OXIMETRY BY DYNAMIC NUCLEAR-POLARIZATION

This study describes quantitatively the effect of O-2 on the dynamic nuclear polarization (DNP) of water protons. It is shown theoretically how DNP enhancement is altered by the paramagnetism of O-2. The nonlinear effect of oxygen on the enhancement factor has been approximated by a first order expression that provides accurate predictions of the experimental results obtained with phosphate-buffered solutions of 2,2,6,6 tetramethyl-4-oxo-piperidine-N-oxyl. The proton relaxation rates and DNP line width were measured for nitroxide concentrations ranging from 0.5 to 10 mM and for oxygen partial pressures ranging from 0 to 760 mmHg. A method is suggested for the calculation of the nitroxide and oxygen concentrations of a sample, based on two measurements, one with on-resonance EPR irradiation, the other with slightly off-resonance irradiation. Although the correlation between the values for the actual and calculated oxygen concentrations is good, the accuracy of the calculated oxygen concentration values remains poor, because of the large number of parameters involved in this calculation.