The science of nuclear magnetic resonance (NMR) is at the heart of all Magritek products. Some products, such as the Terranova-MRI Earth’s field system and the Rheo-NMR accessory, are the result of research in specialised areas within the field of NMR. The following information is intended to provide a basic overview of the science which forms the foundation of Magritek technology.
Nuclear Magnetic Resonance (NMR) is a powerful technique that has had a huge impact on a wide range of scientific disciplines. At the heart of all NMR techniques is the interaction between the magnetic moment of an atomic nucleus and an externally applied magnetic field. When a nucleus is in the presence of an external magnetic field, the nuclear magnetic moment of the nucleus assumes a discrete alignment with respect to this field. This is a consequence of the quantisation of the magnetic moment. The application of an oscillating magnetic field at a specific frequency, typically in the radio-frequency range, will excite transitions between the different energy states, i.e. different orientations, of the magnetic moment. The specific frequency for which these transitions take place is called the Larmor frequency and is directly proportional to the strength of the external magnetic field.
The dependence of the Larmor frequency on the strength of the magnetic field makes NMR a means of obtaining direct insight into the molecular state of matter. An antenna (usually a coil) is placed around the sample to irradiate it with radio waves at the Larmor frequency. Nuclei in the sample absorb these radio waves and then re-emit them at the Larmor frequency. This emission, called the NMR signal, can then be detected by the antenna.
Only nuclei with a non-zero magnetic moment can undergo NMR. Such nuclei must have an odd number of protons or neutrons. Some common examples include: 1H, 13C, 15N, 31P, 19F.
Within an ensemble of nuclei, each nucleus may experience different magnetic fields because of, for instance, the magnetic field of other atoms on the molecule or on neighbouring molecules. The frequency components of the NMR signal are proportional to the magnetic field strengths experienced by the individual nuclei and so the NMR signal is sensitive to the molecular environments of the nuclear spins. A Fourier transform of the NMR signal yields a frequency spectrum, a record of NMR frequencies present in the signal. The study of the properties and features of this spectrum is a central concern of the field of NMR spectroscopy.
Alternatively, different nuclei within a sample can experience different magnetic fields because of an applied magnetic field gradient. The application of this field gradient introduces a dependence of the NMR frequency of a given nucleus on its position. Therefore the Fourier transform of the NMR signal spatially resolves the spins, generating an image of the ensemble of spins. This method is used in the field of magnetic resonance imaging (MRI).