SABRE Hyperpolarization with up to 200 bar Parahydrogen in Standard and Quickly Removable Solvents.

Parahydrogen (p-H2)-based techniques are known to drastically enhance NMR signals but are usually limited by p-H2 supply. This work reports p-H2-based SABRE hyperpolarization at p-H2 pressures of hundreds of bar, far beyond the typical ten bar currently reported in the literature. A recently designed high-pressure setup was utilized to compress p-H2 gas up to 200 bar. The measurements were conducted using a sapphire high-pressure NMR tube and a 43 MHz benchtop NMR spectrometer. In standard methanol solutions, it could be shown that the signal intensities increased with pressure until they eventually reached a plateau. A polarization of about 2%, equal to a molar polarization of 1.2 mmol L-1, could be achieved for the sample with the highest substrate concentration. While the signal plateaued, the H2 solubility increased linearly with pressure from 1 to 200 bar, indicating that p-H2 availability is not the limiting factor in signal enhancement beyond a certain pressure, depending on sample composition. Furthermore, the possibility of using liquefied ethane and compressed CO2 as removable solvents for hyperpolarization was demonstrated. The use of high pressures together with quickly removable organic/non-organic solvents represents an important breakthrough in the field of hyperpolarization, advancing SABRE as a promising tool for materials science, biophysics, and molecular imaging.

Slip/Stick Viscosity Models of Nanoconfined Liquids: Solvent-Dependent Rotation in Metal-Organic Frameworks.

Artificial molecular machines are expected to operate in environments where viscous forces impact molecules significantly. With that, it is well-known that solvent behaviors dramatically change upon confinement into limited spaces as compared to bulk solvents. In this study, we demonstrate the utility of an amphidynamic metal-organic framework with pillars consisting of 2H-labeled dialkynyltriptycene and dialkynylphenylene barrierless rotators that operate as NMR sensors for solvent viscosity. Using line-shape analysis of quadrupolar spin echo spectra we showed that solvents such as dimethylformamide, diethylformamide, 2-octanone, bromobenzene, o-dichlorobenzene, and benzonitrile slow down their Brownian rotational motion (103-106 s-1) to values consistent with confined viscosity values (ca. 100-103 pa s) that are up to 10000 greater than those in the bulk. Magic angle spinning assisted 1H T2 measurements of included solvents revealed relaxation times of approximately 100-1000 ms over the explored temperature ranges, and MAS-assisted 1H T1 measurements of included solvents suggested a much lower activation energy for rotational dynamics as compared to those measured by the rotating pillars using 2H measurements. Finally, translational diffusion measurements of DMF using pulsed-field gradient methods revealed intermediate dynamics for the translational motion of the solvent molecules in MOFs.

Refined high-pressure tube design for improved resolution in high-pressure NMR spectroscopy.

High-pressure NMR spectroscopy has become a valuable analytical tool in many activities, including the energy sector, chemical research, and biophysics. Many pressure-based applications could benefit from low-field NMR. In this context, a novel, simple, and low-cost high-pressure tube design improving the signal-to-noise ratio by more than 80% is presented.

Compact NMR Spectroscopy for Low-Cost Identification and Quantification of PVC Plasticizers.

Polyvinyl chloride (PVC), one of the most important polymer materials nowadays, has a large variety of formulations through the addition of various plasticizers to meet the property requirements of the different fields of applications. Routine analytical methods able to identify plasticizers and quantify their amount inside a PVC product with a high analysis throughput would promote an improved understanding of their impact on the macroscopic properties and the possible health and environmental risks associated with plasticizer leaching. In this context, a new approach to identify and quantify plasticizers employed in PVC commodities using low-field NMR spectroscopy and an appropriate non-deuterated solvent is introduced. The proposed method allows a low-cost, fast, and simple identification of the different plasticizers, even in the presence of a strong solvent signal. Plasticizer concentrations below 2 mg mL-1 in solution corresponding to 3 wt% in a PVC product can be quantified in just 1 min. The reliability of the proposed method is tested by comparison with results obtained under the same experimental conditions but using deuterated solvents. Additionally, the type and content of plasticizer in plasticized PVC samples were determined following an extraction procedure. Furthermore, possible ways to further decrease the quantification limit are discussed.

Screening Metal-Organic Frameworks for Separation of Binary Solvent Mixtures by Compact NMR Relaxometry.

