Electron Spin Resonance Spectrometer Research Articles

Broad-ferromagnetic-linewidth non-metallic gyromagnetic spheres: A comparison of linewidth characterization methods

The ferromagnetic linewidth obtained via different rectangular cavity ferromagnetic resonance experimental data processing algorithms has been compared. Approaches based on perturbation theory closed-form formulas, resonance frequency dispersion, Q-factor, transmitted power, and an electrodynamic transcendental equation (TDE) have been considered. The results were compared with direct Q-factor measurements in a subwavelength cavity and an electron paramagnetic resonance spectrometer. Three spherical samples, of approximately 1 mm in diameter and different saturation magnetization values, were investigated: monocrystalline gallium-substituted YIG (Ga:YIG, 50 Gs), polycrystalline aluminum-substituted YIG (Al:YIG, 380 Gs), and polycrystalline zirconium–calcium-substituted YIG (Zr,Ca:YIG,1826 Gs). A new approach to processing rectangular cavity data, based on the TDE, has been proposed and validated. Using the proposed approach, it is possible to obtain the accurate value of the linewidth in regimes where strong coupling between the sample and the cavity does not occur but the small perturbation assumption is no longer valid either. There is also no need to decouple the sample from the cavity. The conclusions of this work are useful for resonant characterization of not only magnetic spheres but also other shapes.

Preparation of New ESR Dosimetry System for High Dose Radiation Processing

The main objective of this work is to study the possibility of using the radiation sensitive carrageenan substance for routine process control in irradiation processing applications. The free radicals generated in the prepared carrageenan rods were investigated using the Electron Spin Resonance Spectrometer. The effect of relative humidity during irradiation on the response curve as well as short term and long term stability were investigated. The obtained data suggested the possibility of using the carrageenan substance in dose monitoring ranging from 0.5 to 50 kGy.

Direct Z-scheme CdFe2O4/g-C3N4 hybrid photocatalysts for highly efficient ceftiofur sodium photodegradation

A direct Z-scheme CdFe2O4/g-C3N4 hybrid systems with different weight ratios of CdFe2O4 nanoparticles were successfully designed and constructed for ceftiofur sodium photodegradation. The as-obtained CdFe2O4/g-C3N4 hybrid samples composed of CdFe2O4 nanoparticles and g-C3N4 nanosheets were systematically characterized by different techniques including X-ray diffraction, scanning electron microscope, transmission electron microscope, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV–vis DRS, chemical oxygen demand, inductively coupled plasma mass spectrometer, photochemical test and electron spin resonance spectrometer technique. The optimal photocatalytic activity for ceftiofur sodium photodegradation was achieved by modulating the weight ration between CdFe2O4 nanoparticles and g-C3N4 nanosheets. The result from photocatalytic tests indicate that CdFe2O4/g-C3N4-2 hybrid sample exhibit highly efficient photocatalytic activity towards ceftiofur sodium removal in comparison with pristine CdFe2O4 nanoparticles and pure g-C3N4 nanosheets. Meanwhile the excellent photocatalytic stability of CdFe2O4/g-C3N4-2 hybrid sample was also verified during recycling runs towards photocatalytic ceftiofur sodium degradation. The significant enhancement of photocatalytic activity was attributed to the Z-scheme charge separation and transfer based on the construction of tight heterogeneous interface and well-matched band potentials. We expect this research to provide a new insight into the design and preparation of direct Z-scheme hybrid photocatalysts for environmental remediation.

Cryogenic frequency-domain electron spin resonance spectrometer based on coplanar waveguides and field modulation.

We present an instrument to perform frequency-domain electron spin resonance experiments that is based on coplanar waveguides and field modulation. A large parameter space in frequency (up to 25 GHz), magnetic field (up to 8 T), and temperature (down to 1.6 K) is accessible. We performed experiments on DPPH (2,2-diphenyl-1-picrylhydrazyl) as a standard to calibrate the field modulation as well as on a carbon fiber sample to estimate the overall sensitivity of the instruments. Spectra of a ruby sample in a broad frequency and field range at cryogenic temperatures are recorded with and without field modulation. The comparison reveals the improved signal-to-noise ratio achieved by field modulation.

