SEMA sequence

Fig. 4. Pulse sequences for 2D PISEMA experiments on static or slow-spinning samples. It has been shown that the use of ramped rf pulses for S-spin-lock during the ti period makes the pulse more tolerant to experimental errors. The original PISEMA sequence (A) does not suppress the effects of phase transients. Incorporation of the modified SEMA sequences, (B) and (C), in the ti period of the PISEMA sequence (A) have been shown to suppress the effects of phase transients.

As discussed later, the application of the SEMA sequence is not limited to the 2D PISEMA experiment, but it can be used to design a variety of sophisticated pulse sequences. For example, the selective transfer of magnetization between strongly coupled heteronuclei by SEMA is the key feature of the sequence that can be utilized in experiments such as HETCOR, H-detected experiments," and experiments to selectively measure relaxation parameters of a proton bonded to S nuclei. 

The appearance of the spectra highly depends on the relative orientation of the two tensors. 3D experiments constituting the SEMA sequence have been used to determine the CSA, " N CSA, and... 

In order to increase the sensitivity and resolution, and to reduce the experimental time, of PISEMAMAS technique, it is essential to perform the experiment with fast spinning speeds. While the PISEMA pulse sequence has been successfully used under slow spinning conditions, unfortunately, it cannot be used at high spinning speeds as the heteronuclear dipolar couplings are suppressed. However, changing the S -spin-lock rf field strength (in SEMA) to satisfy the Hartmann-Hahn condition, B s[= " ioi with... 

Fig. 7. A PISEMA pulse sequence for studies under a fast magic angle spinning condition. The modified SEMA pulse sequence in the tx period recovers I S dipolar couplings under MAS. Unlike other SLF pulse sequences, this pulse sequence does not require the synchronization of the tx period with the spinning speed of the rotor.

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