Low-field NMR Measurements Research Articles

Assessment of hydrogen adsorption in high specific surface geomaterials using low-field NMR - Implications for storage and field characterization

Natural and produced hydrogen will play a central role in decarbonizing the economy. High specific surface geomaterials can contribute H2 adsorption capacity, affecting both natural systems and storage strategies. Low-field Nuclear Magnetic Resonance NMR provides unique insights into H2 gas adsorption on nanoporous geomaterials. The cumulative NMR amplitude correlates linearly with the hydrogen mass; furthermore, measurements reveal two relaxations: a short-time relaxation for adsorbed H2 gas, and a long-time pressure-dependent relaxation for free H2 gas within pores. NMR measurements and complementary Grand Canonical Monte Carlo simulations show 2.5-to-4 times higher density in adsorbed H2 than bulk H2. The H2 adsorption capacity varies with mineral composition, but in all cases, it increases with specific surface area. Therefore, hydrogen physisorption in nanoporous geomaterials improves storability with respect to single-phase pure gas storage when high specific surface bentonites (at low pressure) or activated carbon (at all pressures) are the adsorbents. While synthetic adsorbents may exhibit higher H2 storage capacities, the low cost of high specific surface geomaterials, the reduced storage buoyancy, and safer controlled release make these natural adsorbents particularly attractive for large-scale storage applications. The insights gained with low-field NMR measurements in the laboratory are directly applicable to the interpretation of down-hole NMR characterization tools in the field.

Acid-Induced Gelation of Carboxymethylcellulose Solutions.

The present work offers a comprehensive description of the acid-induced gelation of carboxymethylcellulose (CMC), a water-soluble derivative of cellulose broadly used in numerous applications ranging from food packaging to biomedical engineering. Linear viscoelastic properties measured at various pH and CMC contents allow us to build a sol-gel phase diagram and show that CMC gels exhibit broad power-law viscoelastic spectra that can be rescaled onto a master curve following a time-composition superposition principle. These results demonstrate the microstructural self-similarity of CMC gels and inspire a mean-field model based on hydrophobic interchain association that accounts for the sol-gel boundary over the entire range of CMC content under study. Neutron scattering experiments further confirm this picture and suggest that CMC gels comprise a fibrous network cross-linked by aggregates. Finally, low-field NMR measurements offer an original signature of acid-induced gelation from a solvent perspective. Altogether, these results open avenues for the precise manipulation and control of CMC-based hydrogels.

Quantifying Gas Storage and Transport in an Intact Core Sample Using Low-Field NMR and Pressure Measurements

Quantifying Gas Storage and Transport in an Intact Core Sample Using Low-Field NMR and Pressure Measurements

What is the best spectroscopic method for simultaneous analysis of organic acids and (poly)saccharides in biological matrices: Example of Aloe vera extracts?

Several species of (poly)saccharides and organic acids can be found often simultaneously in various biological matrices, e.g., fruits, plant materials, and biological fluids. The analysis of such matrices sometimes represents a challenging task. Using Aloe vera (A. vera) plant materials as an example, the performance of several spectroscopic methods (80 MHz benchtop NMR, NIR, ATR-FTIR and UV–vis) for the simultaneous analysis of quality parameters of this plant material was compared. The determined parameters include (poly)saccharides such as aloverose, fructose and glucose as well as organic acids (malic, lactic, citric, isocitric, acetic, fumaric, benzoic and sorbic acids). 500 MHz NMR and high-performance liquid chromatography (HPLC) were used as the reference methods.UV–vis data can be used only for identification of added preservatives (benzoic and sorbic acids) and drying agent (maltodextrin) and semiquantitative analysis of malic acid. NIR and MIR spectroscopies combined with multivariate regression can deliver more informative overview of A. vera extracts being able to additionally quantify glucose, aloverose, citric, isocitric, malic, lactic acids and fructose. Low-field NMR measurements can be used for the quantification of aloverose, glucose, malic, lactic, acetic, and benzoic acids. The benchtop NMR method was successfully validated in terms of robustness, stability, precision, reproducibility and limit of detection (LOD) and quantification (LOQ), respectively.All spectroscopic techniques are useful for the screening of (poly)saccharides and organic acids in plant extracts and should be applied according to its availability as well as information and confidence required for the specific analytical goal. Benchtop NMR spectroscopy seems to be the most feasible solution for quality control of A. vera products.

