PFG techniques

M. L. Johns, L. F. Gladden 2002, (Sizing of emulsion droplets under flow using flow-compensating NMR-PFG techniques),/. Mag. Reson. 154, 142. 

The D-HMBC pulse sequence can also be used in combination with the pulse field gradient (PFG) technique [12]. Figure 5(c) shows the successful observation of cross peaks between the methyl group at C-5 of an oxazole unit and adjacent carbons in promothiocin. These cross peaks are hidden by the strong t noise of the solvent peak in the HMBC and D-HMBC spectra. The above results clearly indicate that D-HMBC is a quite useful technique for structural studies of complicated natural products. 

It would be impossible to handle ternary solvent systems such as acetonitrile/ water/methanol or mixtures which contain further additives such as triethyla-mine. Therefore most techniques nowadays use modulated shaped pulses for multiple presaturation or pulse frequency generation (PFG) techniques. With basic hardware, it is then possible to suppress an arbitrary number of solvents. 

Pulsed field gradient methods may be used in combination with a spin-echo pulse sequence to measure average molecular displacements in a time In liquids, PFG methods can therefore be used to measure self-diffusion coefficients (i.e. the rate of diffusion due to Brownian motion in the absence of a concentration gradient). In porous media, there is the possibility of obtaining information about the pore geometry because the pore boundaries will influence molecular transport. PFG techniques can measure restricted diffusion and thus provide valuable information on pore sizes in the range 5-100 (im. 

CHARACTERIZATION OF POROUS MEDIA USING NMR METHODS 287 4.1. Diffusion measurements using the PFG technique... 

NMR methods offer a noninvasive method of characterizing porous media. A variety of different techniques may be used to obtain useful information on the pore space. For instance, pore sizes may be measured using the freezing point depression technique for mesoporous solids or by relaxation time measurements for macroporous solids. Other pore space information comes from PFG techniques, while direct imaging of the pore space is possible for large pores. The information from studying the pore space can then be incorporated into appropriate pore network models. 

Diffusion measurements continue to be a recurring theme in the NMR literature. Quite a few papers have been published on diffusion in polymer solutions, mostly with the pulsed field gradient (PFG) technique (122). Diffusion-ordered spectroscopy (DOSY), a 2D method based on PFG, has been used on several polymer systenK (123). A different approach to diffusion n asurement has also been reported (124). 

Measurements have included relaxation times, and absorption spectra and (in cases where lines are already narrowed by translation of protonic species) application of combined magic-angle-rotation and multiple-pulse line-narrowing CRAMPS NMR). For the fastest self-diffusions (D > 10 cm s ) direct measurement of D is possible using the pulsed field gradient (PFG) technique (see Chapters 26 28). 

Measurements of self-diffusion coefficients and flow by means of pulsed field gradient (PFG) techniques have evolved to become a very important tool in the study of a multitude of different problems. The technique as such has recently been described in a review article [2], in which other relevant references may also be found, so here we will merely state that the technique requires no isotopic labeling (avoiding possible disturbances due to the addition of probes) and that it gives component-resolved self-diffusion coefficients with great precision in a minimum of measuring time. The main nucleus studied is the proton, but other nuclei, such as Li, F, Cs and P are also of interest. 

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