Spin-Locking Fourier Transform Experiments

In analogy to the methods described for measuring T), T2 is the reciprocal slope of a semilogarithmic plot of 1/1, — A0) versus t, when the experiment is repeated for a series of different t. 

The spin-locking FT method is illustrated in Fig. 2.33 for o-dichlorobenzene in a 13C 1H experiment avoiding noise modulation of the decoupling frequency during application of the CW field [47]. The results are compared in Fig. 2.33 with Tx measurements in order to demonstrate the relation between Tt and T2 in liquids T[ S T2. 

An NMR spectrometer capable of PFT measurements is a combination of a CW circuit such as found in conventional NMR spectrometers [1-5], a computer controllable pulse generator, and a small digital computer of core memory size 8-256 K. Fig. 2.34 outlines this experimental arrangement. 

Commercially available instruments include electromagnets of 2.114 and 2.35 Tesla (1 Tesla = 10 kilogauss). For these fields, the range of Larmor frequencies for nuclei of interest in organic structure analysis can be obtained from Fig. 2.35, e.g.  

---

Any modification of the magnetization thus arises from relaxation phenomena. The transverse magnetization spin-locked along must end up at its thermal equilibrium value, that is zero. The corresponding evolution is exponential with a time constant denoted by Tip (relaxation time in the rotating frame), very close (if not identical) to T. In practice, the signal is measured (and subsequently Fourier transformed) for a set of x values, in successive experiments, and obeys the equation... 

Co-addition of the S-spin s FID from each of the multiple contact experiments yields, upon ultimate Fourier transformation, a spectrum (from one spin-lock period) having a total sensitivity enhancement even greater than 4x. For the interested reader detailed analyses of the sensitivity considerations and of the CP process itself are presented by Pines, et al. ... 

---