Paramagnetic centres can be found naturally anywhere. Some examples of paramagnetic centres are: free radicals, metal centres in proteins, and in organometallics, defects in materials, paramagnetic centres in semiconductors and electrons in metals.
If not naturally present, there are some possibilities of introducing a paramagnetic centre on purpose. Here is a list of systems normally studied by EPR after introduction of a paramagnetic centres in the systems.
Condensed phases: liquid, crystalline or liquid crystalline (dissolution of stable radicals or its generation by chemical or electrochemical methods)
Protein (spin labelling, metal exchange)
Membranes (spin labelling)
Materials (use of ionizing radiations: ions/electrons/gamma, X and UV-rays , inclusion or exchange of paramagnetic molecules, chemical reduction or oxidation)
Photoexcited molecules in solution and in solid phase(UV irradiation)
Transient radicals produced by irradiation(radical production by UV irradiation