Development of NMR probe for the measurement of slow molecular diffusion

Abstract

Pulsed gradient spin echo nuclear magnetic resonance (PGSE NMR) is one of the most powerful tools used to investigate molecular diffusion in solution. The range of diffusion coefficient that can be measured by the PGSE NMR is determined by the gradient strength, gradient pulsewidth, and allowed molecular diffusion time. For the measurement of slow diffusion, however, the gradient strength of a general commercial probe makes it difficult to provide enough gradient for the measurement of polymeric micelle diffusion dynamics. Thus, in order to measure slow molecular diffusion of polymers and micelles (ca. $10^{-11}m^{2}/s\\sim10^{-12}m^{2}/s$), a 400MHz NMR probe with a strong gradient along the z-direction (680Gauss/cm at 10A) has been developed. An RF solenoid coil, which simulated to have little field inhomogeneity, was first made with 0.8mm enamel-coated wire, which gives an advantage of easy fabrication. For the convenience of tube insertion, RF solenoid coil was substituted by a saddle type coil made of copper plate. Despite strict constraints on the maximum geometry of gradient coil, strength of 680Gauss/cm at 10A was achieved by double-set Maxwell type coil. Temperature control has been designed to permit experimentation in high temperature. The stability and accuracy tests were performed at $60\\,^{\\circ}\\mathrm{C}$. The fluctuation of the temperature right below the NMR sample tube was within $\\pm0.1\\,^{\\circ}\\mathrm{C}$ in about 20 minutes. The performance of the developed probe was confirmed through the diffusion coefficient measurement of water at various temperatures using PGSE NMR. At $24\\,^{\\circ}\\mathrm{C}$, diffusion coefficient came out to be $(2.18\\pm0.02)\\times10^{-9}m^{2}/s$ which is within 3$\\%$ error with known value be $2.242\\times10^{-9}m^{2}/s$. The diffusion of 0.4wt$\\%$ Pluronic F127 in heavy water, whose known critical micelle temperature (CMT) is $27\\,^{\\circ}\\mathrm{C}$, was measured at various temperatures $(...