Pulsed Nuclear Magnetic Resonance Method Research Articles

Improved Methods for Characterizing Emulsions by Low-Resolution Nuclear Magnetic Resonance via Surface Relaxation and One-Dimensional Images

The use of nuclear magnetic resonance (NMR) is an appropriate tool for studying colloids in a non-invasive manner. Droplet size distributions and one-dimensional sample profiles are readily produced to characterize an emulsion, its stability, the size distribution of the dispersed phase, and rheological behavior with respect to parameters as temperature and/or water cut. Here, we present pulsed field Grgdient NMR methods that improve the performance as compared to existing methods. In particular, the so-called multi-echo approach is introduced to enhance the signal-to-noise ratio significantly making it possible to characterize emulsions in a minute or less. Thus, any evolution that takes place in the order of just a few minutes can be monitored. In addition to the multi-echo approach, an improved method for determining the droplet size distribution from a residual emulsion, i.e., in the presence of a bulk water phase, is presented.

Measurement of hydrogen dispersion in rock cores using benchtop NMR

Electrolysis followed by underground hydrogen storage (UHS) in both salt caverns and depleted oil and gas reservoirs is widely considered as a potential option to overcome fluctuations in energy provision from intermittent renewable sources. Particularly in the case of depleted oil and gas reservoirs, a denser layer of cushion gas (N2, CH4 or CO2) can be accommodated in these storage volumes to allow for sufficient system pressure control as hydrogen is periodically injected and extracted. These gases/fluids are however fully soluble with hydrogen and thus with sufficient mixing can undesirably contaminate the extracted hydrogen product. Fluid mixing in a porous medium is typically characterized by a dispersion coefficient (KL), which is hence a critical input parameter into reservoir simulations of underground hydrogen storage. Such dispersion data is however not readily available in the literature for hydrogen at relevant storage conditions. Here we have developed and demonstrated novel methodology for the measurement of KL between hydrogen and nitrogen in a Berea sandstone at 50 bar as a function of displacement velocity (0.007–0.722 mm/s). This leverages off previous work quantifying KL between carbon dioxide and methane in rock cores relevant to enhanced gas recovery (EGR). This used infrared (IR) spectroscopy to differentiate the two fluids, hydrogen is however IR invisible. Hence the required time-resolved quantification of hydrogen concentration emerging from the rock core is uniquely performed here using bench-top nuclear magnetic resonance (NMR). The resultant hydrogen-nitrogen dispersion data as a function of displacement velocity allows for the determination of dispersivity (α = 0.31 mm). This intrinsic rock property compares favorably with previous CO2 dispersion measurements on similar sandstones, hence validating our methodology to some extent. In addition, at very low velocities, determination of the rock core tortuosity (τ, another intrinsic rock property) produces a value (τ = 10.9) that is similar to that measurement independently using pulsed field gradient NMR methods (τ = 11.3).

Magneto-resonance methods of the relaxation rate measuring for the proton-containing flowing fluids composition studying

In the present work, the existing pulsed nuclear magnetic resonance methods for determining the longitudinal relaxation time in a condensed medium are analyzed. There is considered an alternative method of the nuclear magnetic momentums relaxation times determining in the current proton-containing liquid by the functional dependence of the detected signal intensity from the amplitude of the magnetic field modulation in the magnet-analyzer.

Spin Diffusion in 3He, Adsorbed Nanostructured Material MCM-41

The pulsed nuclear magnetic resonance method was used for the first time to study diffusion processes in 3He, adsorbed nanostructured material MCM-41. The material contained bundles of nanotubes with internal cylindrical channels whose diameter was 2.5 nm. The measurements were made using the Hahn–Carr–Purcell spin echo method with a sequence of 90°–τ–180° probing pulses, where τ is the time interval between pulses. Two different diffusion processes were detected in the system, with diffusion coefficients differing by an order of magnitude. This result was confirmed by an independent multi-echo method with a pair of 90°–τ–120° probing pulses, and with a noticeable increase in the duration of the pulses themselves. It was also found that both diffusion coefficients depend on the value of τ, which is a characteristic feature of restricted diffusion. The results obtained set new directions for the further study of this system.

