Effect of dielectric properties of solvents on the quality factor for a beyond 900 MHz cryogenic probe model
Effect of dielectric properties of solvents on the quality factor for a beyond 900 MHz cryogenic probe model
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DOI：
10.1016/j.jmr.2005.01.004
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发表时间：
2005-05-01
影响因子：
2.2
通讯作者：
Maeda, H
中科院分区：
文献类型：
--
作者：
Horiuchi, T;Takahashi, M;Maeda, H
Kelly等人的先前报告[J. Am. Soc.124（2002）12013]指出，水溶液的离子电导率对装备有低温冷却探针的高分辨率NMR中的灵敏度损失产生显著贡献。样品溶液中的损耗包含两个贡献：一个来自离子电导率，另一个来自介电损耗;后者在高频（如900 MHz以上）下尤其重要。在这里，我们研究了930 MHz低温探针模型的品质因数的介电电导率的影响，特别是，它涉及的离子水溶液和有机溶剂，通常用于NMR在生物研究和天然化合物的化学。样品品质因数Q（s）首先随着相对介电常数ε '的真实的部分而增加，然后饱和。在极性有机溶剂中，样品上的横向电场随ε ′减小，导致Q（s）增大。在非极性有机溶剂的情况下，介电电导率如此之小，以致于增加的梯度是陡峭的，导致大得多的Q（s），尽管Q '很小。当样品中的"π"变大时，横向电场的影响可以忽略不计，因此离子水溶液的损耗主要由样品溶液中感应的回路电流决定。由于感应电动势与ε '无关，Q（s）在ε'的高值处饱和。基于用低温探针模型获得的Q（s），低温探针的灵敏度预期如下：通过加载水的灵敏度损失大于66%，即，水的介电电导率的影响在高频下是显著的;极性有机溶剂遭受大得多的损失，这是由于ε '效应增强了有效样品电阻;非极性有机溶剂几乎没有灵敏度损失，因为介电电导率可以忽略不计;反胶束的行为类似。(c)2005年爱思唯尔公司All rights reserved.
A previous report by Kelly et al. [J. Am. Chem. Soc. 124 (2002) 12013] indicated that the ionic conductivity of aqueous solution produces a significant contribution to the sensitivity loss in high-resolution NMR equipped with a cryogenically cooled probe. The loss in a sample solution contains two contributions: one from the ionic conductivity and the other from the dielectric loss; the latter is especially important at high frequencies such as above 900 MHz. Here, we investigated the effect of the dielectric conductivity on the quality factor of a 930 MHz cryogenic probe model; in particular, it deals with the ionic aqueous solutions and organic solvents commonly used for NMR in biological research and the chemistry of natural compounds. The sample quality factor, Q(s), at first increases with the real part of the relative dielectric permittivity epsilon' and then saturates. In the case of polar organic solvents, the transverse electric field on the sample decreases with epsilon', resulting in an increase of Q(s). In the case of non-polar organic solvents, the dielectric conductivity is so small that the gradient of the increase is steep, resulting in much larger Q(s) though the epsilon' is small. The effect of the transverse electric field is negligible if the epsilon' becomes large, thus the loss for ionic aqueous solution is mainly governed by a loop current induced in the sample solution. As the induced electromotive force is independent of the epsilon', the Q(s) is saturated at high values of epsilon'. Based on the Q(s) obtained with the cryogenic probe model, the sensitivity for the cryogenic probe is expected to be as follows: the loss in sensitivity by loading water is more than 66%, i.e., the effect of the dielectric conductivity of water is remarkable at high frequencies; polar organic solvent suffers much larger losses, which is due to the enhancement of the effective sample resistance by the effect of epsilon'; a non-polar organic solvent is nearly free of the sensitivity loss as the dielectric conductivity is negligible; the reverse micelle behaves similarly. (c) 2005 Elsevier Inc. All rights reserved.