Oil dispersed nickel-based catalyst for catalytic upgrading of heavy oil using supercritical water

Upgrading of heavy oil using supercritical water (SCW) has attracted a lot of attention recently due to the unique properties of SCW to act as hydrogen donor source and good solvent for organic matters. In this study, nickel-based oil-dispersed catalyst was used to intensify heavy oil upgrading in SCW. Its catalytic activity on upgrading performance and its influence on environment (from the point of the contamination of wastewater obtained after SCW treatments) were assessed. The result revealed that Ni-based catalysts increased the yield of upgraded oil to 89.73% from 81.99%, while reduced the yield of gases and coke to 8% and 2.27% from 14.92% and 3.09%, respectively. With catalysts, the heavy oil (API gravity 14° and viscosity 2073 mPa·s) was upgraded to light oil (API gravity 26.10° and viscosity 19.8 mPa·s) with a higher H/C ratio and lower content of S, N, and metals. Moreover, an obvious change in the composition and structure of upgraded oil was found, including a significant decrease in resins and asphaltenes content with an increase in saturate content; an increase in C7-C20 alkanes content with a decrease in C21-C30 alkanes; a significant conversion of poly-aromatics into di-aromatics; a decrease in the mean chain length (MCL), aromaticity factor (FCA), aromatic (Car carbons), and secondary + quaternary group (Csq) with an increase in primary group (−CH3, Cp) and tertiary group (Ct) in the structure of resins and asphaltenes, etc., proved by SARA analysis, elemental analysis, FTIR spectra, GC, GC–MS, and 13C NMR spectroscopy. In addition, we found that waste water was less contaminated by oil and paramagnetic ions, confirmed by visual observation, scanning optical microscope, and low-field NMR measurements. All these significant improvements in upgrading performance resulted from the in-situ transformation of oil-dispersed Ni-based catalysts into Ni3S2; Ni9S8; NiS; NiO; and Ni[OH]2 in oil environment. These in-situ formed active phases of Ni acted as electro-catalysts for water splitting reactions to promote the participation of water in upgrading process as hydrogen source, and simultaneously accelerated isomerization, hydrogenation, alkylation, and opening ring reactions, which consequently results in a higher yield of upgraded oil with a better quality as well as less contaminated waste water.

Characterization of oil-in-water pickering emulsions stabilized by β-cyclodextrin systems

Cyclodextrins (CDs) are conical molecules with a hydrophilic surface and a hydrophobic cavity. Thanks to the cavity, CDs are able to form inclusion complexes with a wide variety of guest molecules. A new oil-in-water (O/W) emulsifying system suitable for use in preparing Pickering emulsions with a large range of oils was developed. This system contains β-Cyclodextrin (β-CD) and a natural origin selected surfactant, Polyglyceryl-3 diisostearate (PG3D). The purpose of this work is to better understand the role of the PG3D in the realization and stabilization of these O/W emulsions. With this aim, O/W Pickering emulsions stabilized only by β-CD or β-CD/PG3D blends were prepared, and the effects of the presence of PG3D at different levels on the stability of emulsions were investigated. This original work was implemented using a wide range of complementary analytical methods, especially tensiometry, optical microscopy, laser granulometry, classical NMR and low field NMR measurements. The effectiveness of the β-CD/PG3D emulsifying system for preparing O/W Pickering emulsions appears due to synergistic effects between β-CD and PG3D. These molecules are both acting to stabilize the oil/water interface. The created particles have a high affinity for water with about the same contact angle as found for water. This may explain that only O/W emulsions are obtained using this system as in the case of using β-CD alone. PG3D by decreasing interfacial tension and due to its high wettability of the hydrophobic surfaces helps to get finer emulsions. An optimum β-CD/PG3D ratio must be used to obtain the most stable emulsions.

Fast and Accurate Quantification of Nitrogen and Phosphorus Constituents in Animal Slurries Using NMR Sensor Technology.