Over 50 % 1H and 13C Polarization for Generating Hyperpolarized Metabolites-A para-Hydrogen Approach.

para‐Hydrogen‐induced polarization (PHIP) is a method to rapidly generate hyperpolarized compounds, enhancing the signal of nuclear magnetic resonance (NMR) experiments by several thousand‐fold. The hyperpolarization of metabolites and their use as contrast agents in vivo is an emerging diagnostic technique. High degrees of polarization and extended polarization lifetime are necessary requirements for the detection of metabolites in vivo. Here, we present pulsed NMR methods for obtaining hyperpolarized magnetization in two metabolites. We demonstrate that the hydrogenation with para‐hydrogen of perdeuterated vinyl acetate allows us to create hyperpolarized ethyl acetate with close to 60 % 1H two‐spin order. With nearly 100 % efficiency, this order can either be transferred to 1H in‐phase magnetization or 13C magnetization of the carbonyl function. Close to 60 % polarization is experimentally verified for both nuclei. Cleavage of the ethyl acetate precursor in a 20 s reaction yields ethanol with approximately 27 % 1H polarization and acetate with around 20 % 13C polarization. This development will open new opportunities to generate metabolic contrast agents in less than one minute.

Improvements in Measuring Unfrozen Water in Frozen Soils Using the Pulsed Nuclear Magnetic Resonance Method

The pulsed nuclear magnetic resonance (P-NMR) method is commonly used to determine unfrozen water content in frozen soils. Traditionally, unfrozen water content is calculated using first return data from the free induction decay (FID) signal intensity (SI); however, these data may contain both ice and liquid water responses, thus overestimating unfrozen water. To address this, the authors used the normalization method for calculating unfrozen water. First, the authors refined a P-NMR testing system for greater temperature control. Next, testing using two P-NMR instruments at different facilities yielded repeatable unfrozen water measurements to within 2.6% of each other. Results from a desorption test indicated excellent agreement (with an average difference of 0.33%) between P-NMR-derived unfrozen water and physical gravimetric water content data. Finally, comparing results from the normalization method to the first return data indicated that calculated unfrozen water in a frozen soil is up to 3.3 times greater for a given temperature using the first return data versus the normalization method. Because this difference is significant, the authors recommend further investigation using an independent method to confirm the results.

Water transport in maize roots under the influence of mercuric chloride and water stress: a role of water channels

The influence of inhibitor of water channels, HgCl2, on water diffusion in maize (Zea mays L.) seedling roots was investigated with the pulsed nuclear magnetic resonance (NMR) method. Blocking of water channels decreased the water permeability of cell membranes by 1.5 – 2 times. This effect of HgCl2 was exhibited only in the roots of seedlings grown in a nutrient solution containing Ca2+ and was reversed with Hg-scavenging agent β-mercaptoethanol. Subsequent incubation of Ca2+-deficient roots in the nutrient solution with Ca2+ recovered the sensitivity to HgCl2. The water stress decreased water diffusion rates similarly to HgCl2 and the effects of water stress and HgCl2 were not additive. The obtained data demonstrate the possibilities of the pulsed NMR method for study of the transmembrane water exchange in vivo in connection with water channel functioning.

Continuous-wave NMR imaging of solids.

Current pulsed nuclear magnetic resonance methods of imaging samples such as solids with short spin-spin relaxation times are restricted to use with T2 values longer than approximately 10 microseconds. In the present study a method of imaging ultra-short T2 samples using continuous- wave, swept-field NMR is presented that, in principle, will be able to overcome this restriction. The technique is identical to that used in continuous-wave electron paramagnetic resonance imaging of paramagnetic species and involves irradiating the sample continuously with a radiofrequency excitation in the presence of a strong stationary magnetic field gradient. When the main magnetic field is swept over a suitable range, the variation of the NMR absorption signal with applied magnetic field yields a one-dimensional projection of the object under study along the gradient direction. Two- or three-dimensional image data sets may be reconstructed from projections that are obtained by applying the gradient in different directions. Signal-to-noise ratio can be improved by modulating the magnetic field and employing a lock-in amplifier to recover signal variations at the audio modulation frequency. Preliminary experiments were performed using a 7 Tesla magnet and a 300 MHz continuous-wave radiofrequency bridge with lock-in detection. The apparatus is described and the results of pilot experiments that employed vulcanized rubber samples are presented. The ability of the technique to detect short T2 samples was demonstrated by the presence of a background signal from the Perspex former of the birdcage resonator used for signal reception.

