NMR pulse sequence:
MQ-MAS signal enhancement with RAPT
Rotor Assisted Population Transfer MQ-MAS
Grandinetti and coworkers applied a fast 180° phase alternating pulse train during MAS which can prepare a selectively excited state in which the populations of all states |m> with the same sign of m are equal, resulting in an enhanced central transition polarization of all crystallite orientations in a powder sample during MAS.
The application of RAPT before RIACT(II) improves the sensitivity RIACT(II) experiment by a factor of 1.8 in a spin I = 3/2 system.
On the other hand, the sensitivity is reduced by a factor of 0.6 when RAPT is applied before conventional MQMAS sequences that draw their coherences from the equilibrium polarization associated with |m = ±3/2> (for example two-pulse MQMAS and FASTER).
A new RAPT scheme uses the remaining polarisation of the satellites to obtain a further improvement of the central transition by performing RAPT enhanced experiments several times. This enhanced RAPT sequence consists of a train of Gaussian pulses with alternating off-resonance frequencies, which allows us to explore a wider range of frequency offsets. Gaussian pulses also improve the selectivity of satellite excitation.