Metal-organic frameworks (MOFs) have great potential as an efficient alternative to current separation and purification procedures of a large variety of solvent mixtures-a critical process in many applications. Due to the huge number of existing MOFs, it is of key importance to identify high-throughput analytical tools, which can be used for their screening and performance ranking. In this context, the present work introduces a simple, fast, and inexpensive approach by compact low-field proton nuclear magnetic resonance (NMR) relaxometry to investigate the efficiency of MOF materials for the separation of a binary solvent mixture. The mass proportions of two solvents within a particular solvent mixture can be quantified before and after separation with the help of a priori established correlation curves relating the effective transverse relaxation times T2eff and the mass proportions of the two solvents. The new method is applied to test the separation efficiency of powdered UiO-66(Zr) for various solvent mixtures, including linear and cyclic alkanes and benzene derivate, under static conditions at room temperature. Its reliability is demonstrated by comparison with results from 1H liquid-state NMR spectroscopy.

Terahertz Time-Domain Spectroscopy of Plasticized Poly(vinyl chloride).

Poly(vinyl chloride) (PVC) is today one of the most important commodity polymers. Its broad range of applications is due to the presence of plasticizers whose concentration largely impacts the microscopic and the macroscopic properties. Quantifying the concentration of plasticizer in PVC products is therefore of fundamental importance. Thus, in this paper, the applicability of terahertz (THz) time-domain spectroscopy for the characterization of plasticized PVC is for the first time evaluated in a systematic way. It could be demonstrated that the method is able to distinguish between PVC samples with different types and concentrations of plasticizers. Furthermore, a simple, fast, and efficient method is introduced to quantify the concentration of plasticizer in PVC samples of known plasticizer type but different thermal histories. The presented results are of key importance due to the need of reliable noninvasive and nondestructive analytical methods which can deliver onsite information about the remaining plasticizer concentration inside PVC products. Furthermore, it is expected that the proposed approach can be easily extended to other plasticized polymers.

Nondestructive Quantification of Local Plasticizer Concentration in PVC by (1)H NMR Relaxometry.

The properties of plasticized poly(vinyl chloride) (PVC) , one of the most important polymers today, are strongly dictated by the concentration of plasticizer. Yet, it has been impossible to quantify this concentration at different positions inside a PVC product without its destruction because of a lack of suitable analytical methods. Thus, this paper introduces a simple, fast, and efficient way to determine truly nondestructively the concentration of plasticizer in PVC by single-sided nuclear magnetic resonance (NMR). With the help of correlation curves between the concentration of plasticizer inside nonaged PVC samples and the corresponding volume-averaged NMR parameters, single-sided NMR allows the quantification of the local concentration of plasticizer in aged PVC plates at different depths by spatially resolved relaxation measurements. The presented approach represents a fundamental step toward in situ characterization of plasticized PVC.

Incorporation mechanisms of a branched nonylphenol isomer in soil-derived organo-clay complexes during a 180-day experiment.

The incorporation process of a defined (13)C- and (14)C-labeled nonylphenol isomer (4-(3,5-dimethylhept-3-yl)phenol) into soil-derived organo-clay complexes was investigated. Isolated organo-clay complexes were separated into humic subfractions. Noninvasive ((13)C-CP/MAS NMR) and invasive methods (sequential chemical degradation, pyrolysis) were applied to obtain detailed information about the mode of incorporation, chemical structure, and change of the incorporation character of nonextractable residues in course of incubation. (13)C-CP/MAS NMR measurements of humic acids revealed an increasing incorporation of phenolic compounds during the experimental time which was referred to residues of the introduced (13)C-labeled NP isomer. Detailed investigations by means of sequential chemical degradation indicated a predominant incorporation of nonextractable NP isomer residues via reversible ester (amide) bonds. In course of time, the amount of releasable compounds decreased, pointing to altering processes which affected the mode of incorporation. BBr3-treatment, RuO4 oxidation, and thermochemolysis released only low portions of nonextractable radioactivity giving evidence of strongly incorporated residues. With the comprehensive application of complementary methods (e.g., humic matter fractionation, (13)C-CP/MAS NMR, sequential chemical degradation) it was possible to provide a comparatively detailed insight into the incorporation behavior of the applied NP isomer.

1H-NMR measurements of proton mobility in nano-crystalline YSZ.

We report nuclear magnetic resonance (NMR) results on water saturated, dense, nano-crystalline YSZ samples (9.5 mol% yttria doped zirconia) which exhibit proton conductivity at temperatures as low as room temperature. (1)H-NMR spectra recorded under static and magic angle spinning conditions show two distinct signals. Their temperature-dependent behavior and their linewidths suggest that one can be attributed to (free) water adsorbed on the surface of the sample and the other one to mobile protons within the sample. This interpretation is supported by comparison with measurements on a single-crystalline sample. For the nano-crystalline samples motional narrowing is observed for the signal originating from protons in the sample interior. For these protons, the analysis of temperature and field dependent spin-lattice relaxation time T1 points towards diffusion in a confined two-dimensional geometry. We attribute this quasi two-dimensional motion to protons that are mobile along internal interfaces or nanopores of nano-crystalline YSZ.

