A pulsed and continuous wave 250 MHz electron paramagnetic resonance spectrometer

A 250 MHz electron paramagnetic resonance (EPR) spectrometer was constructed to be an engineering test facility for in vivo EPR imaging of physiological samples and for protein structure determination. Innovations relative to prior low-frequency EPR spectrometers include a four-coil, air-core magnet and gradient coils, a crossed-loop resonator, dynamic Q-switching to decrease dead time in pulsed EPR, and a narrow-band bridge based on circulators. The automatic frequency control system uses a signal separate from the EPR signal to make the frequency control independent of the radiofrequency (RF) phase. The design incorporates multiple excitation and signal paths to facilitate testing of a variety of resonators, two magnets, and both a locally built console described here and a Bruker console. Plug-in cards in the bridge facilitate using reflection or crossed-loop resonators in continuous wave or pulsed EPR modes. In the locally built console there is a microprocessor-controlled interface unit to handle magnetic held modulation and scan, tuning display, and other functions. (C) 2002 Wiley Periodicals, Inc.