Fast Amplitude Modulation MQ-MAS
Madhu and coworkers introduced FAM pulses that increase both 1D MAS and MQ-MAS signals. They extended the investigation of Vega and Naor.
FAM(I) is a sequence composed of repeating [x-pulse, delay tau, -x-pulse, delay tau] unit. It is also called modulated RF pulses.
The main property of FAM(I) pulse train is that it induces a change in the coherence order of magnitude 2.
Some FAM(I) pulse sequences are described in modulated RF pulse MQMAS page.
- CW stands for continuous-wave RF pulse.
- FAM(I) block is denoted by FIn, where n corresponds to the number of pulses.
- FAM(II) block is denoted by FIIn, where n corresponds to the number of pulses.
- The excitation part and the conversion part are separate by a hyphen.
- Bräuniger and coworkers use C for a CW pulse followed by a FAM train.
- Madhu and coworkers introduce PH for "hard" RF pulse, and PS for "soft" RF pulse.
- A group of pulses with an overall 180° phase shift is denoted by an overbar.
FAM(II) consists of a train of pulses of progressively decreasing duration with alternating phases of 0 and pi separated by an uniform interpulse delay.
FAM(I), RIACT, and DFS MQMAS experiments are based on the adiabatic transfer of coherences through the anticrossing of the energy levels. On the other hand, FAM(II) relies on a cumulative direct transfer process.
The trend in the design of MQMAS RF pulse sequences for spins I > 3/2 is the substitution of FAM(I) block for FAM(II) block.