Annealing Kinetics of Ti- and Ge-Related Centers in Quartz

Radiation defects in quartz are widely used in trapped charge chronometry. Analytical function which simulates dose–response curve is a very important for the correct measurement of equivalent dose. Ge- and Ti-related centers in quartz which may be used in electron paramagnetic resonance (EPR) dating have non-monotonic dose–response curves. In addition to accumulation, very significant parameter of radiation centers is their thermal stability. Obtained in this work results show that increase of concentration of paramagnetic Ge- and Ti-centers in quartz at the beginning of annealing may be explained by the existence of electron centers with two captured electrons which are not detected with used EPR spectrometer. Proposed model of formation and recombination processes satisfactorily simulates the experimental data—the dose dependencies and isothermal annealing data. It also explains why first- or second-order kinetics used in previous investigations cannot simulate annealing dependences correctly.

Особенности высокочастотного спектрометра электронного парамагнитного резонанса с модуляцией частоты

A high-frequency electron paramagnetic resonance (EPR) spectrometer with frequency modulation was developed. The advantages of the method for recording the EPR spectra of paramagnetic centers with giant fine-structure splitting in the case of non-Kramers ions in a garnet crystal were demonstrated.

Application of High-Frequency EPR Spectroscopy for the Identification and Separation of Nitrogen and Vanadium Sites in Silicon Carbide Crystals and Heterostructures

The advantage of the high-frequency spectroscopy of electron paramagnetic resonance (EPR) for the identification of nitrogen donors and a deep compensating vanadium impurity in various crystallographic positions of the silicon-carbide crystal is shown. Measurements are performed using a new generation EPR spectrometer operating in the continuous wave and pulsed modes at frequencies of 94 and 130 GHz in a wide range of magnetic fields (–7–7 T) and temperatures (1.5–300 K). A magneto-optical closed-cycle cryogenic system (Spectormag PT), highly stable generators (94 and 130 GHz), and a cavity-free system for supplying microwave power to the sample are used.

Novel ternary p-ZnIn2S4/rGO/n-g-C3N4 Z-scheme nanocatalyst with enhanced antibiotic degradation in a dark self-biased fuel cell

A novel ternary heterojunction nanocatalyst, p-ZnIn2S4/rGO/n-g-C3N4, was synthesized as a low-cost, high-efficiency photoelectrocatalyst. A hybrid composite of p-type ZnIn2S4, n-type g-C3N4, and GO was formed at 80 °C by a simple hydrothermal method. When installed in a dark self-biased fuel cell with cathodic Pt nanocatalysts, the anodic p-ZnIn2S4/rGO/n-g-C3N4 (83%) achieved higher triclosan (TCS) pollutant removal rate than anodic ZnIn2S4/g-C3N4 (52.3%), ZnIn2S4 (35%), and g-C3N4 (18%) after 30 min. Moreover, the p-ZnIn2S4/rGO/n-g-C3N4 not only extended the photo-response range but also accelerated the electron transfer and electron–hole separation in the absence of light or any external energy input. By comparing the different conditions in the fuel cell system, the dark condition was still comparable to the visible light and ultraviolet irradiation during the final monitoring time. scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM) were applied to characterize the morphology, composition, and structure of ZnIn2S4/g-C3N4, ZnIn2S4, and g-C3N4. Furthermore, reactive oxygen species were formed via aeration and activation of molecular oxygen through interfacial electron transfer, and active species were detected via electron spin resonance spectrometer (ESR). Finally, TCS degradation mechanisms in the self-biased fuel cell were proposed.

Применение высокочастотной ЭПР спектроскопии для идентификации и разделения позиций азота и ванадия в кристаллах и гетероструктурах карбида кремния

Abstract The advantage of the high-frequency spectroscopy of electron paramagnetic resonance (EPR) for the identification of nitrogen donors and a deep compensating vanadium impurity in various crystallographic positions of the silicon-carbide crystal is shown. Measurements are performed using a new generation EPR spectrometer operating in the continuous wave and pulsed modes at frequencies of 94 and 130 GHz in a wide range of magnetic fields (–7–7 T) and temperatures (1.5–300 K). A magneto-optical closed-cycle cryogenic system (Spectormag PT), highly stable generators (94 and 130 GHz), and a cavity-free system for supplying microwave power to the sample are used.