Fast quantification of water content in glycols by compact 1H NMR spectroscopy.

Glycols are key chemicals for many applications in different fields of activities. Being highly hydroscopic, glycols contain usually water. The presence of water, even in tiny amounts, can affect their chemical and physical properties. Therefore, the accurate determination of water content is essential for any intended applications. In this context, a novel method using low-field Nuclear Magnetic Resonance (NMR) spectroscopy is introduced. The proposed approach offers a straightforward, fast, low-cost, and versatile solution for water quantification in glycols without the need for reagents or calibration data. It is demonstrated by quantifying the water concentration up to 11 wt% in aqueous ethylene glycol (EG) and triethylene glycol (TEG) mixtures with the help of lineshape analysis of the corresponding proton spectra. The limit of detection, achieved within 1 min of measuring time, was 0.05 wt% for water in EG and 0.15 wt% in TEG. The excellent agreement between the NMR results and those from the Karl-Fischer titration indicates that the proposed NMR-based approach has a great potential to be used as an alternative to the Karl-Fischer method. In addition, it is expected that the same methodology can be applied for water quantification in more complex glycolic solutions and other mixtures.

Analysis of Aging Products from Biofuels in Long-Term Storage.

The long-term aging processes during storage of different heating oils and their blends with biofuels including fatty acid methyl ester, hydrogenated vegetable oil, and power-to-liquids products were followed by different analytical techniques, and the aging products were analyzed. While most standard techniques are time-consuming and labor-intensive and specify only a single property, analyses by benchtop nuclear magnetic resonance spectroscopy proved to be effortless and fast. Moreover, only 0.4 mL of the sample is required for nondestructive NMR measurements. White and waxlike precipitates were found in FAME stored at a cold temperature and identified as esters of glycerol with saturated side chains by chromatographic, thermal, and spectroscopic analyses. At colder temperatures, they reversibly precipitate and can lead to system failure.

Quantification of PVC plasticizer mixtures by compact proton NMR spectroscopy and indirect hard modeling.

A novel approach is introduced for the fast, reliable, and low-cost recognition and quantification of plasticizers in plasticizers mixtures. It uses benchtop 1H NMR spectroscopy and indirect hard modeling, a mechanistic multivariate regression technique. The approach is demonstrated on five different PVC plasticizers having similar spectral signatures in proton NMR spectra. With only 16 scans per spectrum, i.e., 2 min 40 s measurement time, quantification limits down to 0.14 mg mL-1, or 0.35 wt% plasticizer in PVC, were achieved. Apart from the rapid data acquisition, the use of spectral hard modeling enabled the quantification of plasticizer mixtures while using only 4 to 6 training samples per component. Despite strongly overlapping signals in the NMR spectra, various plasticizers were differentiated and quantified, as exemplarily demonstrated for binary mixtures. A commercial PVC specimen with three different layers was also examined, confirming the applicability of benchtop NMR spectroscopy. Additionally, the use of the proposed method to validate official regulations concerning the plasticizer content in PVC is assessed. The presented results demonstrate that the combination of benchtop NMR and spectral hard modeling is a very promising analytical tool for rapid PVC plasticizer recognition and quantification with high analytical throughput. Moreover, the results indicate a high potential for benchtop NMR and spectral hard modeling for microchemical analysis, even for complex samples.

RASER MRI: Magnetic resonance images formed spontaneously exploiting cooperative nonlinear interaction.

The spatial resolution of magnetic resonance imaging (MRI) is limited by the width of Lorentzian point spread functions associated with the transverse relaxation rate 1/T2*. Here, we show a different contrast mechanism in MRI by establishing RASER (radio-frequency amplification by stimulated emission of radiation) in imaged media. RASER imaging bursts emerge out of noise and without applying radio-frequency pulses when placing spins with sufficient population inversion in a weak magnetic field gradient. Small local differences in initial population inversion density can create stronger image contrast than conventional MRI. This different contrast mechanism is based on the cooperative nonlinear interaction between all slices. On the other hand, the cooperative nonlinear interaction gives rise to imaging artifacts, such as amplitude distortions and side lobes outside of the imaging domain. Contrast mechanism and artifacts are explored experimentally and predicted by simulations on the basis of a proposed RASER MRI theory.

Characterization of non-extractable ¹4C- and ¹³C-sulfadiazine residues in soil including simultaneous amendment of pig manure.

