Coherence selection using pulsed field gradients

Coherence selection using pulsed field gradients... 

Coherence Selection Using Pulsed Field Gradients... 

Fig. 2. Pulse sequence for selective reverse INEPT using pulsed field gradients to select the coherence transfer echo. The 180° pulse pair in the middle of the 2r delay is not normally needed for t < 50 ms, and the second proton 180° pulse and first t2 delay maybe omitted if a linear phase gradient in the resultant spectrum can be tolerated. The second field gradient pulse has an area (7c/th) times that of the first.

Figure 5.37. An illustration of signal selection by pulsed field gradients. Using two gradients with ratio Gi G2 of 1 2 selects only the coherence transfer pathway shown, leaving all others defocused and unobservable.

We saw in Chapter 8 how a selective 180° pulse can be placed between two gradients of the same sign and duration to give a pulsed field gradient spin echo (PFGSE) that not only selects the desired coherence but also destroys any other coherences. First, we use a hard 90° pulse to create coherence on all spins, and then the first gradient twists the coherence into a helix (Fig. 8.21). The selective 180° pulse reverses the direction of twist in... 

The availability of pulsed field gradient (PEG) techniques has had the most significant impact in terms of making it possible to perform 2D- and 3D-NMR experiments on polymer samples. These methods have taken the place of traditional radiofrequency phase cycling methods for coherence selection. By optimizing the use of the spectrometer s dynamic range, PEG techniques not only save time but also drastically reduce artifact noise. 

Fig. 11. (A) A conceptual diagram of the WATERGATE subunit, the ir RF pulse is some type of selective tt pulse. (B) Multiple solvent suppression using the double-pulsed field gradient echo. Ihe selective ir pulses are SLP pulses to enable multiple solvent suppression. To avoid breakthrough of undesired coherences, different values for the gradients are used in the first and second echo. The phase cycling is given elsewhere. ...

Fig. 8. Pulse sequence required for selective detection of coherence of order M using magnetic field gradients.

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