Magic angle spinning

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Magic angle spinning (MAS) ( Engl. For rotation / magic angle spinning ) is a technique for improving the signal quality in the solid state - nuclear magnetic resonance spectroscopy (NMR spectroscopy), which is based on the very fast rotation of the sample during the measurement.

The test sample (blue) rotates at a high frequency in the magnetic field B 0 . The axis of rotation is tilted by the magic angle θ m to the magnetic field.

In the case of NMR spectroscopy in solids, there is generally a clear (mostly undesirable) line broadening in the measured NMR spectra , which can make further analysis of the spectra difficult or restrictive. This line broadening is caused by anisotropic interactions (essentially dipolar and quadrupole interactions and the anisotropic part of the chemical shift ) between the atomic nuclei of the sample. In contrast to NMR spectroscopy in solutions or liquids, these anisotropic interactions in solids are not averaged out by statistical (Brownian) molecular movements.

The line broadening caused by dipolar interactions and chemical shifts can be reduced significantly by magic angle spinning. For this purpose, the sample is rotated at speeds of up to 130 kHz around an axis that is tilted (the “magic angle”) with respect to the external magnetic field direction . This angle satisfies the condition . At this angle, measured between the line connecting two interacting nuclei and the external field, the dipolar interaction between the nuclei disappears. Due to the rapid rotation of the sample around this axis, all connecting lines (including those without chemical bonds) assume this angle to the external field on average over time. The magic angle corresponds to the angle between the space diagonal of a cube and an edge, i.e. half the tetrahedron angle of approx. 109.47 °.

The MAS technique was first described in 1958 by E. Raymond Andrew, A. Bradbury and RG Eades and independently thereof in 1959 by I. J. Lowe. The term "Magic Angle Spinning" was coined in 1960 by Cornelis J. Gorter at the AMPERE Congress in Pisa.

For this purpose, the sample is located in a cylindrical rotor sleeve, usually made of ceramic, which floats in a turbine housing called a stator on gas bearings and is accelerated by blowing on its end piece, which is structured as a turbine rotor. In order to avoid supersonic effects and to limit the extreme centrifugal forces that would occur at speeds of several million revolutions per minute, the diameter of the rotor has to be reduced more and more. Commercially available systems with over 100 kHz have diameters well below one millimeter.

Individual evidence

  1. ^ A b Jacek W. Hennel, Jacek Klinowski: Magic Angle Spinning: A Historical Perspective . In: Jacek Klinowski (Ed.): New techniques in solid-state NMR . Springer, 2005, ISBN 3-540-22168-9 , pp. 1–14 , doi : 10.1007 / b98646 ( limited preview in Google Book search).
  2. ^ E. Raymond Andrew: Magic Angle Spinning . In: Anne McDermott (Ed.): Solid State NMR Studies of Biopolymers . John Wiley & Sons, 2010, ISBN 0-470-72122-7 , pp. 83–97 ( limited preview in Google Book Search).
  3. ^ Daniel Canet: NMR, Concepts and Methods . Springer, Heidelberg 1994, ISBN 3-540-58204-5 , pp. 62 ff .
  4. ^ ER Andrew, A. Bradbury, RG Eades: Nuclear magnetic resonance spectra from a crystal rotated at high speed . In: Nature . tape 182 , no. 4650 , 1958, pp. 1659 , doi : 10.1038 / 1821659a0 .
  5. ^ IJ Lowe: Free Induction Decays of Rotating Solids . In: Phys. Rev. Lett . tape 2 , no. 7 , 1959, pp. 285-287 , doi : 10.1103 / PhysRevLett.2.285 .
  6. ↑ e.g. 0.6mm at 126kHz, sample volume 0.4 µl, Penzel, S., Oss, A., Org, ML. et al., Journal of Biomolecular NMR (2019) 73, pp. 19-29 doi : 10.1007 / s10858-018-0219-9

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