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==Applications==
EPR/ESR spectroscopy is used in various branches of science, such as [[biology]], [[chemistry]] and [[physics]], for the detection and identification of [[radical (chemistry)|free radical]]s and paramagnetic centers such as [[F-center|F centers]]. EPR is a sensitive, specific method for studying both radicals formed in chemical reactions and the reactions themselves. For example, when ice (solid H<sub>2</sub>O) is decomposed by exposure to high-energy radiation, radicals such as H, OH, and HO<sub>2</sub> are produced. Such radicals can be identified and studied by EPR. Organic and inorganic radicals can be detected in electrochemical systems and in materials exposed to [[UV]] light. In many cases, the reactions to make the radicals and the subsequent reactions of the radicals are of interest, while in other cases EPR is used to provide information on a radical's geometry and the orbital of the unpaired electron.
Miniature Electron Spin Resonance Spectroscopy with Micro-ESR
Miniaturisation of military radar technologies allowed the development of miniature microwave electronics as a spin off by Caltech University. Since 2007 these sensors have been employed in miniaturized electron spin resonance spectrometers called Micro-ESR.
The high cost, large size, and difficult maintenance of electron spin resonance spectrometers has limited their use to specialized research centers with highly trained personnel. Micro-ESR makes ESR feasible for nonspecialists to determine oxidation by directly measuring of free radicals.
Applications include real-time monitoring of free radical containing Asphaltenes in (crude) oils; Biomedical R&D to measure oxidative stress; Evaluation of the shelf-life of food products;
[[medicine|Medical]] and [[biology|biological]] applications of EPR also exist. Although radicals are very reactive, and so do not normally occur in high concentrations in biology, special reagents have been developed to spin-label molecules of interest. These reagents are particularly useful in biological systems. Specially-designed nonreactive radical molecules can attach to specific sites in a [[cell (biology)|biological cell]], and EPR spectra can then give information on the environment of these so-called [[spin label|spin-label]] or [[spin probe|spin-probes]].
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A type of [[dosimetry|dosimetry system]] has been designed for reference standards and routine use in medicine, based on EPR signals of radicals from irradiated polycrystalline α-[[alanine]](the alanine deamination radical, the hydrogen abstraction radical, and the (CO<sup>-</sup>(OH))=C(CH<sub>3</sub>)NH<sub>2</sub><sup>+</sup> radical) . This method is suitable for measuring [[gamma ray|gamma]] and [[x-ray]]s, electrons, protons, and high-[[linear energy transfer]] (LET) radiation of [[absorbed dose|doses]] in the 1 [[Gray (unit)|Gy]] to 100 kGy range.<ref>{{cite journal|title = Dosimetry Systems|journal = Journal of the ICRU|year = 2008|volume = 8|issue = 5|doi = 10.1093/jicru/ndn027|pages = 29}}</ref>
EPR/ESR spectroscopy can be applied only to systems in which the balance between radical decay and radical formation keeps the free-radicals concentration above the detection limit of the spectrometer used. This can be a particularly severe problem in studying reactions in liquids. An alternative approach is to slow down reactions by studying samples held at [[cryogenic]] temperatures, such as 77 K ([[liquid nitrogen]]) or 4.2 K (liquid helium). An example of this work is the study of radical reactions in single crystals of amino acids exposed to x-rays, work that sometimes leads to [[activation energy|activation energies]] and rate constants for radical reactions.
The study of radiation-induced free radicals in biological substances (for cancer research) poses the additional problem that tissue contains water, and water (due to its [[electric dipole moment]]) has a strong absorption band in the [[microwave]] region used in EPR spectrometers.
EPR/ESR also has been used by archaeologists for the dating of teeth. Radiation damage over long periods of time creates free radicals in tooth enamel, which can then be examined by EPR and, after proper calibration, dated. Alternatively, material extracted from the teeth of people during dental procedures can be used to quantify their cumulative exposure to ionizing radiation. People exposed to radiation from the [[Chernobyl disaster]] have been examined by this method.<ref>{{cite journal|author = Gualtieri, G.|coauthors = Colacicchia, S, Sgattonic, R., Giannonic, M.|title = The Chernobyl Accident: EPR Dosimetry on Dental Enamel of Children|journal = Applied Radiation and Isotopes|year = 2001|volume = 55|issue = 1|pages = 71–79|doi = 10.1016/S0969-8043(00)00351-1|pmid = 11339534}}</ref><ref>{{cite journal|author = Chumak, V.|coauthors = Sholom, S.; Pasalskaya, L.|title = Application of High Precision EPR Dosimetry with Teeth for Reconstruction of Doses to Chernobyl Populations|journal = Radiation Protection Dosimetry|year = 1999|volume = 84|issue =|pages = 515–520|url = http://rpd.oxfordjournals.org/cgi/content/abstract/84/1-4/515|doi =}}</ref>
Radiation-sterilized foods have been examined with EPR spectroscopy, the aim being to develop methods to determine if a particular food sample has been irradiated and to what dose.
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Because of its high sensitivity, EPR was used recently to measure the quantity of energy used locally during a mechanochemical milling process.<ref>{{cite journal|author=Baron, M., Chamayou, A., Marchioro, L., Raffi, J.|year=2005|title=Radicalar probes to measure the action of energy on granular materials|journal=Adv. Powder Technol|volume=16|issue=3|pages=199–212|doi=10.1163/1568552053750242}}</ref>
EPR/ESR spectroscopy has been used to measure properties of [[crude oil]], in particular [[asphaltene]] and [[vanadium]] content. EPR measurement of [[asphaltene]] content is a function of spin density and solvent polarity. Prior work dating to the 1960s has demonstrated the ability to measure [[vanadium]] content to sub-ppm levels.
In the field of [[quantum computer|quantum computing]], [[pulsed EPR]] is used to control the state of electron spin [[qubit|qubits]] in materials such as diamond, silicon and galium arsenide.
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