ABSTRACT: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 field modulation and scan, tuning display, and other functions.
In rapid scan EPR the magnetic field is scanned through the signal in a time that is short relative to electron spin relaxation times. Previously it was shown that the slow scan lineshape could be recovered from triangular rapid scans by Fourier deconvolution. In this paper a general Fourier deconvolution method is described and demonstrated to recover the slow scan lineshape from sinusoidal rapid scans. Since an analytical expression for the Fourier transform of the driving function for a sinusoidal scan was not readily apparent, a numerical method was developed to do the deconvolution. The slow scan EPR lineshapes recovered from rapid triangular and sinusoidal scans are in excellent agreement for lithium phthalocyanine, a trityl radical, and the nitroxyl radical, tempone. The availability of a method to deconvolute sinusoidal rapid scans makes it possible to scan faster than is feasible for triangular scans because of hardware limitations on triangular scans.
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