Sensitivity enhancement methods population transfer

Even if the optimisation of the use of DFS, RAPT or adiabatic inversion pulses is not straightforward for nuclei with low sensitivity, it is nonetheless worth applying one of these methods to improve sensitivity. As long as there is no influence on the CT resonance, these techniques are likely to produce an enhanced CT signal compared to standard spin-echo experiments. Therefore, for Mg (as well as for other insensitive half-integer spin quadrupolar nuclei such as S, K and Ca), it is always advisable to apply some population transfer technique before the excitation of the CT signal. 

A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

While the SPT method, as the name indicates, is a selective experiment, techniques derived from the principle of population transfer of a non-selective nature are also availahle. The first of these to appear was the INEPT experiment [38]. The pulse sequence schematics for INEPT and refocused INEPT are shown in Pig. 8.1. As with the SPT experiment described above, INEPT and its successor, the DEPT ex-p)eriment, both operate with enhanced sensitivity through the perturhation of the Boltzmann populations. This prototypical non-selective experiment is now relatively seldom used as more refined variants have been developed. The INEPT experiment (r = Fig. 8.1A) generates antiphase responses analogous to... 

Rotor-assisted population transfer has been developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarisation associated with satellite transitions to the central m = 1/2 -1/2 transi-... 

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