The optimal processing of animal slurry with a minimal environmental impact either as an organic fertilizer or as an energy source for biogas production fundamentally requires accurate, fast, cost-effective, and mobile analytical techniques for the measurement of nitrogen and phosphorus in large volumes of liquid animal slurry. Based on more than 300 different slurries from different species and origins, we provide here an extensive analysis of low-field NMR and standard laboratory measurements for animal slurry analysis. It is found that low-field NMR provides higher precision than wet chemistry laboratory measurements for ammonium nitrogen and total nitrogen, while it provides slightly lower precision for total phosphorus measurements. Low-field NMR may, through a square-root dependency between time and precision, be adapted for analysis at farms, in slurry tankers/transporters, in biogas digesters, or in laboratories.

A New Apparatus for Coupled Low-Field NMR and Ultrasonic Measurements in Rocks at Reservoir Conditions

A New Apparatus for Coupled Low-Field NMR and Ultrasonic Measurements in Rocks at Reservoir Conditions

Impact of semi-solid formulations on skin penetration of iron oxide nanoparticles

BackgroundThis work aimed to provide useful information on the incidence of the choice of formulation in semi-solid preparations of iron-oxide nanoparticles (IONs). The appropriate analytical methods to assess the IONs physical stability and the effect of the semi-solid preparations on IONs human skin penetration were discussed. The physical stability of IONs (Dh = 31 ± 4 nm; ζ = −65 ± 5 mV) loaded in five semi-solid preparations (0.3% w/v), namely Carbopol gel (CP), hydroxyethyl cellulose gel (HEC), carboxymethylcellulose gel (CMC), cetomacrogol cream (Cet) and cold cream was assessed by combining DLS and low-field pulsed NMR data. The in vitro penetration of IONs was studied using human epidermis or isolated stratum corneum (SC).ResultsReversible and irreversible IONs aggregates were evidenced only in HEC and CMC, respectively. IONs diffused massively through SC preferentially by an intercellular pathway, as assessed by transmission electron microscopy. The semi-solid preparations differently influenced the IONs penetration as compared to the aqueous suspension. Cet cream allowed the highest permeation and the lowest retained amount, while cold cream and CP favored the accumulation into the skin membrane.ConclusionBasic cutaneous semi-solid preparations could be used to administer IONs without affecting their permeation profile if they maintained their physical stability over time. This property is better discriminated by low-field pulsed NMR measurements than the commonly used DLS measurements.

Simplified matching and tuning experimental receive coils for low-field NMR measurements

This article describes theoretical and practical results of experiments with home-made receive coils for an NMR scanner. A receive coil must ensure good sensitivity of the whole receiving chain, therefore the qualities of the coil are analysed together with a preamplifier. The coil must be tuned to the operating frequency of the scanner and matched to the input resistance of the preamplifier for effective processing of the induced signal. A technique for calculating matching device parameters using an optimization procedure is presented. The accuracy of the tuning and matching is analysed in the article and documented in figures. A technique for tuning the coils utilizing the spectral qualities of the noise produced by the receive coil or flowing through it is presented. The results are documented in figures for two coils and compared with assumed theoretical curves. The qualitative as well as the quantitative agreement is good. It has been proved that by fulfilling some conditions, accurate matching is not necessary and the noise spectrum can be utilized for tuning a receive coil.

Comparison of different detectors in low field NMR measurements

Low-field (LF) liquid sample NMR measurements using different detectors are performed at Larmor frequencies fL ranging from 300 Hz to 35 kHz. Five different sensors cooled with liquid nitrogen are compared: a wire-wound coil, a printed planar copper coil, both resonant with capacitance C and read out by a room-temperature amplifier, a high critical temperature Tc thin film rf (bare) SQUID and two tuned SQUID circuits, one with a wire-wound input coil and another with a high-Tc superconducting tape coil. The resonant frequency of each LC circuit is adjusted to fL. The sensitivity of any LC circuit is determined by its inductance, quality factor, and fL. The magnetic field resolution of the sensors was evaluated with a homogenous magnetic field source at different frequencies and with a small source at different positions. We discuss the signal-to-noise ratio of the LF NMR signals recorded by these detectors and find the high-Tc tape coil to be unsuitable for LF NMR measurement. The homogeneity of the magnetic measurement field is deteriorated by this bulk superconducting object. That effect leads to a dramatic reduction of the decay time of the free induction decay (FID) signal. In contrast, the bare SQUID oriented parallel to the measurement field does not influence the FID significantly.

