Determination of absolute concentration of nitroxide radical by radio-frequency EPR imaging

The absolute concentrations of a nitroxide radical in samples in a loop-gap resonator (LGR) were determined by using a radio-frequency (about 720 MHz) electron paramagnetic resonance (EPR) imaging system. EPR imaging of phant oms containing a nitroxide radical, 3-carbamoyl-2,2,5,5-tetramethylpyrrolid in-1-yloxy (carbamoyl-PROXYL), dissolved in various concentrations of an aq ueous sodium chloride solution was made to investigate the influence of die lectric losses and sample position within the LGR. As it was found that the se influences an the signal intensity were sufficiently small (less than 6% ), it is possible to use identical radical solutions in which the radical i s dissolved in a known concentration as an internal marker. Two phantoms co ntaining aqueous solutions of 3 mM (as a marker) and 1, 2, 3, 4, or 5 mM (a s a sample) carbamoyl-PROXYL were placed together in the LGR. From EPR imag es of these phantoms, the absolute concentration of the sample could be cal culated by using the gray-scale value (i.e., the signal intensity) of the m arker and sample within a small margin of error (about 4%).