Seed oil determination without weighing and drying the seeds by combined free induction decay and spin‐echo nuclear magnetic resonance signals

AbstractThe free induction decay (FID) and echo signals in a spin‐echo pulse sequence have been used for seed oil determination without weighing and drying the seeds with nuclear magnetic resonance (NMR) equipment that has a magnetic field of low homogeneity. Earlier known methods, based on use of the FID signal to determine seed oil, become inapplicable when the magnetic field homogeneity is poor, because the angular position of seed significantly affects the signal. The present method, which elegantly eliminates the angular dependence, involves sampling the FID signal at 10 µs after a 90° pulse and the subsequent echo signal at 100 µs formed by applying a 180° pulse at 50 µs. Such short pulse spacing in spin‐echo sequence produces almost a full oil signal. It also eliminates the effects of sample‐to‐sample variation inT2 and molecular diffusion on oil signal. The oil values obtained by this method are in good agreement (correlation for mustard: 0.952; linseed: 0.99; and for peanut: 0.912) with the values obtained by the well established and accurate pulsed NMR method, which is based on the measurement of the FID signal of oil in dried and weighed seeds.

Simultaneous determination of oil and water contents in different oilseeds by pulsed nuclear magnetic resonance

AbstractRoutine analysis of oil and water contents in different oilseeds with the Bruker Minispec pulsed nuclear magnetic resonance (NMR) analyzer (Bruker Analytische Mestechnik, Rheinstetten, Germany) is rapid (16 s measurements), precise, accurate and nondestructive. In 1991, subcommittee SC2 (oil seeds) of the Technical Committee Number 34 [organized by the International Standards Organization (ISO), Geneva, Switzerland] organized an international collaborative study for pulsed NMR analysis of oil content in whole rapessed. An additional study was performed in 1993 to extend the method to sunflower, linseed and soy seeds. The Draft International Standard ISO DIS/10565 (Draft International Standards, International Standards Organization, Geneva, 1993) describes the procedure of the pulsed NMR method for determination of oil content in rapeseed and presents the interlaboratory comparison results based on the calculations described in the Normative ISO 5725 (Draft International Standards, International Standards Organization, Geneva, 1993). The standard was approved in 1992 and will be published in 1993. The interlaboratory collaborative studies showed that the analyses of oil and moisture by pulsed NMR were either comparable or more repeatable than measurements done by the traditional methods. No statistical differences between determinations by the traditional and pulsed NMR methods were found. Simultaneous determination of percent moisture and percent oil content in whole seeds is possible with pulsed NMR by the spin‐echo method. In addition, multiple components of the oil can be detected and quantitated by T2 analysis from the Carr‐Purcell‐Meiboom‐Gill pulse sequence. The instrument is easy to calibrate with whole oilseeds, and the calibration can be checked periodically with the same seeds because the measurement is nondestructive. Pulsed NMR provides a rapid alternative to the long, laborious, traditional methods of analysis and offers substantial long‐term savings of both time and money. Minimal operator training is required once the technique has been established for routine use.

Water distribution in mixtures of soy protein isolates with starch and maltodextrin

AbstractHydration of soy protein isolate and its mixtures with potato starch and starch hydrolyzate is studied by the pulsed nuclear magnetic resonance method. As the water content varies between 0.05 and 3 g per gram solid, the relaxation of magnetization of protons of studied systems is approximated by two exponents with the spin‐spin relaxation times falling within the ranges 2 to 10 ms and 20 to 200 ms. The values of relaxation times varying from 2 to 10 ms are attributed to the limited mobility of water molecules due to their interaction with the binding centers of macromolecules. Variations in the relaxation time from 20 to 200 ms with increasing water content is related to the limited mobility of water molecules as a result of two successive processes, namely filling and contraction of capillaries at low moisture content and formation of a fluctuation network of interactions between macromolecules at high water contents. The hydration of soy protein isolate‐starch mixtures is noted for a number of features. Water is found to redistribute in mixtures in a more hydrophilic component.

