degrees of influence of the nitroxide reduction in the lung and other organs were simulated. It was found that the reaction rate (=log2/half-life) obtained from the lung mainly reflected the reduction of nitroxide radical there. Authors' address: Hidekatsu Yokoyama, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, 463-8560 Nagoya, Japan     

Special aspects of the temperature dependence of EPR absorption of chemically carbonized polyvinylidene fluoride derivatives

The temperature dependences of electron paramagnetic resonance (EPR) absorption of two samples of chemically carbonized derivatives of polyvinylidene fluoride (PVDF) synthesized under different conditions have been measured in the range of 100–300 K. It has been found that the temperature dependence of the integrated intensity of the EPR signal of both samples is nonmonotonic and does not obey the classical Curie dependence characteristic of free radicals. An analytical expression that is consistent with experimental data and suggests the presence of an activation component of paramagnetism in the test samples has been obtained. The presence of a term independent of temperature in this equation also indicates the paramagnetic contribution of free electrons. The magnitude of the activation energy of the singlet–triplet transitions has been evaluated: δ = 0.067 eV. The HYSCORE spectra of chemically carbonized PVDF derivatives have been obtained for the first time.