Electron Spin Resonance Spectroscopy:

EPR spectroscopy can be carried out by either

(i) varying the magnetic field and holding the frequency constant or

(ii) varying the frequency and holding the magnetic field constant (as is the case for NMR spectroscopy)

When the radiation is absorbed, a spectrum as shown in Figure 36.03 is produced. In EPR, a phase sensitive detector is used, which results in the absorption signal as the first derivative. Therefore, the absorption maximum corresponds to the point where the spectrum passes through zero. This is the point that is used to determine the center of the signal.

Figure 36.03: EPR absorption spectrum and its first derivative.