CROSS-POLARIZATION UNDER FAST MAGIC-ANGLE SAMPLE-SPINNING USING AMPLITUDE-MODULATED SPIN-LOCK SEQUENCES

Amplitude-modulated cross-polarization (AMCP) schemes that can greatly improve the transfer efficiency under fast magic angle spinning am di scussed. A novel class of pulse sequences (S-AMCP) that employ an ampl itude-modulated spin-lock field on the S-spin channel only and cw irra diation on the I-spin channel is introduced. It leads to efficient cro ss polarization at the standard Hartmann-Hahn condition while maintain ing good spin-lock properties. S-AMCP does, however, not decrease the sensitivity to exact Hartmann-Hahn matching that can become critical u nder fast rotation. A double amplitude modulation scheme (D-AMCP) that uses amplitude modulation on both radiofrequency channels and signifi cantly broadens the Hartmann-Hahn matching condition is presented. It can, however, be applied only to spin systems with relatively low I-sp in homonuclear dipolar interactions due to the absence of an effective spin-locking field. S-AMCP pulse sequences, in contrast, are generall y applicable and yield cross-polarization efficiencies that are always better than or equal to the standard continuous-wave cross-polarizati on experiment.