Low-field NMR Spectroscopy Research Articles

Quantitative Analysis of Multicomponent Mixtures of Over-the-Counter Pain Killer Drugs by Low-Field NMR Spectroscopy

Marketed pain relief drugs with one to three biologically active components, as well as mixtures of these ingredients, were qualitatively and quantitatively analyzed in an undergraduate student lab using a compact, low-field 1H NMR spectrometer. The students successfully analyzed more than 50 self-made sample mixtures with two or three components as well as the two marketed tablet formulations containing acetylsalicylic acid/l-ascorbic acid, or acetylsalicylic acid/paracetamol (acetaminophen)/caffeine. The NMR-based quantification is an attractive application of the technique, as well as a helpful introduction to NMR spectroscopic applications in life sciences. Problem-based learning on NMR techniques on commonly known drugs provided students the opportunity to develop and improve their skills in solving 1H NMR problems.

Low-field 1H NMR spectroscopy for distinguishing between arabica and robusta ground roast coffees

This work reports a new screening protocol for addressing issues of coffee authenticity using low-field (60MHz) bench-top 1H NMR spectroscopy. Using a simple chloroform-based extraction, useful spectra were obtained from the lipophilic fraction of ground roast coffees. It was found that 16-O-methylcafestol (16-OMC, a recognized marker compound for robusta beans) gives rise to an isolated peak in the 60MHz spectrum, which can be used as an indicator of the presence of robusta beans in the sample. A total of 81 extracts from authenticated coffees and mixtures were analysed, from which the detection limit of robusta in arabica was estimated to be between 10% and 20% w/w. Using the established protocol, a surveillance exercise was conducted of 27 retail samples of ground roast coffees which were labelled as “100% arabica”. None were found to contain undeclared robusta content above the estimated detection limit.

Alginate-based polysaccharide beads for cationic contaminant sorption from water

Massive amounts of agricultural and industrial water worldwide are polluted by different types of contaminants that harm the environment and impact human health. Removing the contaminants from effluents by adsorbent materials made from abundant, inexpensive polysaccharides is a feasible approach to deal with this problem. In this research, alginate beads combined with two types of cellulose, starch or xylan were synthesized. Their average diameters in air- and freeze-dried conditions were assessed by optical microscopy. Differences in morphology were observed by scanning electron microscopy. Their capacity for water uptake, their sorption capabilities for a model cationic pollutant and their charge density was investigated in relationship to their composition and their surface characteristics. Their interaction with water was evaluated using low-field NMR spectroscopy. It was found that nanocrystalline cellulose added the most to the beads’ sorption capacity for cationic contaminants while xylan admixture created the beads with the highest water sorption after lyophilization.

Diels-Alder Hydrogels for Controlled Antibody Release: Correlation between Mesh Size and Release Rate.

Eight-armed PEG, molecular mass 10 kDa, was functionalized with furyl and maleimide groups, respectively; the obtained macromonomers were cross-linked via Diels-Alder chemistry. The mesh size (ξ) of the prepared hydrogels was determined by swelling studies, rheology, and low field NMR spectroscopy. The in vitro release of fluorescein isothiocyanate labeled dextrans (FDs) and bevacizumab was investigated. The average mesh size (ξavg) increased from 5.8 ± 0.1 nm to 56 ± 13 nm during degradation, as determined by swelling studies. The result of the rheological measurements (8.0 nm) matched the initial value of ξavg. Low field NMR spectroscopy enabled the determination of the mesh size distribution; the most abundant mesh size was found to be 9.2 nm. In combination with the hydrodynamic radius of the molecule (Rh), the time-dependent increase of ξavg was used to predict the release profiles of incorporated FDs applying an obstruction-scaling model. The predicted release profiles matched the experimentally determined release profiles when Rh < ξavg. However, significant deviations from the theoretical predictions were observed when Rh ≥ ξavg, most likely due to the statistical distribution of ξ in real polymer networks. The release profile of bevacizumab differed from those of equivalently sized FDs. The delayed release of bevacizumab was most likely a result of the globular structure and rigidity of the protein. The observed correlation between ξ and the release rate could facilitate the design of controlled release systems for antibodies.

Local Flips and Chain Motion in Polyethylene Crystallites: A Comparison of Melt-Crystallized Samples, Reactor Powders, and Nanocrystals

The crystallization and the mechanical properties of polyethylene, which is one of the most important commodity polymers, are influenced by the crystalline α-relaxation. This process originates from the diffusive chain transport through the crystallites as mediated by local 180° flips. Recent studies have stressed the relevance of the chain folding morphology on the chain diffusion, but its relation to the rate of jumps of the individual repeat units has not yet been addressed. In this study, we compare samples with very different morphology, including nanocrystals as a unique new model system, and use proton low-field and carbon-13 high-field solid-state NMR spectroscopy to determine the rate of local jumps and the large-scale crystalline–amorphous diffusion coefficient, respectively. We find that samples with tight folds (reactor powders and nanocrystals) display on average lower activation energies of the local jumps. Nanocrystals stand out in that they feature a significantly broader distribution of l...

