Recent two dimensional nuclear magnetic resonance (NMR) techniques access exchange in pore structures through surface relaxation and diffusion based relaxation [1-4]. This research applies these techniques to measure pore changes due to biofilm growth and the impact this growth has on diffusion transport. The porous media used in this study are model beadpacks constructed from borosilicate glass beads with diameters approximately 100 um. This research shows that through changes in the relaxation rates, NMR can be used to verify biofilm growth in porous media.

Many porous materials have high susceptibility magnetic gradients in the pores, due to the presence of iron or other magnetic materials. Thus if probe liquids are placed in the pores they exhibit fast decaying signals with a short T2*. Usually the actual T2 of the liquids is also reduced, due the presence of paramagnetic ions in the pore walls. The usual solution in NMR is to measure an echo (or echo train) at short times. However, recent work [J. Phys.: Condens. Matter 19, 415117, 2007.] has shown that water/ice systems near a pore wall form rotator phase plastic ice, with T2 relaxation times in the region of 100 to 200 ms. Thus if a NMR cryoporometric measurement is attempted with a measurement time significantly less than 1 or 2 milli-seconds, the result is to make a measurement based on the phase properties of the brittle to plastic ice phase transition, not that of the brittle ice to water phase transition. This gives rise to artefacts of small pore sizes that may not actually be present. This work successfully uses a-polar liquids instead.

We report on proton NMR relaxation of hardened Grey CEM I paste with two controlled degree of relative humidity. The comparison between these two aged materials evidences that the moisture transport only occurs on the meso and capillary scales. The proton population distribution vs T1 follows power laws with a negative exponent in the nanopore-mesopore pore scale, and a positive exponent in the mesopore-capillary pore scale. This opposite exponent sign shows that the spatial hierarchical proton distribution behaves like a surface and volume distribution on short and large scales, respectively.

In this work we report the application of a recently developed experimental protocol using Pulsed Field Gradient (PFG) Nuclear Magnetic Resonance (NMR) techniques to simultaneously assess bulk pore and surface diffusion coefficients in liquid saturated porous catalysts. This method has been developed to study solvent effects on the diffusion of methyl ethyl ketone (MEK) in mesoporous 1 wt% Pd/Al2O3 catalyst trilobes. The selection of solvents used in this work is known to have a complex effect on reaction rates and hence catalyst performance in heterogeneous liquid phase catalysis. Here, we report the bulk pore and surface diffusion characteristics of MEK, water and isopropyl alcohol (IPA) in 1 wt% Pd/Al2O3 catalyst trilobes. The results show that the physicochemical interactions of molecules in the porous catalyst matrix are very different for the different molecules. We also find that the mobility of water appears to be affected strongest by the catalyst surface.

Understanding speciation in solids and solutions is important for environmental and toxicological purposes. Capillary electrophoresis (CE) is a simple rapid separation technique that can be used to identify species in solution. CE is particularly is well suited for rapid separations of metal containing samples. Direct on-capillary measurement of metal compound speciation can be obtained with nuclear magnetic resonance (NMR). The development of a low-cost microcoil CE-NMR system for in situ characterization of samples of interest is discussed. High precision laser lithography is used to produce copper sputtered microcoils that have comparable resistivity and quality factors to that of hand wound microcoils. A portable NMR system coupled with a CE system has the potential to identify chemical species in aqueous solutions. In addition, transient isotachophoresis can separate and pre-concentrate samples of interest to obtain separate chemical peaks for speciation by online NMR analysis. We are developing separation assays to determine the speciation of chemical complexes in solutions with minimal perturbation to the original sample equilibrium. On-line NMR measurements will be made downstream of the UV detector.

The proton nuclear magnetic resonance (NMR) spin-lattice relaxation rate (R1) of hydrated sands is often used to determine porosity characteristics of near-surface aquifers using magnetic resonance sounding. Large variations in R1 have been reported in laboratory measurements on hydrated sands. To understand these variations, the R1 values of several fully hydrated sands were studied as a function of grain diameter (d) and magnetic field strength (BB0). We conclude the variations are a consequence of trace paramagnetic metals in the sand grains. R1 values from magnetic resonance sounding data should not be used to predict void size in aquifers unless the exact chemical composition of the grains is known.

The inversion of noisy NMR T2 echo data into a T2 spectrum is widely recognized as an inherently non-unique process [1]. One approach to quantifying this uncertainty is to use Monte Carlo sampling. Uncorrelated measurement noise combine with the non-negativity constraint on T2 spectral values to yield spectra following a non-negative normal distribution. There are two published samplers for truncated normal distributions [2], of which nonnegative normal samples are a subset, but we show that these converge too slowly to be practical for the T2 spectral inversion problem. This is because they are based on Gibbs’ samplers that update the spectral estimate just one T2 component at a time. When all of the spectral elements are fixed but one, that one has little room for change without violating the noise constraints on the data. Thus each spectral sample can only be slightly different from the preceding sample, indicating a high degree of statistical correlation and slow convergence. Our solution is to simultaneously update two neighboring spectral components at a time, allowing changes due to one spectral component to be offset by changes in its neighbor. Central to this improvement is a fast 2D slice sampler for non-negative normal distributions. This improves convergence by more than two orders of magnitude. Such speedup allows routine Monte Carlo inversion of 1D NMR spectra, and opens the door for the inversion of 2D NMR spectra.

A gold standard for the validation of diffusion weighted magnetic resonance imaging (DW-MRI) in brain white matter (WM) is essential for clinical purposes but still not available. Synthetic anisotropic fiber bundles are proposed as phantoms for the validation of DW-MRI because of their well-known structure, their long preservability and the possibility to create complex geometries such as curved and fiber crossings. A crucial question is how the different material properties and size of the fiber phantoms influence the outcome of the DW-MRI experiment. Several fiber materials are compared in this study. The effect of surface relaxation and internal gradients on the SNR is evaluated. In addition, the dependency of the fiber density and fiber radius on the diffusion properties is investigated.

In this paper, we demonstrate a rapid simultaneous measurement of diffusion constant D, T1 and T2 relaxation times in just two scans. Theoretical standard deviations of D, T1 T2 for a wide range of T1 and T2 were predicted for given sequences with a random experimental error of 3%. By carefully selecting of sequence parameters for samples with different relaxation times, the error propagators in T1, T2, and D can be modified to within 10%.

We demonstrate that pulses derived using Optimal Control Theory (OCT) techniques can be used to significantly enhance the robustness of the Carr-Purcell-Meiboom-Gill sequence (CPMG) [1,2] to inhomogeneities in the static BB0 field. By numerically inverting the Liouville - von Neumann equation, OCT pulses were derived that can be used directly in place of hard pulses in the CPMG sequence to greatly improve the bandwidth of refocusing. To retain the echo stability achieved by the Meiboom-Gill correction to the Carr-Purcell sequence, the refocusing pulses were designed to perform a unitary π-rotation as opposed to just a state inversion transfer. To illustrate this approach we present an example of optimized pulses that show an improved CPMG-like behavior with complete excitation and multiple refocusing over a bandwidth of +/- 2.6 γB1,max B with a pulse duration limited to 10 t180.