Is depends on the stability and lifetime of the radicals (which are usually very low).

You mat try some techniques:

1. Place a sample of the reaction mixture (while reacting) in a capilar tube and insert it in the cavity of the spectrometer.

2. During the reaction, flow the reaction mixture through a flat cell in the cavity.

3. Take a sample of the reacting mixture and add a spin trap (e.g. DMPO), then place it in a tube in the ESR cavity.