Radiofrequency fields in MAS solid state NMR probes.

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Physical Chemistry
• Analytical Chemistry

Background:

• Solenoid coils are essential components in NMR spectrometers, generating radiofrequency (RF) fields for experiments.
• The static distribution of RF fields along the coil axis is well-understood, but dynamic inhomogeneities are less characterized.
• Sample rotation, particularly during magic angle spinning (MAS), introduces complex RF field modulations.

Purpose of the Study:

• To analyze the dynamic radiofrequency (RF) field distribution within a solenoid coil during sample rotation.
• To investigate the impact of RF inhomogeneities on NMR pulse sequences, specifically pulse calibration and cross-polarization experiments.
• To review and propose methods for mapping RF fields for imaging applications.

Main Methods:

• Extensive numerical simulations were employed to model RF field dynamics.
• Analysis focused on the effects of radial RF inhomogeneities induced by sample rotation.
• Methods for RF field mapping using B0 gradients were reviewed, including nutation, cross-polarization, and spin-lock experiments.

Main Results:

• Dynamic radial RF inhomogeneities and phase modulations occur during MAS, especially in coil end regions (±25% amplitude, ±30° phase variation).
• RF inhomogeneity significantly affects pulse calibration and ramped cross-polarization (CP) experiments across various MAS rates.
• RF field mapping techniques provide direct visualization of amplitude distribution and channel correlations.

Conclusions:

• Understanding dynamic RF field distribution is critical for optimizing NMR spectrometer performance.
• Knowledge of RF inhomogeneities enables the design of more robust NMR pulse sequences and experimental protocols.
• RF field mapping techniques offer valuable tools for characterizing coil performance and calibrating experiments.