Selectivity gradients

As before, we note that the resonance frequency of a nucleus at position r is directly proportional to the combined applied static and gradient fields at that location. In a gradient G=G u, orthogonal to the slice selection gradient, the nuclei precess (in the usual frame rotating at coq) at a frequency ciD=y The observed signal therefore contains a component at this frequency witli an amplitude proportional to the local spin density. The total signal is of the fomi... 

On the other hand, Vch coherences which survive the BIRD filter have evolved as HxCz and HyCz AP coherences. They will be therefore labelled together with long-range coupling coherences and would give rise to unwanted [fc i residual peaks in the HMBC spectrum. In the course of the filter delay 8, fortunately, these HxCz and HyCz AP coherences evolve back for the most part into IP coherences Hx and Hy. They are therefore not transformed into multiple quantum coherence by the 90°z 13C pulse and are filtered out by the selection gradients Gi, Gz, and G3. 

Fig. 4. Modified X/Y IMPEACH-MBC pulse sequence used for 19F/15N shift correlation according to Ref. 27. The notation of 90° and 180° pulses is as before. The (d/2 — 180°(Y) — d/2) element represents a variable delay that is incremented concurrently with the decrementation of the accordion delay vd. Pulse phases are x, unless specified x = — x 2 = x, — x 3 = x, x, — x, — x = , — x, — x, x. The bipolar gradients Gs flanking the 180°(Y) pulse can be set to arbitrary power levels, and the relative strengths of the coherence selection gradients G and G2 are determined by G2/G1 =2 Yy/Tx-...

Figure 3. Graph representing power level (gauss/on) and duratum (msec) combinali(ms examined for coherence transfer selection gradient G1 in die HSQC ectia of 2.4mM liibeled...

On the other hand, different approaches should be used in reversed-phase and in normal-phase systems for the prediction of retention with the ternary selectivity gradients where the concentration of solvent 1 is increased and the concentration of solvent 2 simultaneously decreased at a constant (pj. In reversed-phase systems where Eq. (1.18) describes the retention both in binary mobile phases comprised of water and organic solvent 1 and in mobile phases containing water and organic solvent 2 ... 

The validity of Eq. (1.44) was tested on the retention data of various phenols and herbicides on a silica gel column with ternary selectivity gradients of 2-propanol and dioxane in n-heptane. The differences between the experimental elution volumes and the values calculated from Eq. (1.44) were 0.5 ml or less [96]. 

The initial sum of concentrations of the two strong eluents for ternary elution strength gradients , Aj, or their initial concentration ratio, for ternary selectivity gradients can be optimised using a similar approach to that for binary gradient elution. 

Fig. 6.2.16 [Cho2] 2D DANTE-imaging. (a) Timing of signals. The rf excitation is a DANTE pulse train applied in a read gradient Gx- The Hahn echo of the response is detected while both the phase-encoding gradient Gy and the read gradient Gx are turned on. The slice-selection gradient is Gj. (b) Traces in k space.

Sensitivity When used for coherence selection, gradients are able to refocus only one of two p coherence orders. In some applications this means that only one-half of the available signal is detected in contrast to phase-cycled experiments in which both pathways may be retained. Experiments which use PFGs for coherence selection may therefore exhibit lower sensitivity than the phase-cycled equivalent by a factor of typically 2 or V2 depending on the precise experimental details [24,34]. Purging gradients, however, do not cause such a sensitivity loss. 

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