text: This technique complements X-ray crystallography in that it is frequently applicable to molecules in an amorphous or liquid-crystalline state, whereas crystallography, as the name implies, is performed on molecules in a crystalline phase. In electronically conductive materials, the Knight shift of the resonance frequency can provide information on the mobile charge carriers. Though nuclear magnetic resonance is used to study the structure of solids, extensive atomic-level structural detail is more challenging to obtain in the solid state. Due to broadening by chemical shift anisotropy (CSA) and dipolar couplings to other nuclear spins,  without special techniques such as MAS or dipolar decoupling by RF pulses, the observed spectrum is often only a broad Gaussian band for non-quadrupolar spins in a solid.
  Professor Raymond Andrew at the University of Nottingham in the UK pioneered the development of high-resolution solid-state nuclear magnetic resonance. He was the first to report the introduction of the MAS (magic angle sample spinning; MASS) technique that allowed him to achieve spectral resolution in solids sufficient to distinguish between chemical groups with either different chemical shifts or distinct Knight shifts. In MASS, the sample is spun at several kilohertz around an axis that makes the so-called magic angle θ (which is ~54.74°, where 3cosθ-1 = 0) with respect to the direction of the static magnetic field B; as a result of such magic angle sample spinning, the broad chemical shift anisotropy bands are averaged to their corresponding average (isotropic) chemical shift values. Correct alignment of the sample rotation axis as close as possible to θ is essential for cancelling out the chemical-shift anisotropy broadening. There are different angles for the sample spinning relative to the applied field for the averaging of electric quadrupole interactions and paramagnetic interactions, correspondingly ~30.6° and ~70.1°. In amorphous materials, residual line broadening remains since each segment is in a slightly different environment, therefore exhibiting a slightly different NMR frequency.
  Dipolar and J-couplings to nearby H nuclei are usually removed by radio-frequency pulses applied at the H frequency during signal detection. The concept of cross polarization developed by Sven Hartmann and Erwin Hahn was utilized in transferring magnetization from protons to less sensitive nuclei by M.G. Gibby, Alex Pines and John S. Waugh. Then, Jake Schaefer and Ed Stejskal demonstrated the powerful use of cross polarization under MAS conditions (CP-MAS) and proton decoupling, which is now routinely employed to measure high resolution spectra of low-abundance and low-sensitivity nuclei, such as carbon-13, silicon-29, or nitrogen-15, in solids. Significant further signal enhancement can be achieved by dynamic nuclear polarization from unpaired electrons to the nuclei, usually at temperatures near 110 K.