EPR, Electron Paramagnetic Resonance, is a spectroscopic technique only for species which have an unpaired number of electrons: ParaMagnetic compounds.
Sometime, the name for this analyze is also ESR: Electron Spin Resonance.
These include many transitions metal ions, free radicals, etc.
As a result EPR crossed several skills: chemistry, physics, biology, materials science, medical science and others.
What kind of information gives an EPR spectroscopy?
Only EPR spectroscopy is able to detects unpaired electrons and the presence of free radicals. Moreover, EPR has the unique power to identify the detected ParaMagnetic species. The sample we want analyze can be solid as a powder or liquide.
This magnetic resonance technique is very similar to the NMR Spectroscopy, used for species which have a pair number of electrons: DiaMagnetic compounds.
But, EPR doesn’t measure the nuclear transition in the sample as in NMR but detects the transition of unpaired electrons in an applied magnetic field. Effectively, when we submit an external magnetic field B, it creates two distincts energy levels for the unpaired electrons : the α level: Ms= +1/2 and the β level: Ms= -1/2, allowing to measure them as they are driven between the two levels. (look at the schema above)
In order for the transition to occur, the lower β and upper α states have to be exactly matched by the microwave frequency.