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