Low-field NMR Relaxometry Research Articles

High-pressure low-field 1H NMR relaxometry in nanoporous materials

A low-field NMR sensor with NdFeB permanent magnets (B0=118mT) and a pressure cell made of PEEK (4cm outer diameter) were designed for 1H relaxation time studies of adsorbed molecules at pressures of up to 300bar. The system was used to investigate methane uptake of microporous metal–organic frameworks and nanoporous activated carbon. T2 relaxation time distribution of pure methane and of methane under co-adsorption of carbon dioxide show that the host–guest interaction lead to a relaxation time contrasts, which may be used to distinguish between the gas phase and the different adsorbed phases of methane. Adsorption isotherms, exchange of methane between adsorbent particles and the surrounding gas phase, successive displacement of methane from adsorption sites by co-adsorption of carbon dioxide and CO2/CH4 adsorption separation factors were determined from the observed NMR relaxation time distributions.

Preparation of high-impact polystyrene nanocomposites with organoclay by melt intercalation and characterization by low-field nuclear magnetic resonance

Recycled high-impact polystyrene nanocomposites with organoclay were prepared. Clay of the smectite group (montmorillonite) with two types of intercalated compounds was used (Viscogel S4 and Viscogel S7). The polymer nanocomposites were prepared by melt intercalation, applying two shear intensities in a twin screw extruder. The nanostructured materials obtained were characterized by NMR relaxometry, X-ray diffraction, thermogravimetric analysis, melt flow index and mechanical analyses. The results showed that the nanostructured materials presented a mixed intercalated/exfoliated morphology. The organophilic clay, Viscogel S7, generated polymer nanocomposites with better dispersion and distribution (at low concentrations) than those produced with the Viscogel S4. The shear rate was effective for dispersion of the nanoparticles. The materials processed at 600rpm showed better dispersion than those processed at 450rpm. The characterization techniques chosen were effective. They were complementary and permitted comparison among the polymer nanocomposites. The use of low-field NMR relaxometry allowed measurement of the spin–lattice relaxation time of hydrogen (T1H), which provided more precise information on the mobility of the materials, thus complementing and explaining the results obtained by X-ray diffraction.

Simultaneous Determination of Water and Oil in Oil Sludge by Low-Field 1H NMR Relaxometry and Chemometrics

In the process of oil sludge treatment, water and oil contents are of particular interest. In this study, the performance of a rapid, accurate analysis of water and oil in oil sludge through low-field 1H NMR relaxometry, followed by using partial least-squares (PLS) regression models, is reported. In total, 13 oil sludge samples collected from the Hangzhou petroleum refinery are analyzed. Calibration models are developed by PLS regression with full cross-validation on the data obtained using azeotropic distillation by the Dean–Stark method as a reference. The results indicate that calibration is satisfactory (Rcal2 > 0.99) for both water and oil, when using the raw magnetization decay data for PLS regression; the root mean squared errors of cross-validation (RMSECV) are 0.78% and 0.82% for water and oil, respectively. This shows that low-field 1H NMR relaxometry is a rapid, reliable, nondestructive, and solvent free alternative for determining the water and oil contents of oil sludge.

Equilibrium moisture content (EMC) in Norway spruce during the first and second desorptions

Abstract It is a commonly accepted notion that the equilibrium moisture content (EMC) of wood at a given relative humidity (RH) is highest during initial desorption of green wood due to an irreversible loss of hygroscopicity during the 1st desorption. The basis for this notion is investigated by assessing how drying and saturation procedures influence the differences between the 1st and the 2nd desorption curves for Norway spruce (Picea abies (L.) Karst.) sapwood. The study establishes 1st and 2nd desorption isotherms for a variety of initial conditions and it covers the RH range from 60.1% to 99.9%. The state of the water is not affected by oven-drying and rewetting as demonstrated by time domain low field NMR relaxometry. The results challenge the conclusions of earlier studies and indicate that in these studies the 2nd desorption was initiated at much too low EMC and therefore fails to describe a boundary desorption isotherm. Instead, it becomes an intermediate desorption isotherm starting at the adsorption boundary curve and crossing over to eventually meet the desorption boundary curve. The results also show that vacuum drying at room temperature only gives a modest loss of hygroscopicity compared to the green state. Conversely, oven-drying at 103°C results in a more significant loss of hygroscopicity, except for RH above 96% where an increase in EMC surprisingly is seen.

