What is EPR?

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Electron Paramagnetic Resonance (EPR) What is EPR? EPR is a spectroscopic technique that detects chemical species that have unpaired electrons. A great number of materials contain such paramagnetic entities, which may occur either as electrons in unfilled conduction bands, electrons trapped in radiation damaged sites, or as free radicals, various transition ions, bi-radicals, triplet states, impurities in semi-conductors, as well as other types. One of the fundamental roles of any spectroscopic technique is identification of the chemical species being studied. In cases where two or more paramagnetic species co-exist, the spectral EPR lines arising from each can be simultaneously observed. Often definitive identification of the individual species is realized solely from the analysis of the EPR spectrum. Furthermore, EPR spectroscopy is capable of providing molecular structural details inaccessible by any other analytical tool. These capabilities of EPR are a result of the unpaired electron’s spin magnetic moment being very sensitive to local magnetic fields within the sample. These fields often arise from the nuclear magnetic moments of various nuclei that may be present within the bulk medium. Examples of such nuclei are interstitial atoms (or ions) within a crystal or glass matrix, nuclei (such as nitrogen) within the molecular structure that also contains the unpaired electron, and so on. Thus EPR provides a unique means of studying the internal structures in great detail. EPR has been successfully applied in such diverse disciplines as biology, physics, geology, chemistry, medical science, material science, anthropology, to name but a few. Solids, liquids and gases are all accessible to EPR. By utilizing a variety of specialized techniques (such as spin-trapping, spin-labeling, ESEEM and ENDOR) in conjunction with EPR, researchers are capable of obtaining detailed information about many topics of scientific interest. For example, chemical kinetics, electron exchange, electrochemical processes, crystalline structure, fundamental quantum theory, catalysis, and polymerization reactions have all been studied with great success. After over 50 years of development and use, EPR remains one of the most sensitive windows into the chemical (i.e., electronic) nature of matter, being able to detect and identify spins with concentrations in the 10^-9 region. In practice, limits of detection are usually somewhat less than this, due to line-width effects. Absolute concentrations of spin species are possible to determine using EPR, but because of the great number experimental parameters that must be taken into account, relative concentrations of species simultaneously present are more easily obtained. How does EPR work? By application of a strong magnetic field B to material containing paramagnetic species, the individual magnetic moment arising via the electron “spin” of the unpaired electron can be oriented either parallel or anti-parallel to the applied field. This creates distinct energy levels for the unpaired electrons, making it possible for net absorption of electromagnetic radiation (in the form of microwaves) to occur. The situation referred to as the resonance condition takes place when the magnetic field and the microwave frequency are “just right” (i.e., the energy of the microwaves corresponds to the energy difference DE of the pair of involved spin states). Energy-level Diagram For Two Spin States as a Function of Applied Field B. This represents the simplest EPR transition (e.g., free electrons). “Allowed” EPR transitions occur when êDMs ê= 1 (Ms is the magnetic spin quantum number of the spin state). The equation describing the absorption (or emission) of microwave energy between two spin states is DE = hu = gbB where: DE is the energy difference between the two spin states h is Planck constant u is the microwave frequency g is the Zeeman splitting factor b is the Bohr magneton B is the applied magnetic field. For further detailed discussion of EPR spectroscopy, refer to standard references listed on this website. 2000-09-27 10:29 AM