Enhanced EPR sensitivity from a ferroelectric cavity insert

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.