HRMAS  -- High Resolution Magic Angle Spinning Spectroscopy

The line width of an NMR resonance depends strongly on the microscopic environment of the nucleus under study. Interactions such as the chemical shift and dipole- dipole coupling between neighboring spins are anisotropic and impose a dependence on the NMR frequency based on the orientation of a spin or molecule with respect to the main magnetic field direction. Furthermore, the magnetic susceptibility of the sample and susceptibility differences within the sample lead to a broadening of the resonances.

In liquid sample the rapid isotropic motion of the molecules averages the anisotropic interactions, resulting in an isotropic chemical shift frequency and a disappearance of the line broadening due to dipolar couplings. Furthermore the sample geometry, a cylinder parallel to the main magnetic field, is chosen such that the susceptibility broadening is minimized.

In solid samples on the other hand, the lack of molecular mobility results in broad lines. This line broadening can be reduced by spinning the sample rapidly around an axis which is oriented at an angle 54.7° with the direction of the magnetic field. By spinning at this so-called 'Magic Angle' (the angle between the z-axis and the (1,1,1) vector), at a rate larger than the anisotropic interactions, these interactions are averaged to their isotropic value, resulting in substantial line narrowing. In addition, magic angle spinning removes the magnetic susceptibility broadening. In addition to pure solids or pure liquids there is a wide range of materials which exhibit either reduced or anisotropic mobility. Examples include polymer gels, lipids, tissue samples, swollen resins (used as supports in combinatorial chemistry). plant and food samples. While these samples generally have sufficient mobility to greatly average anisotropic interactions, the spectral resolution for the static samples are still much lower than that which is achieved for liquid samples. The excess broadening under static conditions is due to a combination of residual dipolar interactions and variations in the bulk magnetic susceptibility. For a variety of samples, including the aforementioned examples, magic angle spinning is efficient at averaging these left-over components of the solid state line width, and leads to NMR spectra that display resolution approaching that of liquid samples. Such methods have been termed High Resolution MAS (HR-MAS) NMR.