Pulse Field Gradient MQ-MAS
Fyfe and coworkers introduced pulsed magnetic field gradients in MQMAS experiment. The selection of coherences in the MQMAS experiment is performed with PFG. In other words, PFG MQMAS does not need phase cycling for the coherence selection, reducing the number of scans for abundant nuclei.
The major difference from the selection by phase cyclings is that the
effect of the gradient depends on the coherence order p rather than on
the change of coherence order and also on the gyromagnetic ratio gamma:
p1*G1*tau1*gamma1 = -p2*G2*tau2*gamma2.
Gi and taui are the amplitude and the duration of the gradient, respectively. If the coherence orders have the same signs then the two PFGs must be applied in opposite senses.
For 3QMAS experiment, the coherence transfer pathway is selected
by applying two gradient pulses, which fulfill the condition:
3*G1*tau1 = G2*tau2.
0Q -> -3Q -> -1Q is the echo pathway for a spin I = 3/2 and 3QMAS experiment.
RIACT pulse sequence with PFG: The first pulse is a selective Pi/2 pulse followed by a spin-locking pulse of duration tauSL, the third pulse is another spin-locking pulse with tauSL = 1/(4*rotorspeed).
0Q -> +3Q -> -1Q is the echo pathway for spin I = 5/2, 7/2, or 9/2 and 3QMAS experiment.