What is EPR ?
EPR stands for Electron Paramagnetic Resonance and is the older sister of nuclear magnetic resonance (NMR).
It is a spectroscopic method employing magnetic fields and microwave in the frequency range between 1 GHz and 500 GHz to study materials and molecules with unpaired electrons (radicals, transition metal complexes, etc.). It is based on the fact that electrons behave like small magnets and can be turned and twisted in a magnetic field by microwaves.
EPR is successful in obtaining structural information of molecules and details of electron density distributions. Quantitative analytical applications include oximetry and dosimetry.
Since its first discovery in 1944 by the Russian scientist E. K. Zavoisky, EPR methods have constantly increased in number. They are divided in two big categories: continuous-wave (cw) and pulse EPR.
In cw EPR, molecules are irradiated continuously by low-power microwaves. In pulse EPR, microwave is applied in very short and high-power pulses. Methods where in addition radiofrequencies in the range up to 150 MHz are applied for exciting nuclear spins are called ENDOR (electron nuclear double resonance), with both cw and pulse ENDOR experiments being possible.
EPR and its derived methods can be applied both to solutions and solid samples (crystals, frozen solutions, etc.), whereas nowadays solid-state applications dominate.
For a more extended introduction see the pages Basic Concepts of EPR.