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

Aix-Marseille Université, JE2421 TRACES, av. Escadrille Normandie Niémen, case 512, 13397 Marseille cedex 20, France.
Journal of Magnetic Resonance (Impact Factor: 2.51). 02/2008; 190(1):113-23. DOI: 10.1016/j.jmr.2007.10.010
Source: PubMed


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.

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    • "DOSY NMR experiments can also be performed to investigate intermolecular interactions, as illustrated in Fig. 3. It has already been shown in the literature that accurate self-diffusion coefficients could be measured for liquid samples in HR-MAS [21]. As such, 19 F DOSY were recorded on a third sample, containing TFBA (10 mM) and trifluoroethanol (10 mM) in the presence of HSA (2 mM) in Fig. 3. 19 F DOSY HR-MAS NMR spectra recorded on a mixture of TFBA (10 mM) and CF3CH2OH (10 mM) dissolved in a H2O/D2O (90/10, v/v) phosphate buffer (20 mM) at 300 K in the absence (a) and presence (b) of HSA (2 mM). "
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    • "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]. "
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