Echo distortions

A 90° pulse of less than 2.5 jus is also necessary for reasonable excitation of the spectral bandwidth, particularly for the three-pulse sequences (Figures 8.2(b), (c)) where roll-off at the edge of the bandwidth and the effects of virtual signals are magnified. A typical deuterium probe is a 5 or 7 mm horizontal solenoid and for this configuration, pulse power should be about a kilowatt. Several commercial amplifiers supply this power level. Composite pulse sequences [120] extend the spectral bandwidth but are less useful when echo distortion is present. Noticeable roll-off is always present with any pulse sequence and correction of simulations for finite-pulse should always be used [121]. 

Deuteron NMR provides an excellent, albeit expensive, method for the determination of the glass transition, Tg. The quadrupole echo spectrum disappears (due to the echo distortion caused by isotropic motion) when the correlation time for chain motion approaches the quadrupole coupling frequency, typically about 30° above the calorimetric Tg. At higher temperatures, a narrow line appears due to fast isotropic motion. Early on, deuteron NMR and the spin alignment experiment (Figure 8.2(b)) were used to characterize slow motions of polystyrene [3]. More recently, deuteron NMR has been used to characterize... 

In addition to the distortions caused by the probes, there were also distortions caused by filtering the signals within the eddy-current test instruments. To achieve the highest possible dynamics with the test instruments, high-pass filters with a high rate of rise, but also a long reverberation time were used. Thus, the recorded C-scan pictures sometimes shows strong echo effects. 

Second, the target should rather be a large surface, homothetic to the probe shape, producing the strongest back-reflected echo with no front wave distortion and with the same time of flight for all the elements. 

This experiment has two limitations (a) nuclei with short T2 are difficult to be detected and (b) multiplets produce a distorted signal phase after n pulse which inverts the coupled spin states. Both limitations are partially overcome by the stimulated echo experiment (STE).64,65 The main difference with the experiment previously described is that the echo attenuation due to the diffusion competes with Tx rather than with T2. The equation analogous to Equation (13) is now ... 

Raw data gathered from seismic surveys must be processed to compensate for and to remove a variety of distortions unwanted noises created by weathered near-surface rocks, normal time delays, and echoing by rebounding acoustic waves—to provide the clearest possible image of the strata below. Computers can restore these distortions in a fraction of the time that was formerly required to adjust the data painstakingly by hand. Advanced techniques not only permit presentations in three dimensions, but also in color, and to create contour maps and models of subterranean features. However, even with the use of sophisticated tools, there remains a large measure of uncertainty. History has shown repeatedly that a prospective area rejected by one petroleum firm has been accepted by another and proved to be successful. 

Fig. 21. The new five-pulse sequence for recording static 2H exchange spectra.51 The experiment differs from the simple three pulse sequence in Fig. 19 in the addition of r-90°-r (or A — 90° - A) echo sequences before the t and h periods to avoid spectral distortions caused by receiver deadtime and finite pulse width problems. The broader pulses are 90° pulses the narrower ones 54.7° pulses.

The jump-return or 1, 1 method is a very simple and elegant solution because rather than destroying the water signal it simply does not excite water in the first place. We saw in Chapter 8, Figure 8.19 that a null in excitation occurs at the center of the spectral window, and this can be adjusted to put the water peak exactly on-resonance. A jump-return NOESY spectrum of a small protein will be shown later in this chapter. Jump-return and some more complicated variations ( 1,1 - echo and binomial ) are not applicable to all experiments, however, and require some careful tuning and adjustment to work well. They also distort the peak intensities throughout the spectrum and greatly reduce the intensities near the water resonance. 

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