Signal selection with PFGs

Signal selection with PFGs Defocusing and refocusing with PFGs 

In the absence of a field-gradient pulse, the Larmor frequency, tu, of a spin depends upon the applied static field, Bq, such that When the gradient pulse is applied, there is an 

If the gradient is applied for a duration s, the magnetisation vector rotates through a spatially dependent phase angle, 0( ), of 

This modification reflects the fact that a p-quantum coherence dephases at a rate proportional to p. Thus, double-quantum coherences dephase twice as fast as single-quantum coherences, yet zero-quantum coherences are insensitive to field gradients. When the coherence involves different nuclear species, allowance must be made for the magnetogyric ratio and the coherence order for each, such that 

It is therefore apparent that the degree of defocusing caused by a gradient is dependent upon the coherence order and the magnetogyric ratio of the spins involved, and these two features provide the key to signal selection with field gradients. The purpose of gradients in the majority 

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To illustrate the incorporation of PFGs into conventional experiments, the basic COSY sequence is again used as an example. It has already been shown that for the absolute-value COSY experiment, those signals to be retained in the final COSY spectrum follow the coherence transfer pathway illustrated in Fig. 5.38, with N-type selection being the favoured option. For signal selection, it is sufficient to phase-encode coherences before the second 90° pulse and decode these after the pulse, immediately prior to detection. For... 

Recently, Li et al. [16] performed PFG-NMR experiments on oil-in D20 emulsions. D20, with similar chemical properties as H20, was chosen because the NMR resonance frequency of deuterium is quite different from that of hydrogen. Therefore they could select the experimental parameters so that only NMR signals from oil molecules are observed. In their calculations they assumed a log-normal distribution. Because of the very different diffusion coefficients of the two oils used, they were only able to obtain stable converged distribution parameters for the n-octane sample during the non-linear fitting procedure. 

The PFG NMR solvent suppression approach as discussed in this work removes all solvent peaks, impurities from the solvents, residual monomers, and small molecules reaction byproducts. As a result, the PFG NMR method permits the acquisition of a clean spectrum of polymers by LC-NMR and GPC-NMR. One can detect a signal such as the methoxy signal of pMMA (3.6 ppm) in THF which also resonates at about the same frequency. This is not possible with other well established techniques such as WET since the suppression is applied at selected solvent frequencies and all signals, solvent and polymer, in the vicinity of those frequencies will be eliminated. 

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