Electron paramagnetic resonance spectroscopy is sometimes called electron spin resonance, and usually spoken of by the acronym EPR.

In EPR, one measures the frequency at which various molecules absorb photons at or near the microwave frequency range while in a magnetic field. The resultant spectrum is due to the absorbance of energy coupled to a change in the alignment of an electron spin from parallel to antiparallel to said external magnetic field. This ‘resonant frequency’ varies linearly with the strength of the applied magnetic field (a property known as the Zeeman effect.) While you might think that as there are a lot of electrons in most substances, these spectra must be pretty complicated, in one sense this is not true: only unpaired electrons will absorb these photons (paired electrons will merely swap spins, and no energy gets absorbed.) Thus only organic free radicals and transition metals will give an EPR signal.

In practice, a typical EPR experiment is pretty simple: One merely places a solution of the molecule of interest in a resonant cavity (a sample compartment with a length calculated to maximize the photon density at the sample), shoots microwave radiation through the sample, and measures the fraction of energy absorbed using a microwave detector. The microwave energy is most often generated at about 9.5 GHz (a wavelength of about 32mm) by a vacuum tube known as a klystron. The applied magnetic field is varied between 0 and 5-6 Tesla and absorbance is observed when the Zeeman effect brings the resonant frequency of the sample into the emission frequency of the klystron. Why do it this way? Because it is relatively cheap to make a variable strength electromagnet but very expensive to make a variable frequency microwave generator. Klystrons have been relatively cheap since WWII as they were used extensively in early radar installations.

EPR is principally used in three areas:

Chemistry: Synthetic chemists, especially the inorganic variety, use EPR to characterize the molecules they have created.

Biophysics: Many biomolecules contain either organic free radicals (mostly flavins and quinones) or transition metals (see metalloproteins). EPR isn’t currently used much in biomedicine for obvious reasons (think about what happens in your microwave!)

Materials Science: Metallurgists use EPR to look for small cracks or strains in metals (often called ‘point defects’.) This is how they examine wear and tear in things like airplane wings. The same methods are used to examine semiconductors intended for use in things like microchips.