EPR Spectra and Electron Spin Relaxation of O.

It has been known from the pioneering and approximately simultaneous work of Freed and Frankel, Hausser and Deguchi (1959 -1960) that collisions with O in fluid solutions broaden the electron paramagnetic resonance (EPR) spectra of free radicals. Freed showed Hausser the effect of degassing solutions on the hyperfine of DPPH. Hausser and Deguchi published spectra illustrating the effect. The effect of O on CW EPR spectra and on relaxation times have been applied to the problem of measuring O concentration in vivo. Various aspects of the effect of O on biological EPR and on the biological systems have been studied. A large literature has developed about in vivo O measurements using nitroxyl radicals, triarylmethyl radicals (trityl, TAM), carbon particles, and analogues of lithium phthalocyanine (LiPc). All of these measurements correctly assume that the O electron spin relaxation time is short relative to the relaxation time of the sensor molecules. There has been no prior review of the EPR spectra and electron spin relaxation of O in the gas phase, dissolved in fluid solution, and in frozen solutions. A summary of the relevant literature is presented here as a resource for the O measurement community.