Electron spin resonance (ESR) or electron paramagnetic resonance (EPR) spectroscopy is a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion. The basic physical concepts of ESR are analogous to those of nuclear magnetic resonance (NMR), but electron spins are excited instead of nuclear spins. Since ESR cannot be performed on the majority of stable molecules which have all of their electrons paired, it is arguably less widely used than NMR. However, this limitation to paramagnetic species also means that the ESR technique is one of great specificity, since ordinary chemical solvents and matrices do not give rise to ESR spectra.
Continuous wave (CW) ESR spectra are recorded by putting a sample into a microwave (MW) irradiation field of constant frequency and sweeping the external magnetic field B0 until the resonance condition is fulfilled. In the experimental set-up, the MW field is built up in a resonator (typically a rectangular cavity), into which the sample tube is introduced. The recording of reflected MW power as a function of the magnetic field yields the CW ESR spectrum. Amplitude modulation of the magnetic field with a frequency of typically 100 kHz increases the signal-to-noise ratio (S/N) considerably and is responsible for the derivative shape of the spectra.
Bruker EPR theory and practice : http://www.bruker-biospin.com/cw.html