Uniform Field Resonators for EPR Spectroscopy: A Review

Abstract

Cavity resonators are often used for electron paramagnetic resonance (EPR). Rectangular TE and cylindrical TE are common modes at X-band even though the field varies cosinusoidally along the Z-axis. The authors found a way to create a uniform field (UF) in these modes. A length of waveguide at cut-off was introduced for the sample region, and tailored end sections were developed that supported the microwave resonant mode. This work is reviewed here. The radio frequency (RF) magnetic field in loop-gap resonators (LGR) at X-band is uniform along the Z-axis of the sample, which is a benefit of LGR technology. The LGR is a preferred structure for EPR of small samples. At Q-band and W-band, the LGR often exhibits nonuniformity along the Z-axis. Methods to trim out this nonuniformity, which are closely related to the methods used for UF cavity resonators, are reviewed. In addition, two transmission lines that are new to EPR, dielectric tube waveguide and circular ridge waveguide, were recently used in UF cavity designs that are reviewed. A further benefit of UF resonators is that cuvettes for aqueous samples can be optimum in cross section along the full sample axis, which improves quantification in EPR spectroscopy of biological samples.

An analysis of cellulose‐ and dextrose‐based radicals in sweet potatoes as irradiation markers

“…EPR spectroscopies that operate at X‐band has a wavelength around 3 cm (Hagen, 2013). Hence, the samples extended in EPR tubes are exposed to a variety of microwave radio frequency magnetic fields (e.g., change in intensity and wave distribution) (Hyde, Sidabras, & Mett, 2019). The magnetic field intensity follows a cosine function with respect to sample axis (Hyde et al., 2019).…”

“…Hence, the samples extended in EPR tubes are exposed to a variety of microwave radio frequency magnetic fields (e.g., change in intensity and wave distribution) (Hyde, Sidabras, & Mett, 2019). The magnetic field intensity follows a cosine function with respect to sample axis (Hyde et al., 2019). The variation enhances the Gaussian effect in the spectra, which causes loss in the signal intensity (Hyde et al., 2019).…”

“…The magnetic field intensity follows a cosine function with respect to sample axis (Hyde et al., 2019). The variation enhances the Gaussian effect in the spectra, which causes loss in the signal intensity (Hyde et al., 2019). Therefore, more compact samples (close to the center of cavity) are desirable in EPR measurements to maximize the magnetic field strength and have a uniform magnetic field wave distribution without losing signal (Hagen, 2013).…”

Microwaves in Chemistry

“…New developments in microwave spectroscopy have been reviewed, including in dielectric measurements using cavity perturbation applied to the real-time measurement of ammonia storage in halide salts [71] and in metal-organic frameworks (MOFs) [72] for hydrogen storage applications. In microwave magnetic spectroscopy we have highlighted developments in electron paramagnetic resonance, with new methods using loop-gap resonators and dual mode cavities for simultaneous microwave heating [3], [80].…”

“…High field, high frequency EPR therefore requires low loss dielectric resonators since the Q factor of metal cavities becomes compromised on miniaturization owing to their reduced volume to surface area ratios. Other subtleties linked to the design of EPR cavities are to ensure microwave field uniformity over the sample volume and to allow the effective exposure to the sample of the modulation field b 1 (given that the skin depth in copper at 100 kHz is only about 0.2 mm), which has led to the development of lumped-element type resonators such as the split ring (or loop gap) type [80]. Detuning of EPR cavities, with a reduction in Q, is an important factor in the measurement of aqueous samples where the sample can encroach upon the region of finite electric field, leading to large dielectric loss; field separation of electric and magnetic fields is generally better in loop gap resonators compared with distributed (cavity-type) resonators.…”