Acquisition and evaluation of complete relaxation curves

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). 

For the NP sequence, we need Mq = 0 which means that we must keep 

In both cases, / is a numeric factor and denotes the estimated 

An important feature of the above equations is the fact that in the NP sequence we refer to relaxation times at zero-field while in the PP sequence they are referred to as the polarization field Bp. This does often make a big difference since, in many samples, Timax(0) may b much shorter than 

Notice also that proper setting of the parameters RD and Tp requires an approximate knowledge of relaxation times, which are in their own term, the objects of the investigation. This circular problem makes it often necessary to carry out a few preliminary experiments before starting an actual relaxation curve acquisition. Fortunately, the values of RD and Tp do not need to be precise (a 20% tolerance is quite reasonable) so that a simple recurrent estimation process converges to acceptable values in just one or two cycles. 

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