Instrument and Application Development in Saturation Recovery and Rapid Scan Electron Paramagnetic Resonance

Enhanced signal sensitivity by the use of Rapid Scan (RS) electron paramagnetic resonance (EPR), a technique that allows for much faster magnetic field scans than traditional field-swept techniques, has facilitated improved data acquisition for many types of samples. For example, irradiated fingernails for radiation dosimetry have been studied using RS-EPR, which resulted in substantial decreases in detection limits. Samarium-mediated reduction mechanisms in organic synthesis have been investigated by RS-EPR providing evidence for a radical intermediate. Spectra of organic radicals exhibiting both narrow lines and closely spaced hyperfine interactions have been recorded via RS-EPR. Well-resolved spectra can be recorded at a rate of 40 spectra/minute to gain insight into molecular changes on this timescale. RS-EPR performed at low temperatures using a closed cycle helium system and a cryostat containing a region with low electrical conductivity provides very wide (\u3e9000 G) spectra free of passage effects near 5 K, expanding the capabilities of RS-EPR. Recent developments in arbitrary wave form generators (AWGs) provide digital waveform synthesis at high enough frequencies to be used in EPR experiments at ca. 9 GHz (X-band). A new saturation recovery (SR) EPR spectrometer has been constructed with an AWG as the microwave source. Circuit design focuses on implementation of an X-band crossed-loop resonator with a reduced quality factor (Q) to minimize dead time due to resonator ring down processes. Increased accuracy of the AWG instrument relative to conventional sources has made nitroxide spin-lattice relaxation time measurements possible via SR-EPR with S/N high enough to permit separation of electron and nuclear spin-lattice relaxation contributions. These results enabled more accurate estimation of the saturation factor in dynamic nuclear polarization (DNP) experiments