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A collection of paramagnetic centers, such as free radicals, is exposed to microwaves at a fixed frequency.
By increasing an external magnetic field, the gap between the energy states is widened until it matches the energy of the microwaves, as represented by the double arrow in the diagram above.
At this point the unpaired electrons can move between their two spin states.
Since there typically are more electrons in the lower state, due to the Maxwell–Boltzmann distribution (see below), there is a net absorption of energy, and it is this absorption that is monitored and converted into a spectrum.
As the difference between the two intensities is detected the first derivative of the absorption is detected.
By using phase sensitive detection only signals with the same modulation (100 k Hz) are detected. Note field modulation is unique to continuous wave EPR measurements and spectra resulting from pulsed experiments are presented as absorption profiles.The latter is the most common way to record and publish continuous wave EPR spectra.