4486709 Depth and refocusing pulses for use with inhomogeneous radiofrequency coils in nuclear magnetic resonance spectroscopy

In a pulsed nuclear magnetic resonance method of analysis, in which a single radiofrequency pulse is applied using an irradiation coil which provides an inhomogeneous radiofrequency field across the sample volume, the inhomogeneity of this field ensures that some sample regions experience a radiofrequency pulse which is closer to a π/2 pulse angle than for other regions, resulting in a larger signal intensity from the former regions as compared to the latter. If a second pulse is applied after the first, prior to acquisition of the signal, and if the phase of this pulse is alternated through the four phases: 0°, 90°, 180° and 270°, during a series of transients, then, provided the receiver phase is changed by 180° when the second pulse phase is changed by 90°, the signal intensity will accumulate from regions where the second pulse angle is π radians and there will be discrimination against regions where the second pulse angle differs markedly from π radians. This discrimination towards certain sample regions can be improved by adding further pulses after the second pulse provided the phase of each additional pulse is alternated through all four 90° phase shifts, during a series of transients, independently of the phase alternation of the other pulses, and provided that for each individual 90° phase shift, the receiver phase is changed by 180°. In its simplest form, such a series of pulses may be represented by θ;(2θ[±x,±y]) n indicating n phase alternated pulses additional to the first excitation pulse, where these additional pulses are twice as long as the first pulse.

Quadrupolar relaxation parameters for a spin7/2system

Using pulsed nuclear magnetic resonance methods the central component magnetisation recovery curves have been measured for 59Co in a single crystal of K3Co(CN)6 at 295K. By using two different sets of initial conditions it has been possible to measure the parameters W1 and W2 which are related to the transition rates for Delta m=+or-1 and Delta m=+or-2 transitions. Previous relaxation measurements have suggested that an optical mode of the Co(CN)6 octahedron is responsible for spin-lattice relaxation in the temperature interval 100-300K. The present value of 0.2+or-0.06 obtained for the ratio W2/W1, together with theoretical calculations, suggests that a band of several overlapping modes, of wavenumber approximately 412 cm-1, is involved in relaxation.

Quantitative analysis of food products by pulsed NMR

SummaryWater contents in the range 1–5% for skim milk powder and 77–81% for cottage cheese curds have been determined quantitatively by pulsed nuclear magnetic resonance methods. The procedures fro establishing optimum operating conditions for a commercial pulsed NMR process analyser are described, and temperature‐dependences of the analytical results are given. For skim milk powders, inegrated measurement times of 72 sec per sample gave a standard deviation of 0.2% water, while at the other end of the range the cottage cheese samples yielded 0.3% standard deviastion in the water content with measurement times of 10 sec per sample. The results demonstrate the versatility of the pulsed NMR technique for analysis of food products over a very wide range of water contents.

Molecular motions in bisphenol a polycarbonates as measured by pulsed NMR techniques. II. Blends, block copolymers, and composites

Molecular motions in several polyblends and composites based on bisphenol A polycarbonate were investigated over a wide temperature range by means of two pulsed nuclear magnetic resonance methods: the spin-lattice relaxation time T1 and the spin-lattice relaxation time in rotating frame T1ρ. Characteristic changes in the transitions of the polyblends and composites with respect to the transitions of unmodified homopolymers and copolymers were observed. By selecting different types and quantities of materials to modify the matrix (bisphenol A polycarbonate) these changes were analyzed. It was found that the multiple transitions in the composites and polyblends were not always combinations of the transitions present in the constituent materials, but depended on compatibility of the polymers and the type of molecular motions of the individual components. Unlike other methods of investigating polymers in bulk, nuclear magnetic relaxation methods are sensitive to supramolecular structure or morphology. Supplemented with transmission and scanning electron micrographs, the results from these experiments led to the postulation of interaction domains or zones between the two phases in certain nonhomogeneous polymer systems in which the motions in one phase (usually the continuous phase) were affected by the motions in the other phase (usually the dispersed phase). Information on the nature and extent of this interaction was obtained by the NMR relaxation methods. The experimental results reflect not only the presence of separate phases in the nonhomogeneous materials, but also the complex heterogeneity of such systems. The results suggest correlations between internal molecular motions and physical properties of the materials examined. Based on the above concepts, a mechanism of rubber reinforcement was proposed. The impact strength of a rubber-modified polymer is related to the apparent volume of the rubber phase. This volume consists of the actual volume of the rubber plus the affected portion of the surrounding glassy matrix which, assisted by the segmental motions of the rubber, assumes the same motions.