Broadband electron paramagnetic resonance spectrometer from 1 to 15 GHz using metallic coplanar waveguide.

We report a broadband electron paramagnetic resonance (EPR) spectrometer that operates continuously in the frequency range from 1 to 15 GHz. A broadband metallic coplanar waveguide is utilized as the probe. The system is capable of performing EPR measurements in both continuous wave and pulsed modes. Its performance has been tested with a sample, named 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl powder, at room temperature. In the continuous wave mode, the sensitivity of the spectrometer is estimated to be 3.3×1012 spins/gaussHz at 13 GHz. In the pulsed mode, inversion recovery experiments were carried out to obtain the spin-lattice relaxation time of the sample.

Novel normal-state low field microwave absorption in SmFeAsO1−xFx iron pnictide superconductors

We report on the systematic normal and superconducting states microwave absorption studies in polycrystalline samples of SmFeAsO1−xFx using an electron spin resonance (ESR) spectrometer. In the normal state, we observed two absorption spectra; at high fields (~3500 G) and low fields (around 0 G). The high field spectrum is in principle a resonance spectrum corresponding to a saturated state. It was observed that this signal broadens and its peak to peak intensity decreases on cooling the sample from room temperature. Ultimately, the signal becomes ESR ‘silent’ in the superconducting state. On the other hand, the low field microwave absorption (LFMA), centered around zero field, was observed for the first time, in a superconducting material. Based on the line-width, hysteresis and peak to peak signal intensity, we have shown that this spectrum is distinctively different from the already known LFMA signal that occurs in the superconducting state. These results present a system that shows LFMA spectrum in both normal and superconducting state thus providing an excellent platform to analyse the elusive LFMA phenomenon.

Towards Low-Cost, High-Sensitivity Point-of-Care Diagnostics Using VCO-Based ESR-on-a-Chip Detectors

In this paper, we present a new architecture for a voltage-controlled oscillator (VCO)-based electron spin resonance (ESR) detection for a future use in portable, point-of-care ESR spectrometers. The proposed architecture is centered around an application-specified integrated circuit (ASIC) containing a VCO-based ESR detector with two distinct tuning ports with largely different VCO gains to enable wide frequency sweeps and small-signal frequency modulations while keeping the requirements on the digital-to-analog converter driving the ports manageable. In addition, the proposed ASIC features a second VCO for an on-chip frequency downconversion via mixing. To allow for a precise derivation of the operating frequency from an external reference as it is required for quantitative ESR experiments, the two on-chip VCOs are embedded into an offset phase-locked loop. The proposed architecture is verified with ESR experiments on commonly used ESR standard samples (DPPH and BDPA). In these experiments, a spin sensitivity of $1.7\times 10^{9}~{\text {spins}/(\text {G}\sqrt {\text {Hz}})}$ has been achieved at $B_{0} = {450} {\text {mT}}$ , which is comparable to the state of the art, using a permanent magnet and low-cost signal processing on an field-programmable gate array. The presented proof-of-concept experiments clearly demonstrate the potential of the proposed VCO-based ESR detection system for future point-of-care applications.

Extending electron paramagnetic resonance to nanoliter volume protein single crystals using a self-resonant microhelix.

Electron paramagnetic resonance (EPR) spectroscopy on protein single crystals is the ultimate method for determining the electronic structure of paramagnetic intermediates at the active site of an enzyme and relating the magnetic tensor to a molecular structure. However, crystals of dimensions typical for protein crystallography (0.05 to 0.3mm) provide insufficient signal intensity. In this work, we present a microwave self-resonant microhelix for nanoliter samples that can be implemented in a commercial X-band (9.5 GHz) EPR spectrometer. The self-resonant microhelix provides a measured signal-to-noise improvement up to a factor of 28 with respect to commercial EPR resonators. This work opens up the possibility to use advanced EPR techniques for studying protein single crystals of dimensions typical for x-ray crystallography. The technique is demonstrated by EPR experiments on single crystal [FeFe]-hydrogenase (Clostridium pasteurianum; CpI) with dimensions of 0.3 mm by 0.1 mm by 0.1 mm, yielding a proposed g-tensor orientation of the Hox state.

