Magic angle spinning 1911761 219132479 2008-06-13T18:48:32Z 128.12.162.46 Categorizing article [[Image:Bruker_MAS_rotors.jpg|thumb|right|300px|Bruker zirconia MAS rotors (left to right), 7 mm diameter for MAS up to 8 kHz, 4 mm for 18 kHz, 3.2 mm for 23 kHz, 2.5 mm for 35 kHz, 1.3 mm for 70 kHz.]]In [[nuclear magnetic resonance]], '''magic angle spinning''' (MAS) is a technique often used to perform experiments in [[solid-state NMR]] [[spectroscopy]]. By spinning the sample (usually at a frequency of 1-70 [[kHz]]) at the [[magic angle]] θ<sub>m</sub> (ca. 54.74°, where cos<sup>2</sup>θ<sub>m</sub>=1/3) with respect to the direction of the [[magnetic field]], the normally broad lines become narrower, increasing the resolution for better identification and analysis of the spectrum. In any condensed phase, a nuclear spin experiences a great number of interactions. The main three interactions (dipolar, [[chemical shift anisotropy]], [[quadrupolar]]) often lead to very broad and featureless lines. However, these three interactions in solids are time-dependent and can be averaged by MAS. The nuclear dipole-dipole interaction, between [[magnetic moment]]s of nuclei averages to zero. The [[chemical shift anisotropy]], a nuclear-electron interaction, averages to a non-zero value. The quadrupolar interaction is only partially averaged by MAS leaving a residual secondary quadrupolar interaction. In liquids, e.g. a solution of an [[organic compound]], most of these interactions will average out because of the rapid time-averaged molecular motion that occurs. This orientation averaging in solution is mimicked by MAS of a solid. This causes the signal to become much narrower, giving rise to the isotropic value (which is of interest for structural determination of solid materials and compounds) and [[spinning sidebands]] which occur at multiples of the spinning speed and can be used to determine the [[chemical shift anisotropy]] of the nuclei. The physical spinning of the sample is achieved via an air turbine mechanism. These turbines (or rotors) come in a variety of diameters (outside diameter), from 2.0-15.0 mm, and are usually spun on air or nitrogen gas. The rotors are made from a number of different materials such as ceramics e.g. [[zirconia]], [[silicon nitride]] or polymers such as [[poly-methyl-methacrylate]] (PMMA), [[polyoxymethylene]] (POM). The cylindrical rotors are axially symmetric about the axis of rotation. Samples are packed into the rotors and these are then sealed with a single or double end cap. These caps are made from number of different materials e.g. Kel-F, Vespel, zirconia or boron nitride depending on the application required. [[Category:Scientific techniques]] [[Category:Nuclear magnetic resonance]]