Analytical Methods | Nuclear Magnetic Resonance: Principles

The self-diffusional motion of fluid molecules in a porous medium is expected to be different from that in the bulk fluid due to the presence of pore boundaries which restrict the molecules diffusing within the pore space. Thus it is conceivable that the study of restricted diffusion may provide information leading to pore structure characterization. We report the use of pulsed field gradient nuclear magnetic resonance (PFG NMR) techniques to study proton restricted self-diffusion in porous media. Measurements were conducted in model porous media consisting of different sized glass bead packs, as well as porous rocks. Computational analyses were carried out to obtain apparent diffusivities and other characteristics from the experimental results.

A nuclear magnetic resonance advance. Imaging fluorinated anesthetics in the brain.

Surface-to-Volume Ratio of Ganglia Trapped in Small-Pore Systems Determined by Pulsed-Field Gradient Nuclear Magnetic Resonance

Appropriate drilling fluid additives which can build effective filter cakes are key components to minimize formation damage. Today there exists no effective and quantitative technique which can assess the formation damage minimization potential of any novel drilling fluid. This study provides the methodology for such a technique, by integrating HP/HT fluid loss data, derived from standard API HP/HT filter press, with filter cake quality and filtrate penetration depth, derived by Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) techniques. Novel nano-based drilling fluidwas made of de-ionized water, 7 wt% commercial Na-bentonite, and addition of custom made iron oxide (CM Fe3O4) nanoparticles (NP) at 0.5 wt%. Its excellent filtration characteristics were determined at elevated pressures and temperatures using a HP/HT filter press (500 psi/350°F) with ceramic discs, mimicking the formation, as filter media of k=775 mD. NMR T2 relaxation time measurements provided the pore size distribution of the ceramic discs, and a newly developed MRI technique by our team allowed direct visualization on the core-ceramic disc of the produced filter cake and filtrate penetration depth, providing insights on filtration characteristics. Addition of 0.5 wt% Fe3O4 NP resulted in a decrease of fluid loss by 47% compared to that obtained from the base fluid (BF) without the nanoparticles. The excellent filtration characteristics were attributed to the synergistic interaction of NP with bentonite platelets producing a strong microstructure network leading to rigid and very low permeability filter cakes. Spin-spin relaxation time (T2) NMR measurements revealed different pore size distribution of the nano-enhanced drilling fluid showing its tendency to occupy the smallest pores of the filter medium (ceramic disc). MRI T2 measurements revealed the saturation profiles of the drilling fluids inside the ceramic discs, in 3-D space. This allowed the visual representation of the penetration depth of the filtrate produced in the ceramic discs of the different tested drilling fluids. Our integrated characterization approach, gives us tools to analyze inter-particle interactions in the filter cakes at HP/HT conditions in order to explain the significant improvement in fluid loss performance of novel drilling fluids. The information gained in this study is considered to be extremely significant for deriving an effective methodology for assessing formation damage potential of any newly developed drilling fluid which can lead to more efficient and cost-effective drilling activities.

Probing pore characteristics of deformed chalk by NMR relaxation

The transport of the lithium ions is the basis of lithium ion conductivity of currently used electrolytes. Understanding how the transport of lithium ions within the matrix is influenced by the interactions with solvating moieties is needed to improve their performance. Along these lines well-defined model compounds based on cyclotriphosphazene (CTP) and hexaphenylbenzene (HPB) cores, bearing side groups of ethylene carbonate, a common solvent for lithium salts used as electrolytes in Li-ion batteries (Thielen et al. Chem. Mater, 23, 2120, 2011) and blended with different amounts of Lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) have been studied by multinuclear nuclear magnetic resonance (NMR) spectroscopy. The local dynamics of the matrix was probed by 1H and 31P NMR, while the local dynamics of the Li+ cations was unraveled by 7Li and 13C NMR. Transport of both ions was studied by pulsed-field gradient (PFG) NMR. Based on the different temperature dependences of the dynamics the bulk ion transport is not attributed to local dynamics, but to translational diffusion best characterized by PFG NMR. Although the glass transition temperatures of the blends are low, their conductivities are only in the range of typical polymer electrolytes. The results of NMR spectroscopy are in accord with the conjecture that the coordination between the cyclic carbonate functionality and the Li+-ion is too tight to allow for fast ion dynamics.

Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) techniques are widely used in food science, mainly because they are non-invasive techniques. MRI, as a non-destructive technique, allows the study of intact samples and without any preparation of the samples before analysis. In food processing, the monitoring of distribution and water content is a consolidated analysis technique, frequently used on the market in order to preserve appropriate nutritional and health characteristics of food according to quality standards. In a food matrix, the variation of the water content is related to the changes in the internal structure and in the physico-chemical properties that occur during the transformation process. In this study MRI technique is used to evaluate the variation of the water content as a function of the drying time. Pumpkin samples are analyzed at four different drying temperatures of 50, 60, 65, and 70°C. The transverse relaxation time, T2, is used to assess the hydration level of the samples by comparing the information extracted from MR images with the drying kinetics measured by gravimetric method. Moreover, T2 maps are used to correlate the change in water distribution with the change in T2 values. The results show that the global weight loss curves obtained with the standard gravimetric method and with the MRI data are in excellent agreement. This work indicates that monitoring changes in the T2 profile of food (i.e., pumpkin) is a useful method for evaluating moisture profiles and changes induced on the sample during the drying process.

Bertram Schwarzschild described the important contributions made by the 2003 Nobel laureates Paul Lauterbur and Peter Mansfield during the 1970s (Physics Today, December 2003, page 24). They extended magnetic resonance imaging for applications in medicine into two and three dimensions and to faster speeds. Schwarzschild refers to an article by Felix Wehrli (Physics Today, Physics Today 0031-9228 45 6 1992 34 https://doi.org/10.1063/1.881310. June 1992, page 34 ), but Wehrli makes no mention of the earlier use of magnetic field gradients and MRI during the 1950s and 1960s (see Physics Today, January 1993, page 94).Schwarzschild does mention the early use of magnetic field gradients to correlate the nuclear signal (usually proton) with spatial coordinates. He cites work by Erwin Harm and by Edward Purcell and me, 1 1. E. L. Hahn, Phys. Rev. 80, 580 (1950); https://doi.org/10.1103/PhysRev.80.580 H. Y. Carr, E. M. Purcell, Phys Rev. 94, 630 (1954). https://doi.org/10.1103/PhysRev.94.630 but that work is not directly related to internal macroscopic imaging. Hahn used existing gradients to observe diffusion, while Purcell and I intentionally applied additional gradients to study diffusion. For modern internal MRI, another component in addition to the applied gradient is necessary. Unlike for diffusion or flow studies, the object being observed must have spatially inhomogeneous internal structure. To the best of my knowledge, the first time such a field gradient was applied to a nuclear magnetic resonance (NMR) sample having an inhomogeneously distributed internal spin system was during a demonstration I conducted for my PhD thesis. 2 2. H. Y. Carr, “Free Precession Techniques in Nuclear Magnetic Resonance,” PhD thesis, Harvard U., Cambridge, MA (1952); Encyclopedia of Nuclear Magnetic Resonance, vol. 1, Wiley and Sons, Hoboken, NJ (1996), p. 253. My motivation in that demonstration was to help faculty and other graduate students understand the recently discovered and rather complicated NMR fine structure in ethyl alcohol. A special object, built for the demonstration, consisted initially of two and later three small separated groups of protons having numbers of protons in the ratios of 3:2:1. The groups were placed in a straight line parallel to the direction of the magnetic field gradient. Thus that first MRI image or spin map was one-dimensional. Although medical applications for 1D imaging are very few, in physics there are several; the most notable was in the discovery of superfluid helium-3. The 1952 demonstration used the new and faster pulsed NMR technique 3 3. D. D. Stark, W. G. Bradley, Magnetic Resonance Imaging, C. V. Mosby, St. Louis, MO (1988), p. 9. rather than the simpler and slower continuous-wave frequency (or field) sweep technique. 4 4. C. Leon Partain et al. , Nuclear Magnetic Resonance (NMR) Imaging, W. B. Saunders, Philadelphia (1983), chap. 17. The visual observation of the three separated groups of protons provided confidence in the pulsed MRI technique with its spatial image coming from the Fourier transform of the complicated time-dependent free-induction decay. That first MRI demonstration was not reported in the 1954 paper, which concentrated on the basic techniques of the new pulsed NMR used with internally homogeneous samples. Other topics, including gradient echoes, electric shims, and the spinning of the samples to obtain narrower lines for observing very fine structure, were left for later papers.My thesis received significant distribution. In particular, Nuclear Magnetic Resonance Specialties near Pittsburgh, Pennsylvania, where Lauterbur and Raymond Damadian did their initial MRI work, received a copy in 1967. Shortly thereafter, the president of the company requested additional copies for some of the purchasers of the company’s pulsed NMR spectrometers.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. E. L. Hahn, Phys. Rev. 80, 580 (1950); https://doi.org/10.1103/PhysRev.80.580 , Google ScholarCrossref, ISI H. Y. Carr, E. M. Purcell, Phys Rev. 94, 630 (1954). https://doi.org/10.1103/PhysRev.94.630 , , Google ScholarCrossref, ISI2. H. Y. Carr, “Free Precession Techniques in Nuclear Magnetic Resonance,” PhD thesis, Harvard U., Cambridge, MA (1952); Google Scholar Encyclopedia of Nuclear Magnetic Resonance, vol. 1, Wiley and Sons, Hoboken, NJ (1996), p. 253. Google Scholar3. D. D. Stark, W. G. Bradley, Magnetic Resonance Imaging, C. V. Mosby, St. Louis, MO (1988), p. 9. Google Scholar4. C. Leon Partain et al. , Nuclear Magnetic Resonance (NMR) Imaging, W. B. Saunders, Philadelphia (1983), chap. 17. Google Scholar© 2004 American Institute of Physics.

