nuclear magnetic resonance

nuclear magnetic resonance (NMR) The absorption of electromagnetic radiation (radio waves) by certain atomic nuclei placed in a strong and stable magnetic field. This results in a change of orientation of the nuclei, which respond to the magnetic field like miniature bar magnets. The main application of NMR is in a form of spectroscopy (NMR spectroscopy) used for chemical and biochemical analysis and structure determination. There are two methods of NMR spectroscopy. In continuous wave (CW) NMR, the sample is subjected to a strong magnetic field, which can be varied in a controlled way. As the field changes, absorption of radiation occurs at certain points; this produces oscillations in the field, which can be detected. Fourier transform (FT) NMR uses a fixed magnetic field and the sample is subjected to a high-intensity pulse of radiation covering a range of frequencies. The signal produced is analysed mathematically to give the NMR spectrum. The ¹H nucleus is the one commonly studied; other biochemically useful nuclei are ³¹P, ¹³C, ¹⁴N, and ¹⁹F, although these have lower natural abundance than hydrogen and produce weaker signals. The spectrum produced is characteristic of the molecule absorbing the radiation. In medicine, magnetic resonance imaging (MRI) has been developed, in which images of soft tissue are produced. This technique is useful for locating tumours and tissue abnormalities.