Coherence pathway selection

Coherence pathway selection Choosing a corresponding phase cycle so that undesired resonances are suppressed and only the desired magnetization is observed. 

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. 

There is a key observation that makes coherence pathway selection possible by phase cycling. Starting with a pure 1+ (p = 1) coherence, consider the effect of changing the phase of a 90° pulse on the phases of the resulting coherences ... 

Better coherence pathway selection is achieved by cycling more than one of the pulses in the sequence. For a DQF-COSY sequence, we could set N to 2, 2, and 4 for the three 90° pulses, thus making the last pulse the most selective so that we can allow both Ap = — 3 and Ap = 1 while blocking the NOESY Ap = —1. So we need to cycle the first two pulses with 180° phase shifts (360°/2) and the final pulse through a 90° phase shift (360°/4). To do all of these phase shifts independently will require 16 scans (2x2x4) because it requires two steps to sample all possible 180° phase shifts and four steps to sample all possible 90° phase shifts. Because these must be sampled independently, the number of scans required... 

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, ... 

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

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. 

A 2D NMR experiment can lead to a data set that is either phase modulated or amplitude modulated as a function of fj, depending on the particular experiment and coherence pathways selected. A regular ID spectrum consists of absorption A(p) and dispersion peaks corresponding to the real and imaginary parts of the spectral lines, respectively. In 2D experiments, phase modulation in fj results in twisted 2D real lineshapes as a result of the Fourier transformation of bi-exponential time domain... 

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... 

Only the /lz term leads to cross-peaks by chemical exchange, so the other term will be ignored (in an experiment this is achieved by appropriate coherence pathway selection). The effect of the first part of the sequence is to generate, at the start of the mixing time, Tmix, some z-magnetization on spin 1 whose size depends, via the cosine term, on tl and the frequency, Qv with which the spin 1 evolves during /). The magnetization is said to be frequency labelled. 

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