Low frequency epr surface probe based on dielectric resonator

A new surface probe for low frequency in vivo EPR studies was designed and constructed based on a dielectric resonator. Dielectric resonators previously have been used in X-band EPR for small samples to take advantage of its well separated microwave magnetic and electric field distributions and high Q. In this work we have built a dielectric resonator for use as a surface probe for in vivo EPR at low frequency (1.2 GHz). The core of the probe was a commercially available dielectric ceramic resonator supported by a PTFE ring. The resonator was coupled to a 50 Ω line by an inductive loop which was interfaced by a quarter wave length transformer. The quarter wave length transformer improved the matching and stabilized the resonant frequency. The resonator was surrounded by a silver coated brass shielding. The shielding blocked the penetration of the field modulation and therefore there was no contribution from any intrinsic EPR signal from the dielectric material. The Q factor before loading samples was 3,600 which was much higher than the Q factors of ～ 1000 obtained by other types of surface probes. The use of animals or biological samples decreases the Q factor to about 330 but this is at least two times higher than occurs with other types of surface probes. The sensitivity of the dielectric surface probe in air decreased more slowly than with other types of surface probes. This would be especially useful for in vivo studies in which the animal has an irregular surface at the site(s) of measurement and/or there is a significant air gap between the probe and the animal. This dielectric resonator can be inexpensively constructed and is robust, and sensitive for in vivo EPR spectroscopy and imaging.