Entanglement Effects in Elastomers: Macroscopic vs Microscopic Properties

This Perspective highlights how entanglement effects on rubber elasticity can be unveiled by a combination of different macroscopic and microscopic methods, taking advantage of new developments in proton low-field NMR spectroscopy as applied to bulk and swollen rubbers. Specifically, the application of a powerful yet routinely applicable double-quantum method, combined with a back-extrapolation procedure over results measured at different degrees of swelling, allows one to characterize the recently introduced “phantom reference network” state, which only reflects contributions of actual cross-links and topologically trapped entanglements. We further present an assessment of the qualitative yet popular Mooney–Rivlin analysis of mechanical data, where the influence of entanglement contributions on the fitted, purely empirical parameters C1 and C2 is reconsidered in the context of different tube models of rubber elasticity. We also review the impact of entanglements on results of equilibrium swelling experim...

Dynamics in Crystallites of Poly(ε-caprolactone) As Investigated by Solid-State NMR

We investigate the molecular dynamics within the crystallites of poly(e-caprolactone), PCL, crystallized from the melt by means of high-field 13C and low-field 1H NMR spectroscopy, addressing the question of whether it can be classified as a “crystal-fixed” polymer without chain motion through the crystallites. We address fast, slow, and intermediate-regime (microseconds to milliseconds time scale) motions by means of high-resolution of 13C DIPSHIFT and CODEX MAS experiments as well as low-resolution static 1H FID and MSE measurements over a range of temperatures. The DIPSHIFT data provide information on motionally averaged 13C–1H dipole–dipole couplings and indicate the presence of fast (≤1 μs) methylene group librational motions within the crystalline phase, where the amplitudes increase with increasing distance from the rather rigid ester groups. The CODEX experiments, addressing slow (≥ms) local rotations of the chemical-shift anisotropy tensors, suggest the absence of slow intracrystallite chain dyna...

Characterisation of multiple emulsions by NMR spectroscopy and diffusometry

An important and often discussed question in emulsion science is the structure of (multiple) emulsions because of the direct correlation of structure and emulsion properties. The well known W1/O/W2 type of double emulsions consists of a quasi continuous water phase (W2), in which an emulsion, consisting of oil- (O) and inner water phase (W1), is dispersed. As these systems are often opaque and dilution additionally influences the system’s equilibrium states, NMR is exploited for its use to give insight into the geometrical structure as well as into the composition.Pulsed Field Gradient NMR (PFG-NMR) is known for determination of droplet size distributions. Signals of the different phases, for example oil and water, can be discriminated either by chemical shift, relaxation or diffusion. Regarding multiple emulsions, molecular dynamics interferes, and often the phases cannot be considered as being independent. Molecular exchange can be observed, which is confirmed independently by PFG-NMR, NMR-relaxometry and Confocal Laser Scanning Microscopy (CLSM).Apart from size distribution, also the composition, that is the concentration of the dispersed phases, is of interest, especially in the context of process development and storage stability. In the case of single emulsions, it can be determined by NMR spectroscopy, also by the recently re-investigated low-field NMR spectroscopy. Concerning multiple emulsions, diffusion or relaxation offers the additional differentiation of chemically similar phases.

Solid particles in an elastomer matrix: impact of colloid dispersion and polymer mobility modification on the mechanical properties

The reinforcement of elastomers by inorganic fillers, a concept of very high technological importance, is commonly understood to result from the presence of a mechanical network of partially aggregated filler particles. The non-linear mechanical properties, in particular the decrease of the modulus at high strain (Payne effect), are further interpreted to be a consequence of the breakdown of this filler network. There are, however, many open questions concerning the actual nature of the interparticle connections, where a modified polymer layer forming “glassy bridges” constitutes one possibility. In this work, we address this issue with a suitable silica-filled model elastomer, where we characterize the silica dispersion by SANS in combination with reverse Monte-Carlo modeling, and the mobility modification of the polymer by low-field proton NMR spectroscopy. In our samples, we identify a glassy layer as well as a region of intermediate mobility (possibly modified Rouse modes). Based on the structural information from SANS, we are able to quantify the amount of interparticle connections, and correlate it with the magnitude of the Payne effect taken from shear rheology. This works only if we assume that these connections comprise both, the glassy layer as well as the region of intermediate mobility. The amount of glassy immobilized polymer only does not suffice to explain the mechanical properties.

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.