Relaxation times and in situ kinetic analysis during network evolution of epoxy via a nickel catalyst of imidazole

A research work has been conducted to investigate relaxation time changes with curing schedule, time, and concentration of DGEBA cured with Im 6NiBr 2. Our results demonstrate that low field NMR relaxometry is a facile and easy protocol to evaluate storage stability of one-pot epoxy based compositions. The DGEBA–Im 6NiBr 2 mixtures consistently display a greater degree of latency and a slower rate of the relaxation time decrease over the parent DGEBA–imidazole mixture. The results obtained on storage stability using NMR method are in reliable agreement to those reported previously using DSC and FT-IR approaches [1]. High field NMR techniques at elevated temperatures were utilized to determine the rate constants of both polymerization stages. The results exhibit that the rate constant of the first step is more sensitive, in comparison to the second stage, to the isothermal temperature used. By reference to the reaction between DGEBA and Im 6NiBr 2, 1H NMR at elevated temperature was shown to be a simple technique, available for on-line process monitoring, for obtaining the degree of cure enabling kinetic parameters. This study highlights the usefulness of the high temperature bulk 1H NMR technique in kinetic studies of epoxy-metal organic curing agent systems.

Probing of wood–cement interactions during hydration of wood–cement composites by proton low-field NMR relaxometry

Proton NMR T2 relaxometry has been applied to investigate phenomena involved in wood–cement composites during hydration. The transformation of capillary pore water into hydrates and gel pore water, as well as the microstructural changes occurring in the cement matrix, was continuously monitored during the first 28 days of hydration. Water in wood and its transfer into the matrix as cement hardens were also evidenced with the method. It has been found, for example, that some of the water in the mixture is retained in wood in the form of bound or free water, depending on the initial water content. By measuring the area under the different peaks, the consumption of water during hydration can be measured and the advancement of the hydration process can be evaluated via the hydration advancement coefficient α. The cement hardening within the composite has been also studied in the presence of calcium chloride, an accelerating agent. The acceleration was clearly evidenced at the early stage of the hydration process. The influence of extractives has been evaluated by comparing the hydration behaviour of composites prepared from Eucalyptus saligna (low extractives content) and Afzelia bipendensis (high extractives content), and a new compatibility index based on NMR relaxometry measurements has been proposed.

Detection of magnetic environments in porous media by low-field 2D NMR relaxometry

The 2D 1H NMR correlation maps of longitudinal ( T 1) and transverse ( T 2) relaxation times prove sensitive in monitoring the distribution of magnetic pore environments in porous systems. The comparison with Mössbauer data establishes a direct correspondence between the susceptibility-induced effects observed in the T 1– T 2 maps for pore–filling water and the Fe(III)-bearing magnetic compounds.

Effect of starter culture and fermentation temperature on water mobility and distribution in fermented sausages and correlation to microbial safety studied by nuclear magnetic resonance relaxometry

Water mobility and distribution in fermented sausages produced with differences in pH development as a result of the use of three different starter cultures (T-SPX, F-1, or F-SC-111) and two fermentation temperatures (24 degrees C, or 32 degrees C) were studied using low-field proton NMR relaxometry. Changes in the distribution and mobility of water in fermented sausages upon fermentation and drying were detectable by NMR T(2) relaxation, and the progress in the drying process could be followed as a shift towards faster relaxation times. In addition, the distribution of water in the sausages was significantly affected by the pH decline. The sausages were spiked with Listeria monocytogenes, Salmonella, and Escherichia coli VTEC, and partial least squares regressions revealed that 90% of the variation in reduction of Salmonella and VTEC could be explained by the NMR T(2) relaxation decay. Consequently, the study demonstrated that NMR relaxometry is a promising technique for elucidating process parameters and microbial safety in the production of fermented meat products.

