**Author details**

298 Advanced Aspects of Spectroscopy

Because the 1H signal for the AEM membrane is not readily observable under HR-MAS conditions, the diffusion rates obtained for the resolved solvent resonances were not biased by the polymer membrane. Figure 10B shows the signal decay for the associate methanol environment as a function of gradient strength for the three different IEC levels. The magic angle gradients were used to perform diffusion measurements utilizing a PFG Stimulated Echo with dipolar gradients and spoil gradient with a Δ=100ms (Figure 9B). The signal decay shows that there is a correlation between diffusion rate and IEC, exhibiting a faster diffusion rate with increasing IEC values. A more detailed analysis of this work is forth coming, but this example demonstrates the power of combining HR-MAS and PFG diffusion experiments.

**Figure 10.** A) 1H HR-MAS NMR spectra with the assigned free [F] and associated [A] water and methanol environments. B) The diffusion rates for the associated methanol in three different anion exchange membranes with varying ion exchange capacity (IEC) values. The colored peaks in the 1H HR-MAS NMR spectra correlate to the colored symbols in the diffusion plot of the associated methanol

The application of HR-MAS NMR to the characterization of materials or material interfaces that exist in the semi-solid range has been demonstrated. A wide variety of different material systems have been explored, showing that this technique can provide resolution and dynamic information where standard solution or solid state NMR techniques were unsuccessful. HR-MAS NMR is a powerful tool for the detailed characterization of modified surfaces and surface adsorbed species. This technique also provides a direct probe of differences in local mobility as reflected by line width variations. Through the combination of the enhanced resolution afforded by HR-MAS with pulse field gradient (PFG) capabilities, selective filtering and diffusion measurements of complex heterogeneous materials can also be realized. The ability to resolve and obtain diffusion rates for multiple environments in materials will prove beneficial for understanding the diffusion process in mixed chemical systems. While HR-MAS NMR is considered a mature, relatively routine technique, the application to the materials field is expected to continue being an active area of development. It is hoped that this review will encourage researchers to explore the

application of HR-MAS NMR techniques to their different material systems.

peak of IEC = 2.2 (█), 1.9 (●), and 1.7 mequiv/g (▲).

**4. Conclusions** 

Todd M. Alam\* and Janelle E. Jenkins *Sandia National Laboratories, Department of Nanostructured and Electronic Materials, Albuquerque, NM, USA* 
