Spin-Lattice Relaxation and Signal to Noise in PFT NMR Spectroscopy

7 Spin-Lattice Relaxation and Signal to Noise in PFT NMR Spectroscopy 

An obvious way to avoid signal noise attenuation for slowly relaxing carbons is to add a delay time to the pulse interval. This relaxation delay should be at least in the order of 3 7]. However, accumulation of pulse interferograms becomes time consuming by this method. 

The third method involves a three pulse sequence, 90 — r — 180° — x — 90°, with a repetition time of tr s. This pulse sequence refocuses the magnetization vector M0 into its equilibrium position within the repetition time, thus representing a pulse driven relaxation acceleration. This technique, known as DEFT NMR [23, 24] (driven equilibrium Fourier transform NMR) can be understood by following the behavior of the magnetization vector Mq under the influence of the pulse sequence in the rotating frame of reference (Fig. 2.17(a-e)). 

Starting with equilibrium of nuclear spins (a), a 90° pulse rotates the magnetization vector M0 by 90° (b). Due to spin-spin relaxation, the spins dephase in (c), causing an FID 

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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