Chelating phytanyl-EDTA amphiphiles: self-assembly and promise as contrast agents for medical imaging

Chelating amphiphiles of ethylene diamine tetraacetic acid (EDTA) have been synthesized by conjugation to one or two C20 isoprenoid-type chain/s (phytanyl) via an ester bond. EDTA-mono and bis phytanyl (EDTA-MP and EDTA-BP) have the ability to sequester a variety of metal ions, including transition metal ions and paramagnetic metal ions such as manganese (Mn) and gadolinium (Gd). Both amphiphiles and their metal complexes were examined for ordered self-assembled structures in the solid state and as hydrated lyotropic liquid crystalline phases. For the neat materials small and wide angle X-ray scattering (SAXS/WAXS) revealed lamellar structures with molten hydrophobic chains for the mono-phytanyl conjugate, however, neither Gd-EDTA-MP nor Mn-EDTA-MP showed any ordered phases. The bis-phytanyl conjugate displayed a less ordered structure, while its Mn complexed conjugate demonstrated an ordered lamellar phase with molten chains. Lyotropic liquid crystalline structures of these amphiphiles were examined by cross polarizing optical microscopy (POM) and SAXS methods in the presence of water or sodium acetate solution (100 mM, 200 mM, 500 mM and 1 M). EDTA-MP formed a lamellar liquid crystal (Lα), whilst higher order inverse hexagonal (HII) and micellar cubic phases (III) with Fd3m symmetry were displayed by the bis- conjugate. Dispersions of these amphiphiles buffered in sodium acetate solution yielded submicron sized particles with analogous nanostructures (liposomes, hexosomes), which were examined by Synchrotron SAXS and visualized with Cryo-TEM. The influence of pH upon the morphology of the nanostructures was critical, with hexosomes of EDTA-BP (pH = 5.7) converting to liposomes at pH > 6. Dispersed Mn and Gd complexed particles were examined as contrast agents (CAs) for magnetic resonance imaging by low field NMR measurements. Mn and Gd complexed nanostructured particles displayed improved relaxivity over Mn-EDTA and Gd-EDTA, respectively. The formation of highly ordered nanostructured self-assembly materials by these chelating amphiphiles combined with their ability to sequester a variety of metal ions, including paramagnetic metal ions, suggests their potential application as contrast agents for MRI.

Application of low field and solid-state NMR spectroscopy to study the liquid/liquid interface in porous space of clay minerals and shales

In petroleum research understanding displacement, redistribution, and adsorption of oil and water plays an important role. To study complex multi-component systems such as liquid/liquid/mineral interactions in the porous space of clays and shales we applied low field (2 – 15 MHz) and high resolution (300 MHz) NMR spectroscopy. The detailed NMR analysis shows that the results from low field NMR measurements are in good correlation with the solid-state data. Consequently the process of liquid/liquid displacement can be characterised by considering the relaxation times, signal amplitudes and chemical shifts together.

Suppression of ringing in the tuned input circuit of a SQUID detector used in low-fieldNMR measurements

In low-field NMR measurements, we employ a high temperature superconductingquantum interference device (SQUID) as a detector with an inductively coupledliquid-nitrogen-cooled LC tuned input circuit. However, ringing across the LCcircuit appears after the sudden switch-off of the prepolarizing magnetic field. Thisringing leads to instability of the SQUID readout and prevents the acquisition ofshort-relaxation-time signals. We developed and tested two simple and effective FET-basedQ switch circuits with adjustable parameters which suppress the ringing. Each of theseQ switches makes it possible to record free induction decay signals with a Larmor frequencyof 1.2 kHz and an effective relaxation time constant of 30 ms. A gradually changing currentcaused by the release of charges stored in the p–n junction of the FET, which delays theQ value recovery of the LC circuit, can only be observed by the SQUID because of itsfrequency-independent sensitivity.

