Pulsed field gradient magic angle spinning NMR self-diffusion measurements in liquids

Abstract

Several investigations have recently reported the combined use of pulsed field gradient (PFG) with magic angle spinning (MAS) for the analysis of molecular mobility in heterogeneous materials. In contrast, little attention has been devoted so far to delimiting the role of the extra force field induced by sample rotation on the significance and reliability of self-diffusivity measurements. The main purpose of this work is to examine this phenomenon by focusing on pure liquids for which its impact is expected to be largest. Specifically, we show that self-diffusion coefficients can be accurately determined by PFG MAS NMR diffusion measurements in liquids, provided that specific experimental conditions are met. First, the methodology to estimate the gradient uniformity and to properly calibrate its absolute strength is briefly reviewed and applied on a MAS probe equipped with a gradient coil aligned along the rotor spinning axis, the so-called 'magic angle gradient' coil. Second, the influence of MAS on the outcome of PFG MAS diffusion measurements in liquids is investigated for two distinct typical rotors of different active volumes, 12 and 50 microL. While the latter rotor led to totally unreliable results, especially for low viscosity compounds, the former allowed for the determination of accurate self-diffusion coefficients both for fast and slowly diffusing species. Potential implications of this work are the possibility to measure accurate self-diffusion coefficients of sample-limited mixtures or to avoid radiation damping interferences in NMR diffusion measurements. Overall, the outlined methodology should be of interest to anyone who strives to improve the reliability of MAS diffusion studies, both in homogeneous and heterogeneous media.

Diffusion studies in confined nematic liquid crystals by MAS PFG NMR

“…6. Opposite to [32], we could not find a significant influence of the rotation frequency in our experiments. Fig.…”

“…Second, the enhanced transverse relaxation time under MAS conditions allows for a sufficient time for the implementation of the pulsed magnetic field gradients. The application to liquid systems should be performed carefully since the measured self-diffusion coefficient tends to deviate from its intrinsic value at higher MAS frequencies [32].…”

HR-MAS NMR Spectroscopy in Material Science

“…1) for the fast diffusing components has been highly attenuated, while the slow diffusing components have significant signal intensity remaining. Practical aspects of diffusion measurements using HR-MAS have been previously discussed by Viel et al [84] A significant finding from this study was that sample volume played a role in the reliability of diffusion rates measured. It was shown that small volumes (~12μL) exhibit reproducible diffusion rates, while larger volumes (~50μL, a full 4 mm rotor) produced inconsistent or unreliable data.…”

Chromatographic NMR with size exclusion chromatography stationary phases

“…In order to utilise this chromatographic approach in high-resolution diffusion NMR methods, two techniques have been presented to lessen this observed line broadening. The first is high resolution magic angle spinning (HR-MAS) to reduce the contributions from sample heterogeneity and susceptibility broadening, as has been successfully demonstrated by Calderelli and co-workers when using HPLC stationary phases [10,14]. The second, alternative, approach relies on reducing broadening caused by differences in magnetic susceptibility by aiming to match the susceptibility of the solvent to that of the stationary phase employed [12].…”