Detection of Free Oxygen Reduction Reaction Intermediates Generated at Pt Nanoparticles By Electron Paramagnetic Resonance Spectroscopy

In the search for clean energy conversion systems, PEM fuel cells have attracted a lot of attention over the last decades. Nanostructured electrocatalysts such as porous platinum nanoparticles (NPs) are considered one of the most promising advances to lower the cost of these fuel cells and make them more commercially attractive. The efficiency of these catalysts is limited by their activity towards the oxygen reduction reaction (ORR), and most research is thus focused on understanding reaction mechanisms and improving the activity. The mechanism is well understood for alkaline conditions, but there is still some debate for that under acidic conditions. It has been shown indirectly[1],[2] that radical intermediates formed during the reaction can desorb from the catalyst surface, but direct proof is still absent. More importantly, these free radicals are suspected to be responsible for the degradation of the carbon support, leading to the deactivation of the electrocatalyst NPs[3]. We propose an approach to detect and identify the free radicals produced during the ORR using electron paramagnetic resonance (EPR) spectroscopy. EPR spectroscopy combined with spin trapping has been shown to be useful for detecting radicals produced in electrochemical reactions[4],[5]. This method relies on binding the free oxygen radicals to the spin trap DMPO, forming the stable radical complex DMPO-OH•. The DMPO-OH complex can be detected using an EPR spectrometer. Custom-made glassy carbon RDE’s with a large geometrical surface area have been designed and were loaded with Pt nanoparticles via double pulse electrodeposition. The large surface area is necessary in order to produce sufficiently large amounts of free radicals to be detectable in the EPR spectrometer. A DMPO-OH• signal is formed and increases in strength for the duration of the ORR experiments. Furthermore, we observe a direct correlation between the charge produced during the experiment and the strength of the DMPO-OH• signal. This approach, capable of detecting the free oxygen radicals produced during the ORR, is employed to shed light on the ORR mechanism at carbon electrodes loaded with Pt NPs in acidic conditions.

Molecular oxygenates from the thermal degradation of tobacco and material characterization of tobacco char

Abstract This contribution investigates the formation of selected oxygenated molecular compounds with smoking temperatures, particulates and free radicals from the burning of tobacco selected from two cigarettes (SM1 and ES1), in a poor oxygen environment - an oxidative atmosphere (5% oxygen in N2), in the temperature region 200–700 °C at 2 s contact time. Molecular carbonyls were determined using a Gas chromatography (GC) hyphenated to a mass selective detector (MSD). Fourier transform infrared (FTIR) spectroscopy afforded the determination of functional groups adsorbed on tobacco particulates. Free radicals in tobacco smoke particulates were investigated using an electron paramagnetic resonance spectrometer (EPR) whereas particulate size of tobacco smoke was estimated using image J. Aztec (UK) software coupled to an Oxford detector (UK) facilitated elemental analysis of tobacco char. At ∼ 400 °C, 2-methylcyclopentenone gave a maximum yield of 7.95 μ g while 2-methyl-2-propanol had a maximum yield of 14.75 μ g . The absorption band at 1740 cm−1 confirms the presence of carbonyl groups ( − C = O ) adsorbed on tobacco smoke particulates. Tobacco particulates generated at 600 °C showed an isotropic g-value of 2.0028 and a particle size distribution between 18 and 30 nm. It was noted that SM1 cigarette had very high radical, oxygen and nitrogen content and therefore considerably more toxic than ES1 cigarette. Tobacco char was largely carbon (∼66–69%) – Energy dispersive X-ray (EDX) data hence the major reason carbon based radicals dominated the oxygen centered radicals at elevated temperatures. Generally, tobacco free radicals are very reactive intermediates and therefore precursors for detrimental ailments associated with cigarette smoking. These findings support the fact that research based policy framework is essential in minimizing the burden associated with tobacco abuse.

