Enhanced EPR sensitivity from a ferroelectric cavity insert

We report the development of a simple ferroelectric cavity insert that incr eases the electron paramagnetic resonance (EPR) sensitivity by an order of magnitude when a sample is placed within it. The insert is a hollow cylinde r (length 4.8 mm, outside diameter 1.7 mm, inside diameter 0.6 mm) made fro m a single crystal of KTaO3, which has a dielectric constant of 230 at X-ba nd (9.5 GHz). Its outside dimensions were chosen to produce a resonant freq uency in the X-band range, based on electromagnetic field modeling calculat ions. The insert increases the microwave magnetic field (H-1) at the center of the insert by a factor of 7.4 when placed in an X-band TM110 cavity. Th is increases the EPR signal for a small (volume 0.13 muL) unsaturated nitro xide spin label sample by a factor of 64 at constant microwave power, and b y a factor of 9.8 at constant H-1. The insert does not significantly affect the cavity quality factor Q, indicating that this device simply redistribu tes the microwave fields within the cavity, focusing H-1 onto the sample in side the insert, thus increasing the filling factor. A similar signal enhan cement is obtained in the TM110 and TE102 cavities, and when the insert is oriented either vertically (parallel to the microwave field) or horizontall y (parallel to the DC magnetic field) in the TM110 cavity. This order-of-ma gnitude sensitivity enhancement allows EPR spectroscopy to be performed in conventional high-Q cavities on small EPR samples previously only measurabl e in loop-gap or dielectric resonators. This is of particular importance fo r small samples of spin-labeled biomolecules. (C) 2001 Academic Press.