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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Electron paramagnetic resonance | 5/7 | https://en.wikipedia.org/wiki/Electron_paramagnetic_resonance | reference | science, encyclopedia | 2026-05-05T10:04:27.171248+00:00 | kb-cron |
=== Medical and biological === Medical and biological applications of EPR also exist. Although radicals are very reactive, so they do not normally occur in high concentrations in biology, special reagents have been developed to attach "spin labels", also called "spin probes", to molecules of interest. Specially-designed nonreactive radical molecules can attach to specific sites in a biological cell, and EPR spectra then give information on the environment of the spin labels. Spin-labeled fatty acids have been extensively used to study dynamic organisation of lipids in biological membranes, lipid-protein interactions and temperature of transition of gel to liquid crystalline phases. Injection of spin-labeled molecules allows for electron resonance imaging of living organisms. A type of 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−(OH))=C(CH3)NH+2 radical). This method is suitable for measuring gamma and X-rays, electrons, protons, and high-linear energy transfer (LET) radiation of doses in the 1 Gy to 100 kGy range. EPR can be used to measure microviscosity and micropolarity within drug delivery systems as well as the characterization of colloidal drug carriers. 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.
=== Material characterization ===
EPR/ESR spectroscopy is used in geology and archaeology as a dating tool. It can be applied to a wide range of materials such as organic shales, carbonates, sulfates, phosphates, silica or other silicates. When applied to shales, the EPR data correlates to the maturity of the kerogen in the shale.
EPR spectroscopy has been used to measure properties of crude oil, such as determination of asphaltene and vanadium content. The free-radical component of the EPR signal is proportional to the amount of asphaltene in the oil regardless of any solvents, or precipitants that may be present in that oil. When the oil is subject to a precipitant such as hexane, heptane, pyridine however, then much of the asphaltene can be subsequently extracted from the oil by gravimetric techniques. The EPR measurement of that extract will then be function of the polarity of the precipitant that was used. Consequently, it is preferable to apply the EPR measurement directly to the crude. In the case that the measurement is made upstream of a separator (oil production), then it may also be necessary determine the oil fraction within the crude (e.g., if a certain crude contains 80% oil and 20% water, then the EPR signature will be 80% of the signature of downstream of the separator).
EPR 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. Similarly, material extracted from the teeth of people during dental procedures can be used to quantify their cumulative exposure to ionizing radiation. People (and other mammals) exposed to radiation from the atomic bombs, from the Chernobyl disaster, and from the Fukushima accident have been examined by this method.
Radiation-sterilized foods have been examined with EPR spectroscopy, aiming to develop methods to determine whether a food sample has been irradiated and to what dose.
Relationship to Dynamic Nuclear Polarization (DNP)
Dynamic Nuclear Polarization (DNP) is a form of signal enhancement in NMR spectroscopy in which microwaves also irradiate the system, transferring polarization of radical species within samples to target nuclei. Developing methods that use radicals that have been optically polarized typically use a combination of transient and pulsed EPR in order to characterize the magnitude of their polarization.
Transient EPR
Transient EPR (also known as time-resolved EPR or trEPR) is a method of EPR spectroscopy used to study photoinduced paramagnetic species in real time in a time-resolved manner with nanosecond to microsecond resolution. Unlike continuous wave EPR, in transient EPR , molecules are placed into an excited state with the use of a laser while data is simultaneously collected on a nanosecond timescale. In a standard 1D transient EPR time trace experiment, the magnetic field strength B0 and microwave frequency are kept constant. In order to build an EPR time-trace profile sufficient data must be collected so the magnetic field parameter can be changed. This is repeated until all resonances have been recorded. The primary data set recorded is two dimensional: the transient EPR signal amplitude with respect to time and magnetic field. Transient EPR is used to characterize the polarization of developing optically pumped polarizing agents that have potential applications in DNP.