Pulsed field gradients signal selection

The HMQC or HSQC sequences may be transformed into their ID equivalents by simply removing the incremental t time period (Fig. 6.22) so that the experiment becomes just a heteronuclear filter. Only magnetisation that has passed via the X spin will be observed in the final spectrum and again the suppression of all unwanted signals is greatly improved by the use of pulsed field gradients. The selective observation of C-labelled glycine in an aqueous mixture is illustrated in Fig. 6.23. 

All standard pulse sequences for these experiments include some mechanism for selecting xH-X pairs and suppressing the other XH signals (phase cycling and/or pulsed field gradients, see below). As a further advantage, residual signal intensity of protons not bound to X (because of imperfect suppression) will not lead to a cross peak in the xH,X plane after Fourier transformation, but merely contribute to axial peaks at the spec-... 

The idea of back transformation of a three-dimensional NMR experiment involving heteronuclear 3H/X/Y out-and-back coherence transfer can in principle be carried to the extreme by fixing the mixing time in both indirect domains. Even if one-dimensional experiments of this kind fall short of providing any information on heteronuclear chemical shifts, they may still serve to obtain isotope-filtered 3H NMR spectra. A potential application of this technique is the detection of appropriately labelled metabolites in metabolism studies, and a one dimensional variant of the double INEPT 111/X/Y sequence has in fact been applied to pharmacokinetics studies of doubly 13C, 15N labelled metabolites.46 Even if the pulse scheme relied exclusively on phase-cycling for coherence selection, a suppression of matrix signals by a factor of 104 proved feasible, and it is easily conceivable that the performance can still be improved by the application of pulsed field gradients. 

This process was described in Section 6-3 as a simple 90° pulse that left unwanted signals along the z axis. In practice, coherence numbers are selected either by phase cycles or by pulsed field gradients. The process of phase cycling, described in Section 5-8, can select a particular phase coherence by exploiting differences between phases. Pulsed field gradients generally perform the operation more efficiently. (See Section 6-6.)... 

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