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Uniform signal enhancement in MAS NMR of half-integer quadrupolar nuclei using quadruple-frequency sweeps. 2018
We introduce two MAS schemes that allow manipulating the satellite-transition (ST) populations of half-integer quadrupolar nuclei, and which both exhibit improved robustness to the quadrupolar coupling constant (CQ). These schemes, called quadruple frequency sweep (QFS) or quadruple WURST (QWURST) are the sums of two DFS or four WURST to efficiently invert the ST populations of nuclei subject to large or small quadrupole interactions, simultaneously. These quadruple sweeps methods only require 6% more rf-power than the double sweeps ones. We demonstrate, both numerically and experimentally, that the QFS and QWURST schemes benefit from robustness to CQ and rf amplitude and offset and hence achieve uniform enhancement of the CT signal for 27Al nuclei subject to different quadrupole interactions. Although the version of QFS with repetitive accumulation can achieve higher enhancement in the S/N of the 27Al MAS spectrum, the final sensitivity gains mainly depend on the longitudinal relaxation time of different 27Al sites. We also confirm that these schemes provide an improved acceleration of the 31P-{27Al} coherence transfer in PT-J-HMQC experiments.
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