Overview of low-field NMR measurements using HTS rf-SQUIDs

High-resolution low-field nuclear magnetic resonance (LF-NMR) investigations of liquid samples, recorded using a HTS radio frequency (rf) superconducting quantum interference device (SQUID) are overviewed in this paper. The measurements were performed either in a magnetically shielded room (MSR) or in the Earth’s Magnetic Field (EMF). In MSR, measurements with Larmor frequencies ( f L) ranging from 2 Hz to 40 kHz were demonstrated. The natural spectral linewidth of water and the scalar coupling spectra of 2,2,2-trifluoroethanol and of fluorobenzene were determined. An additional nitrogen-cooled resonant LC input circuit was also introduced in higher f L (>10 kHz) region to enhance the signal-to-noise ratio up to an order of magnitude. The influence of the speed of decay of the polarizing magnetic field ( B P) on the free-induction-decay (FID) signal was analyzed quantitatively. In EMF, NMR spectra of different liquid samples were recorded. To compensate the line broadening due to EMF fluctuations we used frequency-adjusted averaging.

Enhancement of SQUID-detected NMR signals with hyperpolarized liquid 129Xe in a 1 microT magnetic field.

We report an enhancement of proton NMR signals by a factor of 10(6) by cross polarization with hyperpolarized liquid 129Xe in an ultralow magnetic field of 1 microT. The NMR signals from cyclopentane, acetone, and methanol are detected using a commercial high-T(c) SQUID magnetometer with a signal-to-noise ratio of up to 1000 from a single 90 degrees tipping pulse. This technique allows a wide range of low-field NMR measurements and is promising for the detection of intermolecular scalar spin-spin couplings. Scalar intermolecular couplings can produce a shift of the average NMR frequency in a hyperpolarized sample even in the presence of rapid chemical exchange.

NMR of hyperpolarised 3He gas in aerogel

We report preliminary low-field (2 mT and 0.1 T) NMR measurements on room temperature laser polarised 3He gas imbibed into a 98% porous silica aerogel. The hyperpolarised helium is introduced into the cell in which the aerogel had been grown, and large NMR signals can be recorded. Long polarisation lifetimes ( T 1≃50 s) have been observed, revealing a very weak specific wall relaxation in the cell. NMR of hyperpolarised 3He gas is thus a potential tool to measure physical parameters inside the aerogels which are used to study quantum fluids in confined geometries. This has been demonstrated using a multi-echo technique to characterize spin diffusion in an applied magnetic field gradient. Spin diffusion was found to be isotropic and its pressure dependence was studied. An estimate of the mean free path for gas atoms in the aerogel is deduced from this behaviour.

Probing silica aerogel structure with spin-polarised helium-3 gas

We report on systematic low-field NMR measurements on room temperature 3 He gas imbibed into a 98% porous silica aerogel. Laser polarised helium is used to obtain large NMR signals at 0.1 T even for small amounts of gas. We use a multi-echo technique in a controlled magnetic field gradient to characterize spin diffusion. Measurements are carried out for pressures P ranging from 10 mbar to 1 bar (mean free path of free 3 He ranging from 19 to 0.19 μm respectively). In a free gas, the damping rate 1/T 2 of the amplitude of the echoes is checked to be proportional to the known diffusion coefficient (T 2 P). Damping rates 2-5 times smaller than those in a free gas are measured within the aerogel. This reduction is quite significant at 1 bar, suggesting that 3 He- 3 He and 3 He-silica collisions occur with comparable frequencies at this density. However, a stong dependence of T 2 on P is still observed at low pressure, revealing a significant reduction of diffusion due to binary atomic collisions even for the lowest gas density. Measurements over such a large range of gas densities indirectely probe collisional effects over a wide range of sizes up to tens of μm. In our sample, results are not consistent with a single geometrical mean free path for 3 He-silica collisions.

Low field NMR measurements in 3He-A

We are making simultaneous measurements of the longitudinal and transverse resonances in fields of the order of 30 gauss in a parallel plate geometry, defined by mylar films 100 μm apart, so that l is well oriented perpendicular to the static magnetic field. Comparison of our measurements with the axial phase frequency relation indicates that the A-phase is definitely axial at 29.3 bars pressure.