Augmentation of Oxygen Transport by Various Hemoglobins as Determined by Pulsed Field Gradient NMR

It is well known that the internal gradient (gi) that exists within pores haunts the diffusion coefficient (D) as measured by the pulsed-field gradient (PFG) nuclear magnetic resonance (NMR). Several PFG-NMR methods developed to determine accurate D were not successful. Then, the steady-state diffusion coefficient (Dapp,∞) for the cation [C4mim]+ of [C4mim][Tf2N] [1-butyl-3-methylimidazolium][bis(trifluoromethylsulfonyl)imde] ionic liquid confined in ordered mesoporous carbon (OMC) was determined by comparing Dapp,∞ obtained from 1H PFG-NMR performed with three different stimulated echo sequences: STE, alternating pulsed-field gradient (APFG), and magic pulsed-field gradient (MPFG) under the two external magnetic field strengths, B0 = 9.4 and 14.1 T. The measured Dapp,∞, which is an order of magnitude smaller than D of bulk [C4mim][Tf2N], is in good agreement between APFG and MPFG in both B0 = 9.4 and 14.1 T. However, the strong gi artifact, which caused apparent diffusion coefficient (Dapp), depending strongly and weakly on B0 and temperature, respectively, in diffusion time-dependent Dapp, Dapp(Δ), obtained from a sequence with monopolar gradients (STE), was suppressed by using sequences employing bipolar gradients (APFG and MPFG) in the region of steady-state diffusion. However, the incompletely suppressed gi artifact resulting in the different behaviors of the early part of Dapp(Δ) between the sequences leads to α ≈ 0.6 and 0.9 in MPFG and APFG, respectively, in the relationship between mean-squared displacement and diffusion time: ⟨z(t)2⟩ = 2Dtα, where α = 0.5 and 1 for one-dimensional single-file diffusion and 3D bulk diffusion, respectively. The above observations clearly show that the diffusion behavior of ions/molecules within the pores and pore structure, such as the surface-to-volume ratio (Dapp(Δ)=D0[1−49πSVD0Δ]) and tortuosity (T ≡ D0/Dapp,∞), is possible to be misunderstood, especially in systems with a non-negligible gi. This work demonstrates that it may be necessary to test several PFG sequences under multiple external magnetic fields for the correct determination of the diffusion behavior of ions or molecules in the pores with a larger internal gradient, gi.

