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  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 state spectroscopy 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 m =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 1 , lipids 2 , tissue samples 3,4 , swollen resins (used as supports in combinatorial chemistry)5,6,7 , plant and food samples. While these samples generally have suf-ficient 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 8 . 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.

Bruker has developed a series of dedicated probes for standard bore magnets, to accommodate the rapidly expanding field of HR-MAS. These probes are available in double and triple resonance modes and come equipped with a deuterium lock channel. The probes have automatic sample ejection and insertion capability, with the availability of an optional sample changer, enabling fully automated sample runs. A B0 gradient, directed along the magic angle, is optional.

Figure 1 demonstrates the gain in resolution that can be achieved with a HR-MAS probe, compared to a standard high resolution probe. The spectra are proton spectra of a human lipoma tissue, obtained at a frequency of 500 MHz.

 

Figure 1. 500 MHz proton spectra of a human Lipoma tissue. The top spectrum is acquired in a conventional high resolution probe (spinning at 20 Hz // B0 ), while the lower spectrum is acquired in a HR-MAS probe (spinning at 5 kHz).