Coherence level

Bain A D 1984 Coherence levels and coherence pathways in NMR. A simple way to design phase cycling procedures J. Magn. Reson. 56 418-27... 

Before the application of a pulse, only equilibrium magnetization exists, directed toward the z-axis corresponding to the zero coherence level for all coherence pathways. When the pulse is applied, two coherence levels, + 1 and —1, are created during the evolution period that evolve into M and respectively, where [Pg.74]

Apparently, all coherence pathways will therefore start at zero coherence levels and end at -t-1 coherence levels since the quadrature receiver is sensitive only to the +1 polarization, only the single-quantum coherence is detected. 

Below the pulse sequence we can show the desired coherence level, p, at each stage of the pulse sequence. This diagram defines the coherence pathway that is desired for a particular NMR experiment. Coherence order is mixed for Cartesian product operators... 

The next step is to calculate the receiver phases. Each time we change the phase of a pulse, the receiver phase must be advanced by the appropriate amount so that the desired coherence (resulting from the selected coherence level change Ap) is summed in the receiver and not canceled. For more than one pulse, we can add together the phase shifts required at the receiver according to... 

This pathway is permitted in the phase cycled experiment, but the data from this pathway is lost in the gradient version. In this sense, gradients are sometimes too selective because they generally allow only one coherence level even when two or more pathways are equivalent in terms of the signal we want to see. 

Loss of Sensitivity Due to Overselectivity. Gradient selection means that only a single coherence level can be present at the time of each gradient. With phase cycling we apply a mask with holes at regular intervals, so that more than one coherence level... 

It is often helpful to summarize in a diagram the changes in coherence level as a function of the discrete periods within the experiment, as illustrated in Fig. 11.2. We begin at equilibrium, where the density matrix (Iz) has only diagonal terms... 

Figure 2.4. An example of a coherence level diagram for a 3Q experiment with symmetrised...

Appendix 6 Product-Operator Formalism and Coherence-Level Diagrams 323... 

Figure A6-5 Coherence-level diagram for the inversion recovery experiment.

Finally, let us reexamine the 2D INADEQUATE experiment (Section 6-4) in terms of its coherence-level diagram (Figure A6-9). This experiment utilizes pulses only at the frequency of the insensitive nucleus, such as C. The initial 90° pulse generates single quantum coherence, and the periods t are selected to produce an antiphase disposition of " C nuclei with C coupling partners. The center-band signal from C nuclei lacking adjacent... 

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