Radiofrequency fields in MAS solid state NMR probes

Zdeněk Tošner, Armin Purea, Jochem O. Struppe, Sebastian Wegner, Frank Engelke, Steffen J. Glaser, Bernd Reif

Research output: Contribution to journalArticlepeer-review

34 Scopus citations

Abstract

We present a detailed analysis of the radiofrequency (RF) field over full volume of a rotor that is generated in a solenoid coil. On top of the usually considered static distribution of amplitudes along the coil axis we describe dynamic radial RF inhomogeneities induced by sample rotation. During magic angle spinning (MAS), the mechanical rotation of the sample about the magic angle, a spin packet travels through areas of different RF fields and experiences periodical modulations of both the RF amplitude and the phase. These modulations become particularly severe at the end regions of the coil where the relative RF amplitude varies up to ±25% and the RF phase changes within ±30°. Using extensive numerical simulations we demonstrate effects of RF inhomogeneity on pulse calibration and for the ramped CP experiment performed at a wide range of MAS rates. In addition, we review various methods to map RF fields using a B0 gradient along the sample (rotor axis) for imaging purposes. Under such a gradient, a nutation experiment provides directly the RF amplitude distribution, a cross polarization experiment images the correlation of the RF fields on the two channels according to the Hartmann-Hahn matching condition, while a spin-lock experiment allows to calibrate the RF amplitude employing the rotary resonance recoupling condition. Knowledge of the RF field distribution in a coil provides key to understand its effects on performance of a pulse sequence at the spectrometer and enables to set robustness requirements in the experimental design.

Original languageEnglish
Pages (from-to)20-32
Number of pages13
JournalJournal of Magnetic Resonance
Volume284
DOIs
StatePublished - Nov 2017

Keywords

  • Cross-polarization
  • Magic angle spinning
  • Nutation experiment
  • Radiofrequency amplitude and phase modulation
  • Radiofrequency field inhomogeneity
  • Solenoid coil
  • Solid state NMR

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