Paulson Eric K, Martin Rachel W, Zilm Kurt W
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, CT 06520-8107, USA.
J Magn Reson. 2004 Dec;171(2):314-23. doi: 10.1016/j.jmr.2004.09.009.
Homogeneous radio frequency (RF) fields are important for sensitivity and efficiency of magnetization transfer in solid state NMR experiments. If the fields are inhomogeneous the cross polarization (CP) experiment transfers magnetization in only a thin slice of sample rather than throughout the entire volume. Asymmetric patterns have been observed in plots of the CP signal versus RF field mismatch for an 800 MHz solid-state NMR probe where each channel is resonated in a single-ended mode. A simple model of CP shows these patterns can be reproduced if the RF fields for the two nuclei are centered at different places in the coil. Experimental measurements using B1 field imaging, nutation arrays on extremely short NMR samples, and de-tuning experiments involving disks of copper incrementally moved through the coil support this model of spatially offset RF fields. We have found that resonating the high frequency channel in a double-ended or "balanced" mode can alleviate this field offset problem, and have implemented this in a three-channel solid state NMR probe of our own design.
均匀射频（RF）场对于固态核磁共振实验中磁化转移的灵敏度和效率至关重要。如果场是非均匀的，交叉极化（CP）实验仅在样品的薄片中转移磁化，而不是在整个体积中。在一个800 MHz固态核磁共振探头的CP信号与RF场失配的图中观察到了不对称模式，其中每个通道以单端模式共振。CP的一个简单模型表明，如果两个原子核的RF场在线圈中的不同位置居中，则可以再现这些模式。使用B1场成像、极短核磁共振样品上的章动阵列以及涉及铜盘逐渐穿过线圈的失谐实验进行的实验测量支持了这种空间偏移RF场的模型。我们发现，以双端或“平衡”模式使高频通道共振可以缓解这种场偏移问题，并已在我们自己设计的三通道固态核磁共振探头中实现了这一点。
