PFGs in high-resolution NMR

The principal benefits arising from the use of field gradients for coherence pathway selection as opposed to conventional phase-cycling may be 

The elimination of protons bound to carbon-12 in proton-detected correlations becomes trivial (suppression ratio of 1 100) and even the selection of natural abundance fragments becomes feasible 

Despite their undoubted advantages, PFGs have a number of fundamental 

Suppression of a solvent resonance The ability of PFGs to completely destroy an NMR resonance makes them ideally suited for methods of solvent suppression, where the aim is to selectively remove the solvent resonance but retain all others. The most widespread use of these methods is in the studies of molecules in protonated water where gradient-based suppression methods prove most effective (Chapter 10). 

Quality On each transient, only the desired signal is detected, avoiding the need for signal addition/subtraction in the steps of a phase cycle. Difference artefacts are thus not encountered, and the noise often associated with these in 2D spectra is reduced, providing cleaner spectra. 

Speed As there is no requirement to complete a phase cycle, experiment times are dictated by sensitivity and resolution considerations only. When sensitivity is not limiting, 2D experiments can be acquired with a single transient per ti increment, leading to 

---

Contents 1. Introduction 160 2. Methods to Achieve High-Resolution NMR Spectra in the Presence of Delaying Media 161 2.1. MAS for magnetic susceptibility averaging 161 2.2. Chrom-NMR without MAS 162 3. Mobility PFG 162 3.1. Diffusion NMR and its representation 162 3.2. Diffusion in multiphasic environments 164 3.3. Specificity of diffusion PFG measurements in porous media 165 4. Examples of Chrom-NMR 166 4.1. Comparison with Liquid Chromatography (LC) 166 4.2. Non-chromatographic mobile phases 169 5. Conclusions 171 Acknowledgements 172 References 172... 

Block copolymers of polystyrene and PFOA, PS-6-PFOA, in CO2 were also extensively studied by SAXS, SANS (46, 47), high-pressure, high-resolution (HPHR) NMR (48) and pulsed magnetic field gradient (PFG) NMR (49). Neutron scattering data (46) showed that the micellar size increases with the PFOA mass, when holding MW of the PS component constant. The effect of CO2 density (0.27 to 0.84 g.cm ) at constant temperature (60 °C), and of temperature (29.5 to 76.2 °C) at constant CO2 density (0.733 g. cm ) was studied on PFOA/CO2 systems by HPHR NMR (48). 

Jenkins et al. used high-resolution H MAS NMR in combination with 2D exchange NOESY and PFG NMR diffusion experiments to characterize the methanol swollen polymer anion-exchange membranes. High-resolution MAS NMR observed resonances from water and methanol in both free environment and membrane-associated environment within the anion-exchange membranes. H high-resolution MAS PFG NMR experiments identified different molecular diffusion environments in the solvent, while H 2D NOESY NMR experiments confirmed spatial contacts between membrane-associated species and the membrane [186]. 

---