Validation of NMR relaxation exchange time measurements in porous media

Abstract

Two-dimensional T(2)-T(2) NMR relaxation exchange spectroscopy has been applied to model porous media composed of mixtures of nonporous borosilicate and soda lime glass spheres in water. The spheres had a mean diameter of 100 microm, thus providing an approximately constant characteristic pore dimension throughout the structures, while the use of two glass types ensured that water in different pore-space regions had significantly different T(2) relaxation rates. The packed beds were constructed in various ways with controlled glass type domain sizes to rigorously validate a model for region-to-region exchange of water. From the determined exchange times, the corresponding length scales were calculated based on the molecular self-diffusion of water; these agreed to better than +/-25% with the expected domain sizes. Furthermore, exchange distances on the order of the pore size were observed in thoroughly mixed systems. Depending on the relaxation rates present in the sample, this technique can provide estimates of length scales ranging from microns to millimeters.

Development, simulation, and validation of NMR relaxation-based exchange measurements

“…The relative merits of this model system include its flexibility in terms of the relative pool sizes, relaxation rates, and exchange rates between pools. In addition, the urea model system has a distinct advantage over previous validation studies in borosilicate and soda lime glass spheres 11 because the relative pool sizes are known a priori from the solution stoichiometry. This allows one to invert the model from CPMG data, yielding a gold standard against which each of our methods could be compared.…”

“…A sparse 2 ϫ 2 grid of exponential terms with relaxation times T 2 a and T 2 b ͑derived from equilibrium CPMG data͒ was used for these fits. This eliminated the need to integrate the area under the peak amplitudes, which has previously been shown 11 to be a major source of uncertainty in this type of analysis. IR-REXSY decay data ͑at each mixing time͒ were fitted with Eq.…”

Rapid encoding of with spectral resolution in n-dimensional relaxation correlations

“…The error bars in Fig. 6 were derived from the accuracy of the integration process: see [10,11] for further details. The errors are significantly larger in the T 2 -T 2 experiment because the peaks are not resolved discretely and multiple off-diagonal peaks are present, complicating the analysis.…”

“…This method is also applicable to observing exchange between different chemical environments due to enhanced relaxation in magnetic susceptibility induced gradients, as was demonstrated in packed beds of glass spheres [11]. In chemically complex porous samples, or for high-field measurements, enhanced relaxation mechanisms (e.g., exchange or magnetic field gradients) can be difficult to distinguish.…”

Multi-dimensional inverse Laplace spectroscopy in the NMR of porous media

“…A nice validation of T 2 exchange experiments has been carried out by Mitchell et al [13]. Their model porous medium comprised mixtures of non-porous borosilicate and soda lime glass spheres in water.…”