<Table> <Tr> <Td> </Td> <Td> This article includes a list of references, but its sources remain unclear because it has insufficient inline citations . Please help to improve this article by introducing more precise citations . (November 2016) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article includes a list of references, but its sources remain unclear because it has insufficient inline citations . Please help to improve this article by introducing more precise citations . (November 2016) (Learn how and when to remove this template message) </Td> </Tr> <P> Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei . The sample is placed in a magnetic field and the NMR signal is produced by excitation of the nuclei sample with radio waves into nuclear magnetic resonance, which is detected with sensitive radio receivers . The intramolecular magnetic field around an atom in a molecule changes the resonance frequency, thus giving access to details of the electronic structure of a molecule and its individual functional groups . As the fields are unique or highly characteristic to individual compounds, in modern organic chemistry practice, NMR spectroscopy is the definitive method to identify monomolecular organic compounds . Similarly, biochemists use NMR to identify proteins and other complex molecules . Besides identification, NMR spectroscopy provides detailed information about the structure, dynamics, reaction state, and chemical environment of molecules . The most common types of NMR are proton and carbon - 13 NMR spectroscopy, but it is applicable to any kind of sample that contains nuclei possessing spin . </P> <P> NMR spectra are unique, well - resolved, analytically tractable and often highly predictable for small molecules . Different functional groups are obviously distinguishable, and identical functional groups with differing neighboring substituents still give distinguishable signals . NMR has largely replaced traditional wet chemistry tests such as color reagents or typical chromatography for identification . A disadvantage is that a relatively large amount, 2--50 mg, of a purified substance is required, although it may be recovered through a workup . Preferably, the sample should be dissolved in a solvent, because NMR analysis of solids requires a dedicated magic angle spinning machine and may not give equally well - resolved spectra . The timescale of NMR is relatively long, and thus it is not suitable for observing fast phenomena, producing only an averaged spectrum . Although large amounts of impurities do show on an NMR spectrum, better methods exist for detecting impurities, as NMR is inherently not very sensitive - though at higher frequencies, sensitivity is higher . </P>

What region of the electromagnetic spectrum is used in nmr spectroscopy