Date of Award
Chemistry and Biochemistry
Gareth R. Eaton
Sandra S. Eaton
CW, EPR, rapid-scan, spin-trapping, X-band
The advantages of rapid-scan EPR relative to CW and pulse techniques for samples with long longitudinal relaxation time T1 (Ns0 defects in diamond, N@C60, and amorphous hydrogenated silicon), heterogeneous samples (crystalline 1:1 á,ã-bisdiphenylene-â-phenylallyl (BDPA):benzene), lossy samples (aqueous nitroxyl radicals), and transient radicals (5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO)-superoxide adduct) were studied.
For samples with long relaxation times, CW (continuous wave) EPR is challenging due to power saturation and distortions from passage effects. In rapid-scan EPR, the field is swept through resonance in a time that is short relative to T2. In rapid-scan EPR, the magnetic field is on resonance for a short time relative to CW EPR. Because of this, the energy absorbed by the spins, for the same microwave B1, is less than in conventional CW spectra, and the signal does not saturate as readily. For samples with long electron relaxation times, pulse techniques can also be challenging, particularly if T2 is long and T2* is short. Rapid-scan EPR is a powerful alternative to CW and pulse EPR because it is a straight-forward technique that does not require the high power of pulse EPR. For the samples studied, improvements in signal-to-noise ranging from factors of 10 to 250 were observed.
Rapid-scan can also be used to extract relaxation information from a sample. The rapid-scan spectra for lithium phthalocyanine (LiPc) and 15N-PDT (4-oxo-2,2,6,6-tetra-perdeuteromethyl-piperidinyl-15N-oxyl-d16) were simulated to determine T2. The extraction of T2 from the rapid-scan spectra of BDPA was also attempted. Through our difficulty in simulating the rapid-scan spectra of BDPA, we realized that commercial BDPA was not a homogeneous sample. The experiments studying BDPA demonstrated that rapid-scan experiments can give insight into the relaxation of a sample that might not otherwise be evident with conventional CW EPR.
Finally, rapid-scan EPR at X-band was applied to spin trapping experiments. Superoxide was generated by the reaction of xanthine oxidase and hypoxanthine and trapped with BMPO. Spin trapping with 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO) to form BMPO-OOH adduct converts the short-lived superoxide into a more stable spin adduct. The detection limit for spin-trapped superoxide was compared between CW and rapid-scan EPR. The signal-to-noise ratio was more than 40 times greater for rapid-scan than for CW EPR. We also demonstrated detection of superoxide produced by Enterococcus faecalis at rates that are too low for detection by CW EPR.
Mitchell, Deborah Gale, "X-band Rapid-scan EPR" (2013). Electronic Theses and Dissertations. 436.
Recieved from ProQuest
Deborah Gale Mitchell
Chemistry, Physical chemistry, Biochemistry