What is EPR?

Electron spin resonance (EPR) is a spectroscopic technique based on the interactions of unpaired electron spins in a sample with microwaves in a magnetic field. It has widespread use in chemistry, biochemistry and physics applications.

Electrons possess a property called “spin”, resulting in an angular momentum. Because the electron is charged, there is associated with the angular momentum a magnetic moment which points in the opposite direction to the angular momentum vector. In an external magnetic field, the spin precesses around the field direction at the Larmor frequency and thus a component of the magnetic moment is either parallel or anti-parallel to the field direction. If a microwave field of this frequency is applied to a spin containing sample, then the spins can change their direction relative to the magnetic field. This results in absorption of the microwave field, which may be measured.

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The energy of the transition will be affected by the local environment of the spins. Thus, EPR spectra can yield information about the structure and composition of a sample.

A typical continuous wave (CW) EPR spectrometer consists of a source of microwave radiation, a cavity into which the sample is placed (to enhance the size of the microwave field), a detector to measure the microwave signal reflected from the cavity and a magnet to generate the external field. A small modulation field allows the use of phase sensitive detection.

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Typically, the microwave frequency is kept constant while the magnetic field is swept. It is also possible to carry out pulsed EPR. By applying very short pulses (the order of nanoseconds) the bandwidth of the incident microwave field is increased, allowing all, or a large part of, the spectrum to be excited at once. By using Fourier transform or spin echo techniques the spectrum may be obtained or hyperfine or electron dipolar interactions can be measured.