<P> All isotopes that contain an odd number of protons and / or neutrons (see Isotope) have an intrinsic nuclear magnetic moment and angular momentum, in other words a nonzero nuclear spin, while all nuclides with even numbers of both have a total spin of zero . The most commonly used nuclei are and, although isotopes of many other elements (e.g., Li,,,,, O,, Na, Si,, Cl, Cd, Xe, Pt) have been studied by high - field NMR spectroscopy as well . </P> <P> A key feature of NMR is that the resonance frequency of a particular simple substance is usually directly proportional to the strength of the applied magnetic field . It is this feature that is exploited in imaging techniques; if a sample is placed in a non-uniform magnetic field then the resonance frequencies of the sample's nuclei depend on where in the field they are located . Since the resolution of the imaging technique depends on the magnitude of the magnetic field gradient, many efforts are made to develop increased gradient field strength . </P> <P> The principle of NMR usually involves three sequential steps: </P> <Ul> <Li> The alignment (polarization) of the magnetic nuclear spins in an applied, constant magnetic field B . </Li> <Li> The perturbation of this alignment of the nuclear spins by a weak oscillating magnetic field, usually referred to as a radio - frequency (RF) pulse . The oscillation frequency required for significant perturbation is dependent upon the static magnetic field (B) and the nuclei of observation . </Li> <Li> The detection of the NMR signal during or after the RF pulse, due to the voltage induced in a detection coil by precession of the nuclear spins around B. After a RF pulse, precession usually occurs with the nuclei's intrinsic Larmor frequency and, in itself, does not involve transitions between spin states or energy levels . </Li> </Ul>

Difference between continuous wave and fourier transform nmr