Novel Experimental Approach To Evaluate Filler−Elastomer Interactions

Novel information on filler−elastomer interactions is obtained by combining solid-state 1H low-field NMR spectroscopy and equilibrium swelling experiments. Multiple-quantum (MQ) NMR experiments provide detailed quantitative molecular information on the cross-link density of the elastomer matrix in a variety of filled systems, indicating generally weak filler effects on the overall cross-link density and on the network homogeneity. Swelling experiments, as well as mechanical data, are additionally influenced by the matrix−filler and filler−filler interactions. Our approach is based on comparing cross-link densities from NMR and (Flory−Rehner) swelling experiments, for which a masterline is always found in unfilled elastomers. In filled elastomers two different scenarios are observed. If there are no interactions between the polymer chains and the filler surface, no deviations from the masterline are detected because the swelling capacity of the composite is governed by the bulk polymer. Deviations from the masterline (reduced swelling) are exhibited by those composites that have strong rubber−filler interactions. In these cases, some fraction of the polymer is connected to the filler surface, which thus behaves like a giant cross-link, and the overall degree of swelling is thus reduced as compared to the bulk polymer. The novel experimental approach was used to evaluate filler−elastomer interactions in different composites and nanocomposites.

Dissolution Mechanism of Crystalline Cellulose in H3PO4As Assessed by High-Field NMR Spectroscopy and Fast Field Cycling NMR Relaxometry

Many processes have been proposed to produce glucose as a substrate for bacterial fermentation to obtain bioethanol. Among others, cellulose degradation appears as the most convenient way to achieve reliable amounts of glucose units. In fact, cellulose is the most widespread biopolymer, and it is considered also as a renewable resource. Due to extended intra- and interchain hydrogen bonds that provide a very efficient packing structure, however, cellulose is also a very stable polymer, the degradation of which is not easily achievable. In the past decade, researchers enhanced cellulose reactivity by increasing its solubility in many solvents, among which concentrated phosphoric acid (H(3)PO(4)) played the major role because of its low volatility and nontoxicity. In the present study, the solubilization mechanism of crystalline cellulose in H(3)PO(4) has been elucidated by using high- and low-field NMR spectroscopy. In particular, high-field NMR spectra showed formation of direct bonding between phosphoric acid and dissolved cellulose. On the other hand, molecular dynamics studies by low-field NMR with a fast field cycling (FFC) setup revealed two different H(3)PO(4) relaxing components. The first component, described by the fastest longitudinal relaxation rate (R(1)), was assigned to the H(3)PO(4) molecules bound to the biopolymer. Conversely, the second component, characterized by the slowest R(1), was attributed to the bulk solvent. The understanding of cellulose dissolution in H(3)PO(4) represents a very important issue because comprehension of chemical mechanisms is fundamental for process ameliorations to produce bioenergy from biomasses.

Structure of Poly(vinyl alcohol) Cryo-Hydrogels as Studied by Proton Low-Field NMR Spectroscopy

The network structure of poly(vinyl alcohol) (PVA) hydrogels obtained by freezing−thawing cycles was investigated by solid-state 1H low-field NMR spectroscopy. By the application of multiple-quantum NMR experiments, we obtain information about the segmental order parameter, which is directly related to the restrictions on chain motion (cross-links) formed upon gelation. These measurements indicate that the network mesh size as well as the relative amount of nonelastic defects (i.e., non-cross-linked chains, dangling chains, loops) decrease with the number of freezing−thawing cycles but are independent of the polymer concentration. The formation of the PVA network is accompanied by an increasing fraction of polymer with fast magnetization decay (∼20 μs). The quantitative study of this rigid phase with a specific refocusing pulse sequence shows that it is composed of a primary crystalline polymer phase (∼5%), which constitutes the main support of the network structure and determines the mesh size, and a secondary population of more imperfect crystallites, which increase the number of elastic chain segments in the polymer gel but do not affect the average network mesh size appreciably. Correspondingly, progressive melting of the secondary crystallites with increasing temperature does not affect the network mesh size but only the amount of network defects, and melting of the main PVA crystallites at ∼80 °C leads to the destruction of the network gel and the formation of an isotropic PVA solution.

Detection of heterogeneities in dry and swollen polymer networks by proton low-field NMR spectroscopy.

We report on the implementation of modern proton multiple-quantum NMR methods for the characterization of molecular order and dynamics in polymer networks and melts on cost-efficient low-resolution low-field NMR instrumentation. The method permits the extraction of chain order parameter distributions, and is therefore sensitive to a heterogeneity of cross-links and other topological constraints. Data from samples with bimodal network chain length distributions acquired at 20 MHz (0.47 T) are in quantitative agreement with results obtained at high field (500 MHz). It is shown for the first time that the chain order distribution is broadened upon swelling, providing evidence for the nontrivial nature of the swelling process.

