EPR Spectrometer of Very High Sensitivity

A recording electron paramagnetic resonance absorption spectrometer of very high sensitivity, operating at a wavelength of 3 cm, is described. It is of the reflection-cavity-in-magic-T-bridge type, using a superheterodyne receiver. Its special features are: (1) a two-cavity klystron oscillator with superior frequency stability and more available power; (2) a synchrodyne system to derive local oscillator power, thus avoiding the difficulty of tracking two independent oscillators; (3) a balanced magic-T mixer which considerably reduces the noise output; (4) a double-input low-noise intermediate frequency amplifier at 30 Mc; and (5) a tunable high-Q sample-carrying cavity with an almost identical cavity to balance it in the magic-T bridge. Arrangements for an oscilloscopic display of the absorption vs magnetic field and a continuous recording of the derivative of absorption vs magnetic field have been incorporated. Sensitivity of the microwave bridge system and of the spectrometer have been tested. About 1.1×10−11 mole of diphenyl picryl hydrazyl dissolved in purified benzene, which corresponded to approximately 1.06×10−14 mole of crystalline sample, has been detected by the spectrometer.