Date of Award
Chemistry and Biochemistry
Sandra S. Eaton, Ph.D.
Arbitrary waveform generator, EPR, Electron paramagnetic resonance, Imaging, Pulse shaping, Rapid scan
EPR is a powerful biophysical tool that can be used to measure tumor physiology. With the addition of magnetic field gradients, the spectral properties of paramagnetic species can be mapped. To facilitate EPR imaging, methods and instrumentation at frequencies between 250 MHz and 1 GHz were developed.
At low spin concentrations, the rapid scan background signal is often many times larger than the EPR signal of interest. To help remove the background contribution, a data acquisition procedure that takes advantage of a cross-loop resonator and bipolar power supplies was developed at 250 MHz. In this procedure, two scans are collected. Relative to the first scan, in the second scan the magnetic field (B0) is reversed and the phase of the rapid scan field is offset by 180°. This results in an inversion of the EPR signal and no net change to the background. The difference between the scans is calculated to cancel the background and enhance the EPR signal by the square root of 2. The procedure was also applied to data at 700 MHz and 980 MHz.
A table-top arbitrary waveform generator (AWG) based rapid scan and pulse spectrometer was designed to operate at frequencies between 700 MHz and 1 GHz using both cross-loop and reflection resonators. The frequency range was selected to provide adequate signal to noise and an imaging penetration depth appropriate for imaging mice. Characterization of the spectrometer including the noise figure, gain, magnetic field homogeneity, source noise, resonators, and gradient fields is reported. To demonstrate the imaging capabilities of the instrument, rapid scan images were collected of nitroxide and trityl radicals in vitro up to 4 dimensions.
New methods were tested that use rapid scan, frequency steps, and field jumps to measure electron spin lattice relaxation (T1) at 1 GHz. Overall good agreement of the relaxation times was observed between the new methods and conventional techniques. However, the uncertainty associated with the rapid scan method is greater due to the low number of points that define the recovery curve. In the frequency stepped method, the resonator bandwidth limits samples to ones with narrow lines. Preliminary results of the field jump method are presented.
Finally, excitation bandwidth and power requirements of a new exponential sine shaped pulse produced with an AWG are compared to conventional rectangular pulses at 1.5 GHz. For the same amount of power, a higher resonator Q can be utilized with the exponential sine pulse yielding higher sensitivity and an increased excitation uniformity.
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Buchanan, Laura A., "Development of Low Frequency Electron Paramagnetic Resonance Methods and Instrumentation for Biological Applications" (2018). Electronic Theses and Dissertations. 1535.
Received from ProQuest
Laura A. Buchanan