Carboxylates and sulfates of polysaccharides for controlled internal water release during cement hydration

Calcium and aluminium carboxylates and sulfates of polysaccharides form hydrogels. Four Al- and three Ca-salt hydrogels of such polysaccharides, which were found to be stable enough to be mixed into fresh Portland cement pastes of low initial water-to-cement ratio of 0.275, were compared with respect to their water retardation potential using non-destructively operating low-field 1H NMR relaxometry. All of the investigated hydrogels release their water to the cement mainly during the accelerated period of cement hydration. At a degree of hydration of about 0.7 the water in the hydrogels is consumed completely by the hydration reactions. Based on Powers’ hydration model the development of the volumetric phase distribution of the investigated hydrating cement with internal curing by the hydrogels was quantified. Within the hardening cement the Ca-salt hydrogels are more stable than the Al-hydrogels which is concluded from the changes of the T2 relaxation times during the cement hydration.

Water Distribution and Microstructure in Enhanced Pork

Water characteristics and meat microstructure of NaHCO3-enhanced pork were compared with NaCl- and Na4O7P2-enhanced pork using low-field proton NMR relaxometry, advanced microscopy techniques, and traditional meat quality measurements. Porcine samples were enhanced at 4 degrees C for 48 h with sodium salts individually and in the following combinations: (i) 5% NaCl, (ii) 5% Na4O7P2, (iii) 3% NaHCO3, (iv) 5% NaCl and 5% Na4O7P2, (v) 5% NaCl and 3% NaHCO3, (vi) 5% Na4O7P2 and 3% NaHCO3, and (vii) 5% NaCl, 5% Na4O7P2, and 3% NaHCO3. Independently of the marinade used, the water-binding capacity was improved, cooking loss was reduced, and the yield was enhanced compared with nonmarinated pork samples. This was also reflected in the water mobility within the samples measured by proton NMR relaxometry. Visualization of samples by confocal laser scanning microscopy (CLSM) revealed salt-dependent microstructural changes in the green pork samples treated with NaHCO3, giving rise to nearly complete disintegration of overall structures. High-resolution visualization by atomic force microscopy (AFM) further suggested that a higher cooking loss in sodium chloride-enhanced samples could be ascribed to less solubilization and higher heat-induced protein denaturation compared with phosphate- and bicarbonate-enhanced samples.

NMR relaxometry during internal curing of Portland cements by lightweight aggregates

The hydration of an ordinary Portland cement of low water-to-cement ratio under the influence of internal curing by addition of water-saturated lightweight aggregates (LWA) is investigated by non-destructive low-field NMR relaxometry. The methodology, which was recently developed to investigate internal curing by addition of a water-saturated polyelectrolyte gel, is applied to follow the transition of water from the mineral aggregates into hydrating cement paste and to detect the time dependence of the water consumption by the hydration reaction. By the changes of the transverse relaxation times of the physically bound water, the compaction of the pore space of the hardening cement is estimated qualitatively. The water retention potential of two types of LWA and the temporal moisture requirement of the cement during internal curing are determined without and with superplasticizer.

Relationship between water mobility and distribution and sensory attributes in pork slaughtered at an age between 90 and 180 days

Water mobility and distribution were measured in M. longissimus dorsi from 41 pigs slaughtered at an age of 90, 140, 161 or 182 days using low-field proton NMR relaxometry, and in order to investigate the impact on sensory attributes, a sensory evaluation of the pork was performed in parallel. The sensory evaluation demonstrated a significant effect of slaughter age on juiciness of the meat, and a final juiciness score of 8.4, 7.7, 7.2 and 7.8 was obtained for meat from 90, 140, 161 and 182-day old pigs, respectively. The NMR measurements revealed that the higher juiciness in meat slaughtered at an age of 90 days could be ascribed to a longer relaxation time of the extramyofibrillar water, corresponding to more mobile water, in the fresh meat of 90-day old pigs compared with the older pigs. In the cooked meat the higher juiciness of meat from 90-day old animals could be ascribed to a more homogenous distribution of the myofibrillar water compared with meat from older pigs. In contrast, the NMR measurements showed no effects that could explain the higher juiciness in meat from pigs slaughtered at an age of 182 days compared with meat from pigs slaughtered at an age of 140 or 161 days. Possibly the increase in juiciness when the age at slaughter is increased from 161 to 182 days should be ascribed to an increase in intramuscular fat content, which was not evident in the NMR measurements.

