Voehler Markus W, Collier Galen, Young John K, Stone Michael P, Germann Markus W
Department of Chemistry, Center in Molecular Toxicology, Vanderbilt University, Nashville, TN 37235, USA.
J Magn Reson. 2006 Nov;183(1):102-9. doi: 10.1016/j.jmr.2006.08.002. Epub 2006 Sep 1.
The pursuit for more sensitive NMR probes culminated with development of the cryogenic cooled NMR probe. A key factor for the sensitivity is the overall resistance of RF circuitry and sample. Lowering the coil temperature to approximately 25 K and the use of superconducting coil material has greatly reduced the resistance contribution of the hardware. However, the resistance of a salty sample remains the same and evolves as the major factor determining the signal-to-noise ratio. Several approaches have been proposed to reduce the resistance contribution of the sample. These range from encapsulating proteins in a water cavity formed by reverse micelles in low viscosity fluids to the optimal selection of low mobility, low conductivity buffer ions. Here we demonstrate that changing the sample diameter has a pronounced effect on the sample resistance and this results in dramatic improvements of the signal-to-noise ratio and shorter pi/2 pulses. We determined these parameters for common 5 mm NMR tubes under different experimental conditions and compared them to the 2, 3 and 4 mm tubes, in addition, 5mm Shigemi tubes were included since these are widely used. We demonstrate benefits and applicability of studying NMR samples with up to 4M salt concentrations in cryogenic probes. Under high salt conditions, best results in terms of short pi/2 pulses and high signal-to-noise ratios are obtained using 2 or 3mm NMR tubes, especially when limited sample is available. The 4 mm tube is preferred when sample amounts are abundant at intermediate salt conditions.
对更灵敏的核磁共振（NMR）探针的追求随着低温冷却NMR探针的发展而达到顶峰。灵敏度的一个关键因素是射频电路和样品的总电阻。将线圈温度降至约25K并使用超导线圈材料大大降低了硬件的电阻贡献。然而，含盐样品的电阻保持不变，并成为决定信噪比的主要因素。已经提出了几种方法来降低样品的电阻贡献。这些方法包括将蛋白质包裹在由低粘度流体中的反胶束形成的水腔中，以及优化选择低迁移率、低电导率的缓冲离子。在这里，我们证明改变样品直径对样品电阻有显著影响，这会导致信噪比显著提高和π/2脉冲缩短。我们在不同实验条件下确定了常见5mm NMR管的这些参数，并将它们与2mm、3mm和4mm的管子进行比较，此外，还包括5mm的Shigemi管，因为它们被广泛使用。我们证明了在低温探针中研究盐浓度高达4M的NMR样品的益处和适用性。在高盐条件下，使用2mm或3mm NMR管可获得最短π/2脉冲和最高信噪比的最佳结果，尤其是在样品有限时。在中等盐条件下样品量充足时，首选4mm的管子。
