NMR pulse sequence:
MQ-MAS signal enhancement with modulated RF




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Modulated Radio Frequency MQ-MAS

Vosegaard and coworkers investigated several MQMAS sequences involving modulated RF pulses, which are also called FAM(I) pulses.

FAM(I) sequence involves a train of equally spaced pulses of equal duration and alternating phases of 0 and pi (denoted x and x bar).

The idea behind the x and x bar pulse trains derives from the cosine amplitude modulated mixing period proposed by Vega and Naor for 3Q to 1Q conversion in single crystals of spin-3/2 systems. It relies on selective irradiation on the satellite transitions.

One important property of FAM pulses is that they are selective for delta p = 2 coherence order changes and have little beneficial effect upon either the p = +3 to -1 (delta p = -4) conversion or the p = 3 to 0 (delta p = 3) conversion.

First sequence

Two-pulse MQMAS with modulated RF sequence

For I = 3/2, modulated pulses mainly transfer the echo pathway signal. This selectivity has the drawback that it inhibits the acquisition of the anti-echo pathway signal which is necessary in the hypercomplex or TPPI approaches to pure absorption lineshapes.

Some distortions of the lineshapes are observed, showing that this sequence does not provide a pure absorption spectrum.

Second sequence

3Q phase modulated MQMAS with modulated RF shifted-echo sequence for I = 3/2

For spin I = 3/2 nuclei, only the p = 0 -> +3 -> +1Q -> -1Q pathway signal is refocused at positive t2 values. Therefore in shifted-echo experiment, the anti-echo shifts towards lower t2 values with increasing t1.

Third sequence

3Q phase modulated MQMAS with modulated RF shifted-echo sequence for I = 5/2

This figure represents the 3Q-MAS with the shifted echo approach and FAM for conversion pulse, applied to a spin I = 5/2.

For 3Q MAS with I = 5/2, most of the observed NMR signal (90%) resulted from the echo pathway; hence phase modulated sequence is sufficient.

The sensitivity is approximately doubled compared to the single-pulse mixing sequence.

This approach provides us with a pure absorption 2D lineshape.

ACQUISITION: F1-QF

Shearing transformation is avoided.

Fourth sequence

5Q phase modulated MQMAS with modulated RF split-t1 sequence for I = 5/2

This figure represents the 5Q-MAS with the shifted echo approach and FAM for conversion pulse, applied to a spin I = 5/2.

The first pulse trains have a fixed modulation frequency tuned near the singularity of the outer satellite transitions.

The second pulse trains have a fixed modulation frequency tuned near the singularity of the inner satellite transitions.

The sensitivity is approximately doubled compared to the single-pulse mixing sequence.

This approach provides us with a pure absorption 2D lineshape.

ACQUISITION: F1-QF

Shearing transformation is avoided.

 

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