Spectrometer quality factor, resonator

Good X-band resonators mounted into a spectrometer and with a sample inside have approximate quality factors of 103 or more, which means that they afford an EPR signal-to-noise ratio that is over circa three orders of magnitude better than that of a measurement on the same sample without a resonator, in free space. This is, of course, a tremendous improvement in sensitivity, and it allows us to do EPR on biomolecules in the sub-pM to mM range, but the flip side of the coin is that we are stuck with the specific resonance frequency of the resonator, and so we cannot vary the microwave frequency, and therefore we have to vary the external magnetic field strength. 

In a reflection mode spectrometer of high finesse or quality factor Q, the reflected power when matched on resonance is many decibels below the incident power, which reduces the noise floor by many decibels with respect to a transmission mode resonator. When the ESR sample is resonant, the residual resonator mismatch changes, which causes the reflected power to change, and a small signal on a low background is presented to the detector. In order for the signal to be detected, however, it must be discriminated from the radiation incident on the resonator, just as in a conventional reflection mode ESR spectrometer. 

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