Molecular motions in bisphenol a polycarbonates as measured by pulsed NMR techniques. I. Homopolymers and copolymers

Molecular motions in homopolymers and copolymers based on bisphenol A polycarbonate (BAP) were investigated over a wide temperature range by means of two pulsed nuclear magnetic resonance methods: the spin-lattice relaxation time T1 and the spin-lattice relaxation time in rotating frame, T1ρ. Nuclear magnetic relaxation measurements of bisphenol A polycarbonate indicated the existence of four transitions in the solid amorphous state. This included the glass transition Tα, two transitions associated with localized motions of the main chain, Tβ and Tγ, and a transition due to side-group motion, Tδ. The results offer new evidence for the transition Tβ, which is ascribed to localized segmental motions of the polycarbonate chain. The physical properties of the polymer (high impact strength and a large fractional free volume in the glassy state) can be explained by the presence of the two secondary transitions Tβ and Tγ, which are both associated with main-chain motions. The presence of the bulky bromine on a bisphenol A unit of a polymer chain in a position ortho to the carbonate groups restricts all motions of the main chain, thus increasing Tα, Tβ, and Tγ. As a result of an increase in the secondary transition temperatures, impact strength is reduced at least 20-fold as compared with the unsubstituted material. The results for tetrabromobisphenol A polycarbonate and copolymers of bisphenol A with tetrabromobisphenol A and bisphenol A with tetrachlorobisphenol A polycarbonate confirm the idea that the impact strength of the polymer is related to the secondary transitions (T < Tg) arising from the main-chain motions. Copolymers of bisphenol A polycarbonate with tetrabromobisphenol A polycarbonate showed a single glass transition temperature whose value lay between those of the homopolymers. The results indicated multiple secondary transitions which corresponded to the transitions of the homopolymers with magnitudes proportional to the comonomer content.

A study of the interaction of manganese ions with ATP by 31P Fourier-transform nuclear-magnetic resonance.

The spin‐lattice relaxation rates of the phosphorous nucelei in ATP have been measured as a function of temperature in the presence of manganese perchlorate using pulsed nuclear magnetic resonance methods. Detailed interpretation of the results in terms of the dipolar interaction between the electron and nuclear spins leads to the conclusion that the α‐phosphorous atom is more distant from the manganese ion than the β‐ or γ‐phosphorous atoms, which are equidistant from the metal ion within experimental error. The rate constant for the manganese ion dissociating from the complex is found to be 150 ms−1 at 25°C.

Data by NMR and Representations of Self-Diffusion Coefficients in Carbon Tetrafluoride and the Determination of Intermolecular Force Constants

Self-diffusion coefficients were measured by the pulsed nuclear magnetic resonance method for pure carbon tetrafluoride for the conditions −30 to 75°C and 20–440 atm. As predicted by the dilute gas theory, the density—diffusivity product (ρD) versus density (ρ) at constant temperature was found to be almost constant up to a reduced density of about 0.4, above which ρD started decreasing sharply. Based on the corresponding states principle, three semiempirical representations for the data were developed and compared. The low-density data were used, along with previously published second virial coefficient data, to evaluate intermolecular force constants for the Lennard-Jones (6–12) potential and the modified Buckingham (6-exp) potential.

Upper Limits to Electric-Field-Induced Nuclear Magnetic Dipole—Dipole Couplings in Polar Liquids

The alignment of polar molecules in liquids by an external electric field is investigated by pulsed nuclear magnetic resonance methods. Sensitive rotating-frame techniques are described for the detection of the expected induced nuclear magnetic dipole—dipole coupling. In no case could an induced coupling be detected. The upper experimental limits for the induced couplings disagree with the predictions of the Lorentz localfield model. For nitrobenzene and p-nitrotoluene, disagreement is also obtained with the Onsager local-field model by a reduction of almost an order of magnitude in the expected size of the effect. The negative results for these two compounds are indicative of strong local antiparallel ordering of the molecular electric dipoles. The induced coupling in p-nitrotoluene, reported by Buckingham and McLauchlan using standard NMR methods, is found to be absent in this investigation, which employs a measuring technique more sensitive by at least one order of magnitude.

Nuclear Magnetic Resonance of Ni61 in Nickel-Ferrite

The nuclear magnetic resonance (NMR) of Ni 61 in Nickel-ferrite at zero external field has been observed at liquid air temperature by using the pulsed NMR method. The spin echo signal could be observed in the frequency range from 31 Mc/sec to 38 Mc/sec. The center frequency (34.5 Mc/sec) corresponds to the value of about 97 KOe of hyperfine field, which could be accounted for as the sum of the core polarized and the orbital hyperfine fields of Ni 2+ ion. This value is consistent with the hyperfine coupling constant of paramagnetic Ni 2+ ion diluted in Al 2 O 3 .