A New and Robust Method for In‐situ EPR Electrochemistry

AbstractA new and simple design of electrochemical setup for in‐situ generation of free radicals to be measured using X‐band (9.5 GHz) electron paramagnetic resonance (EPR) spectrometer is described. The cell parts are all from commercially available components and requires no specially made glassware. The EPR performance of the setup is demonstrated by in‐situ electrochemical generation of organic radical cations and radical anions such as quinones, perylene‐diimide, pyrene, flavin, tryptophan and tyrosine through oxidation or reduction in solution. The well‐resolved EPR spectra of the radical products were simulated and analyzed and the hyperfine coupling constants have been assigned for the interactions of the unpaired electron with the various ring protons and the nitrogen nuclei.

Verification of lithium formate monohydrate in 3D-printed container for electron paramagnetic resonance dosimetry in radiotherapy

The nondestructive dosimetry achieved with electron paramagnetic resonance (EPR) dosimetry facilitates repetitive recording by the same dosimeter to increase the reliability of data. In precedent studies, solid paraffin was needed as a binder material to make the lithium formate monohydrate (LFM) EPR dosimeter stable and nonfragile; however, its use complicates dosimetry. This study proposes a newly designed pure LFM EPR dosimeter created by inserting LFM into a 3D-printed container. Dosimetric characteristics of the LFM EPR dosimeter and container, such as reproducibility, linearity, energy dependence, and angular dependence, were evaluated and verified through a radiation therapy planning system (RTPS). The LFM EPR dosimeters were irradiated using a clinical linear accelerator. The EPR spectra of the dosimeters were acquired using a Bruker EMX EPR spectrometer. Through this study, it was confirmed that there is no tendency in the EPR response of the container based on irradiation dose or radiation energy. The results show that the LFM EPR dosimeters have a highly sensitive dose response with good linearity. The energy dependence across each photon and electron energy range seems to be negligible. Based on these results, LFM powder in a 3D-printed container is a suitable option for dosimetry of radiotherapy. Furthermore, the LFM EPR dosimeter has considerable potential for in vivo dosimetry and small-field dosimetry via additional experiments, owing to its small effective volume and highly sensitive dose response compared with a conventional dosimeter.

Nonresonant Transmission Line Probe for Sensitive Interferometric Electron Spin Resonance Detection.

Electron spin resonance (ESR) spectroscopy measures paramagnetic free radicals, or electron spins, in a variety of biological, chemical, and physical systems. Detection of diverse paramagnetic species is important in applications ranging from quantum computation to biomedical research. Countless efforts have been made to improve the sensitivity of ESR detection. However, the improvement comes at the cost of experimental accessibility. Thus, most ESR spectrometers are limited to specific sample geometries and compositions. Here, we present a nonresonant transmission line ESR probe (microstrip geometry) that effectively couples high frequency microwave magnetic field into a wide range of sample geometries and compositions. The nonresonant transmission line probe maintains detection sensitivity while increasing availability to a wider range of applications. The high frequency magnetic field homogeneity is greatly increased by positioning the sample between the microstrip signal line and the ground plane. Sample interfacing occurs via a universal sample holder which is compatible with both solid and liquid samples. The unavoidable loss in sensitivity due to the nonresonant nature of the transmission line probe (low Q) is recuperated by using a highly sensitive microwave interferometer-based detection circuit. The combination of our sensitive interferometer and nonresonant transmission line provides similar sensitivity to a commercially available ESR spectrometer equipped with a high-Q resonator. The nonresonant probe allows for transmission, reflection, or dual-mode detection (transmission and reflection), where the dual-mode results in a √2 signal enhancement.

Management of electron paramagnetic resonance spectrometer in a university core laboratory

Management of electron paramagnetic resonance spectrometer in a university core laboratory

Broadband Tunable Electron Paramagnetic Resonance Spectroscopy of Dilute Metal Complexes.

Analysis of the electron paramagnetic resonance (EPR) of transition ion complexes requires data taken at different microwave frequencies because the spin Hamiltonian contains operators linear in the frequency as well as operators independent of the frequency. In practice, data collection is hampered by the fact that conventional EPR spectrometers have always been designed to operate at a single frequency. Here, a broadband instrument is described and tested that operates from 0.5 to 12 GHz and whose sensitivity approaches that of single-frequency spectrometers. Multifrequency EPR from triclinic substitutional (0.5%) Cu(II) in ZnSO4 is globally analyzed to illustrate a novel approach to reliable determination of the molecular electronic structure of transition ion complexes from field-frequency 2D data sets.