Gradient coherence pathway selection

You might think that gradients are an endlessly beneficial technology, but in fact there are a few minor disadvantages of gradient coherence pathway selection ... 

Technically, the inverse experiment used to be very demanding because the excess of protons not coupled to the nucleus of interest (e.g., protons coupled to the almost hundred-fold excess of 12C instead of 13C) needed to be suppressed. Originally, this was achieved by the use of elaborate phase-cycling schemes, but today the coherence pathway selection by gradient pulses facilitates this process. 

Coherence Pathway Selection with Pulsed Field Gradients... 

To see the power of gradients in coherence pathway selection, consider the effect of a gradient on a homonuclear DQC I+I. The DQC undergoes evolution during a delay according to the sum of the two precession frequencies for the two protons Ha and Ht, ... 

Between these pairs there will be a vertical streak (parallel to the F axis) that represents the 12C-bound proton signal. Because the 12C-bound proton signal is not modulated in t, the 13C evolution period, it has no F frequency, and so it just appears at all F frequencies that is, as a vertical streak. This problem can be solved by coherence pathway selection using phase cycling or gradients. 

In practice, HNCO is now carried out by a somewhat more complex pulse sequence than that given in Fig. 12.16 in order to improve its efficiency. Pulsed field gradients are added to aid coherence pathway selection an INEPT transfer from N to K replaces the multiple quantum coherence step and the N evolution is carried out with a constant time experiment. 

The inhomogeneity in Bi, especially when using surface coils, can lead to a spatially dependent population of desired and undesired coherence pathways. Field gradient pulses in combination with shaped RF pulses lead to spatially selective excitation or refocusing and, in such cases, localization can be viewed as a type of coherence pathway selection. The inherent Bi gradients resulting from the inhomogeneous RF fields of surface coils have also been used for water suppression in in vivo experiments. ... 

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

Few two-dimensional experiments naturally produce phase modulated data sets, but if gradient pulses are used for coherence pathway selection it is then quite often found that the data are phase modulated. In one way this is an advantage, as it means that no special steps are required to obtain frequency discrimination. However, phase modulated data sets give rise to spectra with phase-twist lineshapes, which are very undesirable. So, it is usual to attempt to use some method to eliminate the phase-twist lineshape, while at the same time retaining frequency discrimination. 

Fig. 8.19 Schematic representation of the gradient heteronuclear multiple quantum coherence or GHMQC pulse sequence. The gradient version of this experiment now in use [114] is derived from the earlier non-gradient experiment described by Bax and Subramanian [113]. Coherence pathway selection is obtained by the application of gradients in a ratio of 2 2 1 as shown. Other ratios are also possible, as considered in the reports of Ruiz-Cabello et al. [115] and Parella [116]. The experiment creates heteronuclear multiple quantum coherence with the 90° C pulse that precedes evolution. Both zero and double quantum coherences are created and begin to evolve through the first half...

Before the development of pulsed field gradient sequences (see below), most NMR pulse sequences included a phase cycle in which the phases of at least one pulse and the receiver were varied systematically. This was needed for one or more of several reasons, e.g. suppression of unwanted signals, suppression of artifacts due to hardware imperfections and/or incomplete return to equilibrium between scans and coherence pathway selection in multidimensional NMR. One example of each the first two kinds of phase cycle is briefly discussed below. 

Parella T (1996) High quality ID spectra by implementing pulsed-field gradients as the coherence pathway selection procedure. Magnetic, Resonance in chemistry 34 329-347. 

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