Sample magnetization

A pulse is a burst of radiofrequency energy that may be applied by switching on the Rf transmitter. As long as the pulse is on, a constant force is exerted on the sample magnetization, causing it to process about the Rf vector. 

The so-called static dephasing regime (SDR) predicts a linear dependence of I/T2 on the magnetic moment of the particles in solution (22, 25). This regime describes the transverse relaxation of homogeneously distributed static protons in the presence of static, randomly distributed point dipoles. The free induction decay rate, I/T2, is then proportional to the part of the sample magnetization due to the dipoles (p), M = rap, where ra is the concentration... 

As shown in Fig. 22, the resulting procedure, referred to as a multi-block experiment, produces a two-dimensional data set, such as an array of FIDs (its exact nature depends upon the signal acquisition method). The data of each x-block are then reduced to a single quantity, S(t) which should be proportional either to the total sample magnetization Ma(x) or to one of its components. Since the vertical scale of the relaxation curve is irrelevant, we can identify S(t) with Ma(x) at the exact time of detection (usually just after the first excitation pulse). 

A little less obvious is the setting of the recycle delay RD (for NP-type sequences) or Tp (for PP-type sequences) which is linked through the factor f in Eqs. (3) and (4) to the estimated relaxation time Ti ax of the slowest-decaying component of sample magnetization at a specific field. One cannot influence the sample relaxation times, of course. On the other hand, the relaxation times usually dominate the overall duration of a single... 

All this points to the factor f, which guarantees that the sample magnetization at the beginning of each block is the same with a relative precision of e. However, the actual reproducibility is much better than this, since we do not really need Mq to be exactly zero (in NP) or Mp (in PP) but only that they be the same for all T-blocks of a multi-block experiment. Theoretically, considering that the acquisition period of the previous block normally destroys the longitudinal magnetization and the subsequent sequence of events until the start of the next relaxation period is the same for every T-block, any value of f should be theoretically acceptable. Further investigation of these aspects is currently under way. 

The shape of a simple, low-resolution FID is usually not suitable for discriminating between various sample magnetization components except, perhaps, in the co-presence of a very fast-decaying component and a very slow-decaying one. 

Fig. 28. FFC Inversion Recovery sequence. In the upper case the sample is first prepolarized in a filed Bp, then switched to the acquisition field Ba where the first RF pulse of 180° is applied and the sample magnetization is inverted. The field is then switched to B,. and the sample is allowed to relax for the variable time t. Finally, the field is switched again to the acquisition value and the magnetization is sampled by any of the sample-detection methods (here, a simple FID following a 90° RF pulse). Notice that, as shown in the lower diagram, in the special case when Bp = Ba it is possible to neatly avoid the extra switching interval prior to the inversion pulse.

Mach disc. See Ion sampling Magnetic sector. See Mass analyser Mass analyser ... 

Fig. 2.17. Driven equilibrium FID (a) — (b) — (c) — (d) — (e) — (a) the sample magnetization following a 90", t, 180°, r. 90° pulse sequence (f) and (g) comparison between the FID signals following repetitive 90 pulses and those following repetitive 90c, r, 180°, r, 90" pulse sequences example 13C FID of enriched carbon tetrachloride, 13CC14.

Figure 11. Angular dependence of the largest projection of the sample magnetization vector to the (001) plane [1,2].

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