射频不均匀性及其对交叉极化魔角旋转（CPMAS）的控制
RF inhomogeneity and how it controls CPMAS.
作者信息
Gupta Rupal, Hou Guangjin, Polenova Tatyana, Vega Alexander J
机构信息
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA.
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA.
出版信息
Solid State Nucl Magn Reson. 2015 Nov;72:17-26. doi: 10.1016/j.ssnmr.2015.09.005. Epub 2015 Sep 25.
In this report we discuss the effect of radiofrequency field (RF) inhomogeneity on cross-polarization (CP) under magic-angle spinning (MAS) by reviewing the dependence of the CP-detected signal intensity as a function of the position in the sample space. We introduce a power-function model to quantify the position-dependent RF-amplitude profile. The applicability of this model is experimentally verified by nutation spectra obtained by direct signal detection, as well as by CPMAS signal detection, in two commercial MAS probes with different degrees of RF inhomogeneity. A conclusion is that substantial sections of a totally filled rotor, even in a probe with rather good homogeneity, do not contribute at all to the detected spectra. The consequence is that in CPMAS-based recoupling experiments, such as the CP-with-variable-contact-time (CPVC), spatial selectivity of the Hartmann-Hahn matching condition overcomes complications that could be caused by RF inhomogeneity permitting determination of accurate spectral parameters even in cases with high inhomogeneity.
在本报告中，我们通过回顾交叉极化（CP）检测信号强度随样品空间位置的变化关系，讨论了射频场（RF）不均匀性对魔角旋转（MAS）下交叉极化的影响。我们引入了一个幂函数模型来量化与位置相关的RF幅度分布。通过在两个具有不同程度RF不均匀性的商用MAS探头中，通过直接信号检测以及CPMAS信号检测获得的章动光谱，对该模型的适用性进行了实验验证。得出的结论是，即使在具有相当好均匀性的探头中，完全填充转子的相当大部分对检测光谱根本没有贡献。其结果是，在基于CPMAS的重新耦合实验中，如可变接触时间的CP（CPVC），Hartmann-Hahn匹配条件的空间选择性克服了由RF不均匀性可能引起的复杂性，即使在高度不均匀的情况下也能确定准确的光谱参数。
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