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Evolution

Frequency-selective radiofrequency pulses (each at a different frequency) are used to excite the nuclear spins in the various slices, tte slices are excited in sequence, so that spins evolve for a different time in each slice- those in the top slice having the longest evolution time and so on, in a linear progression down through the sample (Fig. 6.1). Whereas the conventional experiment has to employ a fresh scan for each and every evolution increment, the single-scan method simply puts information from each evolution step into a different pigeon hole. We may liken this to a form of parallel processing rather than the more protracted serial processing used in the traditional mode of operation, tte number N is a compromise between the desire to obtain an adequate number of samples during evolution and the restriction imposed by the need to fit N slices into a sample of finite effective length.

At this point the applied field gradient is reversed for an equal duration, thus refocusing the effect of the first gradient pulse. Spin packets that found themselves initially in a positive local field are now situated in an equal negative local field, so the two effects cancel at the end of the bipolar gradient pair. Ms compensation is important because the entire sequence is to be repeated many times, and cumulative effects must be avoided, tte distribution of the slices in space remains unaffected (just as if a horizontal rack of pigeon holes had been relabelled right-to-left rather than left-to-right, without disturbing any of the contents.) Note that spins in a given slice continue to precess at their characteristic NMR frequencies in the main applied magnetic field, essentially unaffected by the bipolar gradient pair, ^is precession is "spectroscopic" rather than "spatial".

After the bipolar gradient pulses, the spins in the various slices have evolved for different time intervals, accumulating different precession phases, ^eir magnetizations can be represented as a helix of vectors in the rotating reference frame (Fig. 6.2). No macroscopic NMR signal is detectable because the various magnetization components of the ensemble mutually interfere. Next a mixing pulse is applied, affecting spins in all the slices equally. Mixing may take many possible forms but suppose, for the purposes of illustration, that we are performing a simple correlation spectroscopy (COSY) experiment (Aue et al. 1976; Bax and Freeman 1981) designed

Fig.6.1. The NMR sample is divided into several slices by selective excitation in an applied magnetic field gradient aligned along the tube axis. The slices are excited in succession -the top slice having the longest evolution time while the bottom slice has the shortest. After the field gradient is extinguished, the spins in different slices "remember" how long they precessed during the evolution interval

Fig.6.1. The NMR sample is divided into several slices by selective excitation in an applied magnetic field gradient aligned along the tube axis. The slices are excited in succession -the top slice having the longest evolution time while the bottom slice has the shortest. After the field gradient is extinguished, the spins in different slices "remember" how long they precessed during the evolution interval

Fig. 6.2. At the end of the evolution stage, the effects of the bipolar field gradient are cancelled, but precession in the main field of the magnet has dispersed the magnetization vectors into a helix. During the subsequent detection interval an applied magnetic field gradient gradually brings these vectors into alignment to form a spin echo

Fig. 6.2. At the end of the evolution stage, the effects of the bipolar field gradient are cancelled, but precession in the main field of the magnet has dispersed the magnetization vectors into a helix. During the subsequent detection interval an applied magnetic field gradient gradually brings these vectors into alignment to form a spin echo to correlate chemical sites connected by a resolvable spin-spin coupling. A nonselective ("hard") radiofrequency pulse is employed, causing coherence to be transferred between interacting sites, tte affected spins are now poised to precess at the frequency of the new site, but they still "remember" their spatial locations because they are labelled according to their evolution history (fa).

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