Millot Y, Man P P
Laboratoire de Chimie des Surfaces, CNRS FRE 2312, Systèmes Interfaciaux à l'Echelle Nanométrique, Université Pierre et Marie Curie, 4 Place Jussieu, Tour 55, Paris Cedex 05, 75252, France.
J Magn Reson. 2001 May;150(1):10-6. doi: 10.1006/jmre.2001.2311.
Spurious signals such as the piezoelectric signal from a ferroelectric crystal or the ringing signal from the NMR probe head tuned for low gyromagnetic ratio nuclei are often observed in pulsed NMR. Both signals are cancelled using the Hahn echo sequence with appropriate phase cyclings. The present paper applies a composite-pulse sequence to cancel the ringing signal. The main advantage of this sequence over the Hahn echo sequence is in the simplicity of optimizing the line intensity: the optimization of only one pulse duration for this sequence but of two pulse durations and the interpulse delay for the Hahn echo sequence. We are interested in half-integer quadrupole spins (I = 3/2, 5/2, 7/2, and 9/2), which means that we must consider the first-order quadrupole interaction during the pulses. For simplicity, we deal mainly with spin I = 3/2 nuclei. Since the central-line intensity depends on the ratio of the quadrupole coupling constant (QCC) to the amplitude of the RF pulse, we can determine the QCC from a featureless lineshape by fitting the variation of the experimental central-line intensity for increasing pulse duration with theoretical results. Contrary to the one-pulse sequence where the central-line intensity is proportional to the pulse duration if the latter is short, there is no such condition with the composite-pulse sequence. In other words, this sequence does not allow us to quantify the relative spin populations in powders. The size of the sample must be much smaller than that of the RF coil in order for the RF magnetic field to become homogeneous for the sample. We used (87)Rb (I = 3/2) in an aqueous solution of RbCl and in RbNb2O5F powder, (131)Xe (I = 3/2) of xenon gas physisorbed in Na-Y zeolite, and (23)Na (I = 3/2) in two well-known powders (NaNO3 and NaNO2) to support our theoretical result.
在脉冲核磁共振中，常常会观察到一些虚假信号，比如来自铁电晶体的压电信号，或者为低旋磁比原子核调谐的核磁共振探头的振铃信号。使用具有适当相位循环的哈恩回波序列可以消除这两种信号。本文应用一种复合脉冲序列来消除振铃信号。该序列相对于哈恩回波序列的主要优势在于优化谱线强度的简便性：对于此序列，只需优化一个脉冲持续时间，而对于哈恩回波序列，则需要优化两个脉冲持续时间和脉冲间延迟。我们关注半整数四极自旋（I = 3/2、5/2、7/2和9/2），这意味着在脉冲期间我们必须考虑一阶四极相互作用。为简便起见，我们主要处理自旋I = 3/2的原子核。由于中心线强度取决于四极耦合常数（QCC）与射频脉冲幅度的比值，我们可以通过将实验中心线强度随脉冲持续时间增加的变化与理论结果进行拟合，从无特征的线形中确定QCC。与单脉冲序列不同，如果单脉冲序列的脉冲持续时间较短，中心线强度与脉冲持续时间成正比，而复合脉冲序列不存在这种情况。换句话说，该序列不允许我们对粉末中的相对自旋布居进行定量。为了使射频磁场对样品变得均匀，样品的尺寸必须远小于射频线圈的尺寸。我们使用了RbCl水溶液中的（87）Rb（I = 3/2）、RbNb2O5F粉末中的（87）Rb、吸附在Na - Y沸石中的氙气的（131）Xe（I = 3/2）以及两种知名粉末（NaNO3和NaNO2）中的（23）Na（I = 3/2）来支持我们的理论结果。
