We report the development of a simple ferroelectric cavity insert that increases the electron paramagnetic resonance (EPR) sensitivity by an order of magnitude when a sample is placed within it. The insert is a hollow cylinder (length 4.8 mm, outside diameter 1.7 mm, inside diameter 0.6 mm) made from a single crystal of KTaO3, which has a dielectric constant of 230 at X-band (9.5 GHz). Its outside dimensions were chosen to produce a resonant frequency in the X-band range, based on electromagnetic field modeling calculations. The insert increases the microwave magnetic field (H1) at the center of the insert by a factor of 7.4 when placed in an X-band TM110 cavity. This increases the EPR signal for a small (volume 0:13 μL) unsaturated nitroxide spin label sample by a factor of 64 at constant microwave power, and by a factor of 9.8 at constant H1. The insert does not significantly affect the cavity quality factor Q, indicating that this device simply redistributes the microwave fields within the cavity, focusing H1 onto the sample inside the insert, thus increasing the filling factor. A similar signal enhancement is obtained in the TM110 and TE102 cavities, and when the insert is oriented either vertically (parallel to the microwave field) or horizontally (parallel to the DC magnetic field) in the TM110 cavity. This order-of-magnitude sensitivity enhancement allows EPR spectroscopy to be performed in conventional high-Q cavities on small EPR samples previously only measurable in loop-gap or dielectric resonators. This is of particular importance for small samples of spin-labeled biomolecules.
Bibliographical noteFunding Information:
This work was supported by grants to D.D.T. from NIH (AR32961, GM27906), the Muscular Dystrophy Association, and the Minnesota Supercomputing Institute. We thank Dr. L. A. Boatner of Oak Ridge National Laboratory, TN, for the gift of the KTaO3 crystals, Dr. A. Gopinath and Mr. D. Olson for assistance in measuring cavity Q, Ms. L. LaConte for a sample of spin-labeled muscle fibers, and Ms R. Bennett for helpful discussions.