Adjustment of Pulse Width

In order to rotate the magnetization vectors of all nuclear spins within the range of Larmor frequencies to be observed, the pulse must not only be adjusted for 90 , so that yB1 tp = 71/2 (eq. 2.2)), but must also be very strong, so that y B, 2 7i A (eq. (2.3)). These requirements give the relation between pulse width and spectral width  

Maximum FID signal for a large spectral width (e.g. A = 5 kHz for 13C) thus requires very short rf pulses (e.g. fp 1/4 5 103 = 50 ps for 13C). Usually, square wave bursts of several ps width are used in PFT 13C NMR, 

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The repetition time tr of the pulse sequence is independent of 7j, which may be different for nonequivalent nuclei. The optimum repetition time has been found to be t, = 4 r [22]. DEFT NMR requires careful adjustment of pulse widths for 90° and 180° pulses and (computer-controlled) pulse programming for accurate timing between pulses and pulse sequences. Other methods for improving signal noise using other pulse sequences and spin echo trains have been described [22, 25]. DEFT NMR, however, appears to be the most efficient method so far, as long as Tj and T2 are of the same order of magnitude. 

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