Spin-locks, efficiency

The spin-locking efficiency under MAS depends on the amplitude and frequency of the spin-locking rf field, MAS frequency and magnitude of the quadrupolar interaction. Vega [80] introduced the adiabatic-passage parameter... 

If we used a continuous-wave spin lock (ROESY mixing scheme) for TOCSY mixing, the only way efficient transfer could occur via 7 couplings within a spin system would be if the two protons have opposite resonance offsets (v0 - vr), where vr is the carrier or reference frequency. This is a special case of the Hartmann-Hahn match ... 

Even more efficient single-scan zero-quantum dephasing is possible if adiabatically switched gradients are used in combination with on-resonance spin-locking. Experimental and theoretical details of this technique can be found in the paper by Davis et al. (1993). 

Efficiency of spin-locks. Examine the efficiency of Zj spin-locks on / and S rf channels using pulse sequences discussed elsewhere. The longest time for which the magnetization can be spin-locked on both channels depends on the strength of the rf power, stability of rf amplifiers, rf field inhomogeneity, effects of phase transients, span of the chemical shift, T p of the nuclei (assuming no spin-lock on the other channel), and the quality of the probe. In addition, on the /-spin channel, it also depends on the degree of suppression of the / / dipolar... 

A rigorous examination of the various MQ MAS sequences has been carried out with reference to sensitivity enhancement in the isotropic dimension and the lineshapes of the corresponding MAS peaks in the anisotropic dimension. An echo efficiency parameter has been defined as an indicator of the performance aspects of the various sequences. A consequence of the systematic analysis has been the combination of a spin-lock pulse for excitation of MQ coherences and an amplitude-modulated pulse for their conversion into observable single-quantum coherences. This approach has resulted in an improved performance over other sequences with respect to both the anisotropic lineshapes and the isotropic intensities. 

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