Many naturally occurring fluids, such as crude oils, consist of a very large number of components. It is often of interest to determine the composition of the fluids in situ. Diffusion coefficients and nuclear magnetic resonance (NMR) relaxation times can be measured in situ and depend on the size of the molecules. It has been shown [D. E. Freed et al., Phys. Rev. Lett. 94, 067602 (2005)] that the diffusion coefficient of each component in a mixture of alkanes follows a scaling law in the chain length of that molecule and in the mean chain length of the mixture, and these relations were used to determine the chain length distribution of crude oils from NMR diffusion measurements. In this paper, the behavior of NMR relaxation times in mixtures of chain molecules is addressed. The author explains why one would expect scaling laws for the transverse and longitudinal relaxation times of mixtures of short chain molecules and mixtures of alkanes, in particular. It is shown how the power law dependence on the chain length can be calculated from the scaling laws for the translational diffusion coefficients. The author fits the literature data for NMR relaxation in binary mixtures of alkanes and finds that its dependence on chain length agrees with the theory. Lastly, it is shown how the scaling laws in the chain length and the mean chain length can be used to determine the chain length distribution in crude oils that are high in saturates. A good fit is obtained between the NMR-derived chain length distributions and the ones from gas chromatography.

In the last few years there has been rapid progress in the application of nuclear magnetic resonance clinical imaging as a diagnostic aid (Partain et al, 1983; Steiner, 1983; Worthington, 1983). Records kept by the operators of NMR imaging equipment indicate that about 1800 individuals have now been examined without incident within the United Kingdom. In view of this progress, an advisory group (Appendix) appointed by the National Radiological Protection Board recommends some modifications to the original advice given in 1981 (NRPB, 1981). The principal modifications are that: (1) It is no longer considered necessary to exclude volunteers with a history of epilepsy or cardiac disease. (2) It is not obligatory that a registered medical practitioner trained in resuscitation techniques be present during an exposure. (3) Higher limits are recommended for rates of change of magnetic flux density when the period of change is less than 10 ms. (4) The radiofrequency limit of 15 MHz has been removed. Theoretical considerations indicate that the interaction of an applied magnetic field with the intrinsic magnetic moments of protons and electrons within a molecule could alter its rate of chemical reaction (McLauchlan, 1981). Chemical reactions involving radical pair intermediates are expected to be sensitive to magnetic fields for this reason and a number of enzyme reactions that might be expected to be affected have been identified (A. Cass, personal communication). However, there is no experimental confirmation of these theoretical possibilities at present.

Decision letter: Dynamic Na+/H+ exchanger 1 (NHE1) – calmodulin complexes of varying stoichiometry and structure regulate Ca2+-dependent NHE1 activation

The diffusion behavior of hydroxyl protons (OH) in quercetin in 100% DMSO-d6 (deuterium dimethylsulfoxide) and a 90% DMSO-d6 solution containing 10% H2O was investigated with 600 MHz 1H pulsed field gradient (PFG) nuclear magnetic resonance (NMR). Only resonances of the 5-hydroxyl protons (OH5) were well resolved in NMR spectra of quercetin for all solutions under study. This phenomenon is explained by the intramolecular hydrogen bonding between OH5 protons and the 4-carbonyl oxygen (CO4). During diffusion experiments, the OH5 protons showed a biexponential diffusion decay, indicating an exchange process with water. As water content in the solvents increased, the lifetime (τ q) of the OH5 protons decreased from 96.7±10.0 ms in 100% DMSO-d6 to 14.3±1.4 ms in the 90% DMSO-d6 solution containing 10% H2O, indicating an increase in the exchange rate (k q = l/τ q) of the OH5 protons. This study demonstrates that the diffusion approach with PFG-NMR is much faster and easier for estimating the short lifetime or fast exchange rate of hydroxyl protons in quercetin.

In this paper, we evaluated the particle dispersion degree of alumina slurry containing a dispersant by solvent nuclear magnetic resonance (NMR) relaxation and compared it with conventional dispersion evaluation methods such as viscosity, particle size, and sedimentation height measurements. The dispersion of slurry was evaluated via numerical analysis of the transverse relaxation time (T2 ). The effect of the changes in different parameters of the experiment in terms of milling time, solid loading, and dispersant amount was investigated by NMR relaxation as well as conventional methods. The results of NMR relaxation measurements revealed that T2 correlates well with other dispersion evaluation methods; thus, it is an efficient technique to evaluate the dispersion of alumina slurry, specifically, when studying the effect of the change in milling time and dispersion amount.