Rutile dielectric loop-gap resonator for X-band EPR spectroscopy of small aqueous samples.

• Single crystal rutile can give 6 times the X-band EPR resonator efficiency of an LGR. • A factor of two higher saturable EPR signal for aqueous sample sizes near 400 nL. • Dielectric in the inner loop of an LGR can perform similarly to a DR. • Experimental results are compared with analytic theory and finite element simulations. • Polycrystalline rutile has background signals that make it unsuitable for EPR use. The performance of a metallic microwave resonator that contains a dielectric depends on the separation between metallic and dielectric surfaces, which affects radio frequency currents, evanescent waves, and polarization charges. The problem has previously been discussed for an X-band TE 011 cylindrical cavity resonator that contains an axial dielectric tube (Hyde and Mett, 2017). Here, a short rutile dielectric tube inserted into a loop-gap resonator (LGR) at X-band, which is called a dielectric LGR (dLGR), is considered. The theory is developed and experimental results are presented. It was found that a central sample loop surrounded by four "flux-return" loops (i.e., 5-loop–4-gap) is preferable to a 3-loop–2-gap configuration. For sufficiently small samples (less than 1 µL), a rutile dLGR is preferred relative to an LGR both at constant Λ (B 1 / P l ) and at constant incident power. Introduction of LGR technology to X-band EPR was a significant advance for site-directed spin labeling because of small sample size and high Λ. The rutile dLGR introduced in this work offers further extension to samples that can be as small as 50 nL when using typical EPR acquisition times. [ABSTRACT FROM AUTHOR]