Recently, we reported on soil fate of SDZ residues amended with pig manure treated with ¹4C-labeled sulfadiazine ¹4C-SDZ). The first objective of the present study was to determine whether this strategy can be substituted by application of ¹4C-SDZ to soil. The second objective was to characterize non-extractable SDZ residues by fractionation, size exclusion chromatography (SEC) and solid state ¹³C-NMR. The fate of ¹4C-SDZ was examined for 28 d, using two soils with and without amendment of pig manure. Mineralization of ¹4C-SDZ was low; extractable residues decreased to 7-30%. Compared to the previous study, results were similar. ¹4C-SDZ derived bound radioactivity was found in HCl-washings, fulvic, humic acids and humin. According to SEC, one bound ¹4C portion (70%) co-eluted with fulvic acids (above 910 g mol⁻¹); the other consisted of adsorbed/entrapped ¹4C-SDZ. The ¹³C-SDZ study was performed for 30 d; humic acids were examined by ¹³C-NMR. A signal (100-150 ppm) was referred to ¹³C-SDZ. SEC and ¹³C-NMR demonstrated rapid integration of SDZ into humics.

Versatile high-pressure gas apparatus for benchtop NMR: Design and selected applications.

A simple, yet highly versatile setup is presented for benchtop NMR analyses of gases at high-pressure. It consists mostly of commercial parts and includes multiple safety features while maintaining a small size to fit into a 1.20 m wide fume hood. Pressures up to 200 bar can be adjusted independently of the sample gas-bottle pressure in a matter of seconds. Mixtures of multiple gases can be produced in situ in a mixing chamber, which also serves to adjust the pressure. The high-pressure hardware and benchtop NMR spectrometer have been tested for long-term stability and repeatability of the measurements. The versatility of the setup is demonstrated by analyzing hydrocarbon-gas with attention to linewidths as well as their 1H relaxation times, by improving the resolution of 1H NMR spectra from solid polymers with pressurized CO2, and by visualizing the ingress of gaseous and supercritical methane into liquid benzene.

Non-destructive analysis of polymers and polymer-based materials by compact NMR.

Low-field nuclear magnetic resonance (NMR) based on permanent magnet technologies is currently experiencing a considerable growth of popularity in studying polymer materials. Various bulk properties can be probed with compact NMR tabletop instruments by placing the sample of interest inside the magnet. Contrary to this, compact NMR sensors with open geometries give access to depth-dependent properties of polymer samples and objects of different sizes and shapes truly non-destructively by performing measurements in the inhomogeneous stray-field outside the magnet system. Some of the sensors are also portable being thus well suited for onsite measurements. The gain of both bulk and depth-dependent microscopic properties are important for establishing improved structure-property relationships needed for the rational design of new polymer formulations. Selected recent applications will be presented to illustrate this potential of compact NMR.

Impact of Ionic Liquids on the Structure and Dynamics of Collagen.

The changes in the structure and dynamics of collagen treated with two different classes of ionic liquids, bis-choline sulfate (CS) and 1-butyl-3-methyl imidazolium dimethyl phosphate (IDP), have been studied at the molecular and fibrillar levels. At the molecular level, circular dichroic studies revealed an increase in molar ellipticity values for CS when compared with native collagen, indicating cross-linking, albeit pronounced conformational changes for IDP were witnessed indicating denaturation. The impedance was analyzed to correlate the conformational changes with the hydration dynamics of protein. Changes in the dielectric properties of collagen observed upon treatment with CS and IDP reported molecular reorientation in the surrounding water milieu, suggesting compactness or destabilization of the collagen. This was further confirmed by proton transverse NMR relaxation time measurements, which demonstrated that the water mobility changes in the presence of the ILs. At the fibrillar level, differential scanning calorimetry thermograms for rat tail tendon collagen fibers treated with CS show a 5 °C increase in denaturation temperature, suggesting imparted stability. On the contrary, a significant temperature decrease was noticed for IDP, indicating the destabilization of collagen fibers. The obtained results clearly indicate that the changes in the secondary structure of protein are due to the changes in the hydration dynamics of collagen upon interaction with ILs. Thus, this study on the interaction of collagen with ionic liquids unfolds the propensity of ILs to stabilize or destabilize collagen depending on the changes invoked at the molecular level in terms of structure and dynamics of protein, which also got manifested at the fibrillar level.

A size-adjustable radiofrequency coil for investigating plants in a Halbach magnet.

A radio-frequency coil with adjustable distance has been developed and tested for in-situ examination of growing plants. The Helmholtz-based coil design reduces laborious tuning and matching efforts encountered with solenoids wound around a growing stem or branch. Relaxation experiments were performed on tomato plants and winter wheat under controlled light irradiation. Changes in signal amplitude and in relaxation times T2 were recorded over day and night cycles. Peaks in distributions of relaxation times were attributed to different tissue components of two different plants.