What is EPR?
Electron paramagnetic resonance (EPR), also known as electron spin resonance (ESR) and electron magnetic resonance (EMR), is the name given to the process of resonant absorption of microwave radiation by paramagnetic ions or molecules, with at least one unpaired electron spin, and in the presence of a static magnetic field. EPR was discovered by Zavoisky in 1944. It has a wide range of applications in chemistry, physics, biology, and medicine: it may be used to probe the "static" structure of solid and liquid systems, and is also very useful in investigating dynamic processes.
The most commonly used EPR spectrometer is in the range of 9-10 GHz (X-band). However, advances in electronics have facilitated the development of spectrometers working at frequencies ranging from several hundred MHz to several hundred GHz.
The IERC currently maintains EPR spectrometers working at 1-2 GHz (L-band) and 2-4 GHz (S-band), 8-10 GHZ (X-Band), 35 Ghz (Q-band) and 95 GHz (W-band).
Multifrequency EPR (1 GHz to 100 GHz) provides an experimental route to study the magnetic interactions in paramagnetic materials. The elucidation of parameters like g, A, D, E, and Q, which characterize these interactions, can lead to an understanding of atomic and molecular structure at magnetic sites. High sensitivity and the ability to investigate small-scale order in powders, polymers, and frozen solutions are key advantages of EPR over a wide frequency range. Additional techniques employing both EPR and NMR methods (ENDOR - electron-nuclear double resonance, and S-band ESE - electron spin echo spectroscopy) that are available at the IERC extend the precision and scope of such investigations. EPR spectroscopy, combined with techniques such as spin trapping, can be used to detect and follow free radical reactions in biological systems.
High-frequency (95 GHz) electron paramagnetic resonance (EPR) instrumentation provides the ability to study very small samples and often provides extraordinary resolution and discrimination between similar species. This is especially true for radical species which have relative small g-shifts and which often give a single unresolved line at X-band frequencies. Radicals that have been studied at W-band include biologically important semiquinone and flavin radicals, as well as tyrosyl and tryptophan radicals. Radical species in solution may show special sensitivity to motion at high frequency. This spectroscopy is particularly valuable for studying both very fast (nanosecond range) and very slow motions, for example with nitroxide probes. It can identify and distinguish various spin-trapping products through the higher g-resolution available at high magnetic fields. It can also detect partitioning of paramagnetic molecules such as nitroxides between different solvents or environments (such as aqueous and lipid) through the environmental g-shifts. Point sensitivity is excellent (in the vicinity of 10^8 spins), even for aqueous samples (better than 2*10^9 spins demonstrated in the current IERC spectrometer), so very small samples can be studied; single cells have been detected with the aid of exogenous spin probes. Zero-field splittings in metal-based systems, e.g., metalloproteins, may be overcome at high microwave frequencies; this potential application is being explored currently. There are plans to extend the high-frequency capabilities of the IERC to include pulsed and ENDOR as well as higher frequencies (to ca. 150 GHz).
Low-frequency EPR spectrometers and special microwave detectors and circuitry provide the capability to obtain spectra from living animals and perfused organs, including the concentration of oxygen in tissues, redox metabolism, distribution of spin-labeled molecules, and biophysical measurement. Special materials developed and being improved at the IERC provide sensitive EPR probes of both oxygen and nitric oxide (NO) in tissues.
Weil and Bolton, Electron Spin Resonance; Elmentary Theory and Pratical Applications, 2nd Edition(2007).
Abragam and Bleaney, Electron Paramagnetic Resonance of Transition Ions (1970).
Pilbrow, Transition Ion Electron Paramagnetic Resonance (1990).