Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, United Kingdom.
J Chem Phys. 2010 Jun 28;132(24):244705. doi: 10.1063/1.3446805.
It is well known that nuclear magnetic resonance (NMR) transverse relaxation measurements of porous media at high magnetic field strengths provide only an effective relaxation time T(2,eff), as opposed to the true T(2), due to molecular diffusion through magnetic field gradients induced by the magnetic susceptibility contrast between the adsorbent and the adsorbate. Here, we deconvolve the diffusion and surface relaxation contributions to measurements of T(2,eff) and thus obtain the true T(2) relaxation time distribution. This technique is applicable within the short time diffusion regime where the diffusion exponent varies as t(E) (3), while the surface relaxation exponent varies as t(E), where t(E) is the echo time in a standard Carr-Purcell Meiboom-Gill measurement. We demonstrate this technique on measurements of water in contact with glass spheres across a range of magnetic field strengths from B(0)=50 mT to 7.4 T. A direct measurement of T(2,eff) suggests that the transverse relaxation rate increases with field strength, in contrast to theoretical predictions. We show that when the effects of the susceptibility induced gradients, which are known to increase with magnetic field strength, are deconvolved from the T(2,eff) measurement, the true T(2) relaxation rate does indeed decrease with increasing field strength. We also apply the T(2) correction in multidimensional NMR experiments using the example of a T(1)-T(2) relaxation correlation. Here, the correction is essential in order to obtain the true T(1)/T(2) ratio as a function of magnetic field strength, which provides a measure of mobility for surface-adsorbed species; without this correction, we see surface residence times overestimated by up to two orders of magnitude. This novel approach enables the accurate determination of T(2) distributions, and hence T(1)/T(2) ratios, on high-field spectrometers that would have previously been deemed inappropriate for the study of liquids in porous media because of the intrinsic susceptibility effects.
众所周知，由于吸附剂和吸附质之间的磁化率对比引起的磁场梯度导致分子扩散，在强磁场下对多孔介质进行核磁共振（NMR）横向弛豫测量仅提供有效弛豫时间 T(2,eff)，而不是真实的 T(2)。在这里，我们对 T(2,eff)测量中的扩散和表面弛豫贡献进行了去卷积，从而获得了真实的 T(2)弛豫时间分布。该技术适用于短时间扩散范围内，其中扩散指数随 t(E) (3)变化，而表面弛豫指数随 t(E)变化，其中 t(E)是标准 Carr-Purcell Meiboom-Gill 测量中的回波时间。我们在一系列磁场强度下（从 B(0)=50 mT 到 7.4 T）对与玻璃球接触的水中的 T(2)进行了测量，演示了这种技术。直接测量 T(2,eff)表明，横向弛豫率随磁场强度增加而增加，与理论预测相反。我们表明，当从 T(2,eff)测量中解卷积磁化率引起的梯度的影响时，这些梯度已知随磁场强度增加，真实的 T(2)弛豫率确实随磁场强度增加而降低。我们还通过 T(1)-T(2)弛豫相关的示例在多维 NMR 实验中应用了 T(2)校正。在这里，为了获得作为磁场强度函数的真实 T(1)/T(2)比值，这提供了表面吸附物种的流动性的度量，校正至关重要；如果没有这种校正，我们看到表面停留时间被高估了多达两个数量级。这种新方法使在以前由于固有磁化率效应而被认为不适合研究多孔介质中液体的高场光谱仪上能够准确确定 T(2)分布，从而确定 T(1)/T(2)比值。
