Comparison of 10 mm and 30 mm Diameter Surface Coil Resonators for 1 GHz EPR

Two surface coil resonators were tested at 1.0 GHz as alternatives to volume resonators that fully contain the sample; a 10 mm diameter coil with Q of 61 and efficiency of 0.044 mT/√W at ~2.5 mm above the coil and a 30 mm diameter coil with Q of 56 and efficiency of 0.0038 mT/√W at ~5 mm above the coil. The 10 mm diameter coil uses power more efficiently, but the 30 mm diameter surface coil detects signal from a larger volume and the B₁ extends further into the sample. The signal intensity as a function of distance from the coil was measured for small samples of lithium phthalocyanine, a 1 mm thick plate of aqueous nitroxide solution, and a 7 mm internal diameter tube containing aqueous nitroxide solution. For a sample localized at a defined distance from the coil the signal intensity can be increased by increasing incident power to compensate for the decreases in B₁ with distance from the coil. However, when the noise is dominated by the source, increasing power increases noise. If incident power is adjusted to compensate for the decrease in B₁ with increasing distance from the coil the S/N for aqueous nitroxide in a 1 mm thick plate, was better for the 10 mm coil than for the 30 mm coil up to about a distance of about 6 mm but that advantage is lost at greater distances from the coil. Samples of nitroxide were used as a phantom to demonstrate 3D spatial imaging with the 30 mm coil.