Spin-spin relaxation time determination of intermediate states in heterogeneous products from free induction decay NMR signals

For technical reasons, spin-spin relaxation times ( T 2) in the 50–600 μs range cannot be determined by a Carr-Purcell-Meiboom-Gill (CPMG) sequence. In complex heterogeneous compounds such as food products, these relaxation times correspond to liquid at the solid-liquid interface and/or an amorphous phase as well as possibly a “solid-like” state. We have developed a process of free induction decay nuclear magnetic resonance (FID NMR) signal treatment which allows the quantitative determination of T 2 ∗ distributions corresponding to these states from low field NMR spectroscopy measurements. Thus, this kind of NMR spectrometry is a valuable alternative to differential scanning calorimetry (DSC) measurements since it can be used as a routine control method in industry.

180° Pulse Imperfection Effects on Fitting of Relaxation Curves Obtained by Low Field NMR Spectroscopy

Abstract Low field NMR spectroscopy is well adapted to the study of heterogeneous media, such as those found in the food industry or in life sciences, because it does not require preparation of samples. CPMG relaxation curves obtained with different pulse spacing times, on samples without exchange phenomenon, are not superimposable. We show that this phenomenon, due to pulse imperfections at 180°, is not a single drift; pulses reach a stationary regime towards the 80th pulse; values of T2 of the order of milliseconds, are then a bit perturbed. For the same reason, with a liquid sample, the total signal of a CPMG curve does not exactly correspond to the total signal obtained with a FID curve. With some fitting programs, by iterating on the value of the offset in order to minimise the standard deviation of the relaxation curve fitting, it is no longer necessary to know the value of the baseline of the CPMG relaxation curves; it enables one to better sample the first part of the curve.

An NMR investigation of the dynamics of a methyl group with tunneling frequency 3.4 MHz

The ground state tunneling frequency ( ν t = 3.39 MHz) of the methyl groups in 2,5-dimethyl-1,3,4-thiadiazole has been measured using low field NMR-spectroscopy. Using a variety of rapid field cycling NMR techniques the temperature dependence of ν t and the correlation time for incoherent reorientation τ c have also been measured. These have enabled the dynamics of the methyl groups to be studied from the quantum regime into the classical regime. These dynamics can be well described with a purely threefold barrier of V k B = 1175 K .

Pressure dependence of methyl tunnelling in solid diacetyl

Methyl tunnelling has been measured in solid diacetyl at low temperatures as a function of pressure using low field NMR spectroscopy. In addition the associated thermally activated rotation at higher temperatures has been measured as a function of pressure by spin-lattice relaxation time (T 1) analysis. The tunnel frequency decreases exponentially from 76 kHz at atmospheric pressure to 9 kHz at 6·25 kbar. The T 1 data can be correlated with the tunnel frequency according to the methyl thermometer model, extending the model to lower tunnelling frequencies than previously demonstrated. A small sixfold symmetric term in the potential hindering rotation can be deduced from the data. The overall height of the hindering potential and its shape are compared with the potential calculated from the crystal structure.

Methyl tunnelling in solid diacetyl

The techniques of dipole-dipole driven low-field NMR spectroscopy have been employed to measure the tunnelling frequency, ν t, of methyl groups in solid diacetyl (2,3-butanedione) at 4 K. The value of ν t is 74±1 kHz from which the height of the barrier to reorientation (assumed to have three-fold symmetry) is deduced to be 1845 K (15.3 kJ mol −1). The intermolecular contribution to the barrier has been calculated by evaluating the lattice sum of pairwise interatomic potentials as a function of methyl group orientation. This analysis reveals that the predominant contribution to the barrier arises from the methyl group of a neighbouring molecule. Temperature dependence measurements of the spin-lattice relaxation time have also been employed to study the dynamics of methyl reorientation in diacetyl and in the related molecule acetonylacetone (2,5-hexanedione).

Physical properties of a minimal infectious RNA (viroid) associated with the exocortis disease

Purification of the pathogenic RNA (viroid) from exocortis disease has been accomplished. Composition analysis indicates the absence of any DNA or protein and a molar nucleotide composition of CMP, 29.4%; AMP, 21.5%; GMP, 28.8%; and UMP, 19.9%. Even though the AMPUMP and GMPCMP ratios approach unity the per cent hyperchromicity (22%) resembled a tRNA-like molecule and not a dsRNA, while the Tm (52°) in 0.1 SSC was higher than the pure tRNA species tested. Nuclease digestion similarly suggested the presence of a highly ordered structure for the pathogenic RNA. We also introduced low field NMR spectroscopy as a new technique in the study of pathogenic RNA's (viroids) of unknown sequence.