Electron Paramagnetic Resonance is the technique that detects only the species with unpaired electrons (paramagnetic species) such as free radicals, many transition metal ions and defects in material. EPR spectrum of a free radical or coordination complex with a single unpaired electron is the simplest of all the spectroscopy studies.
Free radicals are often short-lived (with few exceptions) , but still play crucial roles in many processes such as photosynthesis, oxidation, catalysis, and polymerization reactions.
Conventional continous wave EPR spectroscopy which is not time resolved technique represent a challenge. In such cases various techniques like in situ steady state analysis and spin trapping becomes handy.