Heat-Induced Changes in Myofibrillar Protein Structures and Myowater of Two Pork Qualities. A Combined FT-IR Spectroscopy and Low-Field NMR Relaxometry Study

Low-field NMR T(2) and Fourier transform infrared (FT-IR) measurements were performed on meat samples of two qualities (normal and high ultimate pH) during cooking from 28 degrees C to 81 degrees C. Pronounced changes in both T(2) relaxation data and FT-IR spectroscopic data were observed during cooking, revealing severe changes in the water properties and structural organization of proteins. The FT-IR data revealed major changes in bands in the amide I region (1700-1600 cm(-)(1)), and a tentative assignment of these is discussed. Distributed NMR T(2) relaxation data and FT-IR spectra were compared by partial least-squares regression. This revealed a correlation between the FT-IR peaks reflecting beta-sheet and alpha-helix structures and the NMR relaxation populations reflecting hydration water (T(2B) approximately 0-10 ms), myofibrillar water (T(21) approximately 35-50 ms), and also expelled "bulk" water (T(2) relaxation times >1000 ms). Accordingly, the present study demonstrates that definite structural changes in proteins during cooking of meat are associated with simultaneous alterations in the chemical-physical properties of the water within the meat.

The Effects of Amylose and Starch Phosphate on Starch Gel Retrogradation Studied by Low‐field 1H NMR Relaxometry

AbstractLow‐field Nuclear Magnetic Resonance (23 MHz) was used to study the effects of the degree of phosphorylation, the amylose content and the amylopectin chain length distribution on gel retrogradation for a set of 26 starches, six of which were of crystal polymorph type A, 18 of type B and two of type C. The phosphate content ranged from 0 to 58.8 nmol Glc6P/mg and the amylose content from 0 to 72.1%. The starch pastes (13%, w/w) were measured before and after storage for six days at 35 °C. It was found that Principal Component Analysis (PCA) of the raw Carr‐Purcell‐Meiboom‐Gill (CPMG) relaxation curves from the two measurements (day 1 and day 7) could be used as a simple, illustrative way of describing the retrogradation. Three different behaviours were identified: One group of samples (mostly potato starches) slowly changed from a soft to a more rigid gel from day 1 to 7. A second group (mostly cereal starches) formed a rigid gel already before the first measurement and changed little after that. A third group comprised a few samples containing little or no amylose aged similarly to the first group of samples, but at a much slower rate. For the potato starches, a weak negative correlation (r = ‐0.63) was found between the degree of phosphorylation and the difference between the LF NMR relaxation curves of day 1 and day 7.

Low-field NMR relaxometry: a study of interactions of water with water-repellant soils.

Petroleum-induced water repellency in soils is a problem that has been thought to develop randomly following contamination and then remediation of a site with petroleum. The emergence of the phenomenon can occur within months or years of original contamination and with seemingly no warning. Low-field NMR has been used to study these soils and, specifically, the processes of water uptake that occur in them. Critical aspects in the development of this phenomenon have been identified as well--specifically, a dependence on climatic events in the area and contamination levels that contribute are suggested.

Pore-scale redistribution of water during wetting of air-dried soils as studied by low-field NMR relaxometry.

The kinetics of water uptake and redistribution in several soils and their components are studied using NMR relaxometry. Unlike the normal behavior observed in stable porous media, entry into micropores in the soil is a slow process as compared to entry into macro- and mesopores. This indicates that soils air-dried at ambient temperature include gel phases that have collapsed or reoriented, closing micropores, during drying. Wetting must then include the swelling processes that re-open micropores. This can even exhibit temperature dependence giving an "apparent activation energy" comparable to that of a chemical reaction, for example, ester hydrolysis. The processes of micropore opening may play a role in slow uptake of contaminants into soils.