Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) is one of the most informative techniques for the electronic structure of paramagnetic substances. EPR spectroscopy is particularly useful for studying (bio)chemical systems with strong local spin densities and their interactions with the environment. For these systems, EPR provides information on structure and dynamics and is widely used in chemistry, physics, and biology.

EPR measurements are usually performed in continuous wave ( cw ) or pulsed (Pulse) mode. The cw EPR spectrometer (see figure 1) applies a magnetic field of about 0.35 T and measures microwave absorption (X-band) in the 9-10 GHz range. Typically, the microwaves are kept at a fixed frequency and sweep the magnetic field (from 0 mT - 700 mT for the X-band ).

Figure 1.EPR test system structure