Microelectrode-detected ESR (MEDESR) spectroscopy is a technique that is employed to observe magnetic resonance on the basis of an impedance change of electrodes. The characteristics of MEDESR for nitroxide radicals in electrolyte solutions were analyzed. MEDESR signals were obtained at zero bias-current, suggesting that modulation of the bias-current derived from ESR would not generate MEDESR signals. A good linear relationship between the MEDESR signal intensity and the applied power over a range of up to 150 mW was obtained. The frequency characteristics of the magnetic field modulation indicated that a simple thermal phenomenon would not cause an impedance change in MEDESR. When the radical was distributed on only the electrode surface, no signals were detected. The MEDESR spectra of a nitroxide solution, located outside the electrolytic cell, was detectable. These findings suggest that since MEDESR signals are irrelevant to the radicals in an electrical double layer, they are derived from some changes in the microwaves in the whole resonator. It is speculated, however, that the detecting mechanism of MEDESR is not a simple process of rectifying detection of a microwave change by resonant absorption like in conventional ESR, because the signal intensity of MEDESR is proportional to the irradiation power, and not to the square root as in the conventional ESR.
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