Polarization transfer efficiency in PHIP experiments.

Parahydrogen induced polarization (PHIP) is a hyperpolarization method for NMR signal enhancement with applications in spectroscopy and imaging. Although parahydrogen can be easily enriched up to nearly 95%, the polarization detected on the hydrogenated substrate is substantially lower, where numerous loss mechanisms between the start of the hydrogenation reaction and detection affect polarization levels. The quality of PHIP systems is commonly determined by stating either the polarization degree or the enhancement factor of the product at the time of detection. In this study, we present a method that allows the distinction of polarization loss due to both the catalytic cycle and T1 relaxation of the formed product prior to detection. We determine the influence of homogeneous catalysts and define a rigorous measure of the polarization transfer efficiency (PTE). Our results show that the PTE strongly depends on the concentration of all components and the chemical structure of the catalyst as well as on the magnetic field of detection.

Moisture dynamics in wall paintings monitored by single-sided NMR.

The durability of historic wall paintings is highly dependent on environmental influences such as moisture ingress, salt crystallization and temperature changes. A fundamental understanding of dynamic transport processes in wall paintings is necessary to apply suitable conservation and restoration methods to preserve such objects with high cultural value. Non-invasive, mobile-NMR techniques with single-sided sensors, such as the NMR-MOUSE(®), enable to monitor the moisture content, transport and apparent diffusion constants in wall paintings. We investigated this technique by experiment and modeling to correlate salt crystallization, moisture transport and local diffusion in wall-painting samples. Moreover, the influence of different painting techniques (fresco and secco) and conservation/consolidation methods on moisture transport and diffusion is discussed. The results are compared with results from field measurements on real fresco paintings in Casa del Salone Nero and the Villa of the Papyri, Herculaneum, Italy.

On-line monitoring of chemical reactions by using bench-top nuclear magnetic resonance spectroscopy.

Real-time nuclear magnetic resonance (NMR) spectroscopy measurements carried out with a bench-top system installed next to the reactor inside the fume hood of the chemistry laboratory are presented. To test the system for on-line monitoring, a transfer hydrogenation reaction was studied by continuously pumping the reaction mixture from the reactor to the magnet and back in a closed loop. In addition to improving the time resolution provided by standard sampling methods, the use of such a flow setup eliminates the need for sample preparation. Owing to the progress in terms of field homogeneity and sensitivity now available with compact NMR spectrometers, small molecules dissolved at concentrations on the order of 1 mmol L(-1) can be characterized in single-scan measurements with 1 Hz resolution. Owing to the reduced field strength of compact low-field systems compared to that of conventional high-field magnets, the overlap in the spectrum of different NMR signals is a typical situation. The data processing required to obtain concentrations in the presence of signal overlap are discussed in detail, methods such as plain integration and line-fitting approaches are compared, and the accuracy of each method is determined. The kinetic rates measured for different catalytic concentrations show good agreement with those obtained with gas chromatography as a reference analytical method. Finally, as the measurements are performed under continuous flow conditions, the experimental setup and the flow parameters are optimized to maximize time resolution and signal-to-noise ratio.

Analysis of parahydrogen polarized spin system in low magnetic fields.

Nuclear magnetic resonance (NMR) spectra of spin systems polarized either thermally or by parahydrogen exhibit strikingly different field dependencies. Thermally polarized spin systems show the well-known roof effect, observed when reducing magnetic field strengths which precludes the independent determination of chemical shift differences and J-coupling constants at low-fields. Quantum mechanical analysis of the NMR spectra with respect to polarization method, pulsed state preparation, and transition probabilities reveals that spectra of parahydrogen polarized systems feature an "inverse roof effect" in the regime where the chemical shift difference δν is smaller than J. This inverse roof effect allows for the extraction of both J-coupling and chemical shift information down to very low fields. Based on a two-spin system, the observed non-linear magnetic field dependence of the splitting of spectral lines is predicted. We develop a general solution for the steady state density matrix of a parahydrogen polarized three-spin system including a heteronucleus which allows explaining experimentally observed (1)H spectra. The analysis of three-spin density matrix illustrates two pathways for an efficient polarization transfer from parahydrogen to (13)C nuclei. Examination of the experimental data facilitates the extraction of all relevant NMR parameters using single-scan, high-resolution (1)H and (13)C NMR spectroscopy at low fields at a fraction of the cost associated with cryogenically cooled high-field NMR spectrometers.

Highly stable and finely tuned magnetic fields generated by permanent magnet assemblies.

Permanent magnetic materials are the only magnetic source that can be used to generate magnetic fields without power consumption or maintenance. Such stand-alone magnets are very attractive for many scientific and engineering areas, but they suffer from poor temporal field stability, which arises from the strong sensitivity of the magnetic materials and mechanical support to temperature variation. In this work, we describe a highly efficient method useful to cancel the temperature coefficient of permanent magnet assemblies in a passive and accurate way. It is based on the combination of at least two units made of magnetic materials with different temperature coefficients arranged in such a way that the ratio of the fields generated by each unit matches the ratio of their effective temperature coefficients defined by both the magnetic and mechanical contributions. Although typically available magnetic materials have negative temperature coefficients, the cancellation is achieved by aligning the fields generated by each unit in the opposite direction. We demonstrate the performance of this approach by stabilizing the field generated by a dipolar Halbach magnet, recently proposed to achieve high field homogeneity. Both the field drift and the homogeneity are monitored via nuclear magnetic resonance spectroscopy experiments. The results demonstrate the compatibility of the thermal compensation approach with existing strategies useful to fine-tune the spatial dependence of the field generated by permanent magnet arrays.

Estimation of self-diffusion coefficients of small penetrants in semicrystalline polymers using single-sided NMR.

A simple and fast way to measure proton self-diffusion coefficients of small penetrant molecules in semicrystalline polymers is introduced. The approach takes advantage of the strong static gradient of a mobile single-sided NMR sensor and it is demonstrated on PE samples with varying degrees of crystallinity fully saturated in either toluene or n-hexane. The self-diffusion coefficients were measured using the gradient stimulated echo sequence appended with a CPMG. It is also shown for the first time, with demonstration on PE plates several millimeter thick with different aging histories, that one-dimensional profiles of self-diffusion coefficients as a function of depth can be easily obtained.

Near-zero-field nuclear magnetic resonance.

We investigate nuclear magnetic resonance (NMR) in near zero field, where the Zeeman interaction can be treated as a perturbation to the electron mediated scalar interaction (J coupling). This is in stark contrast to the high-field case, where heteronuclear J couplings are normally treated as a small perturbation. We show that the presence of very small magnetic fields results in splitting of the zero-field NMR lines, imparting considerable additional information to the pure zero-field spectra. Experimental results are in good agreement with first-order perturbation theory and with full numerical simulation when perturbation theory breaks down. We present simple rules for understanding the splitting patterns in near-zero-field NMR, which can be applied to molecules with nontrivial spectra.

Chain dynamics of a weakly adsorbing polymer in thin films.

Thin films of weakly adsorbing poly(dimethyl siloxane) (PDMS) on porous alumina are examined with NMR fast field cycling (FFC) relaxometry and NMR transverse relaxometry. The longitudinal relaxation dispersion of polymer amounts corresponding to approximate monolayer coverage shows substantial deviation from the bulk and is characterized by a particularly weak temperature dependence. Thicker films, however, show relaxation behavior and temperature dependence more similar to the bulk polymer. Transverse relaxation times were found to cover a range of several orders of magnitudes for any sample investigated; their dependence on temperature is a function of the total amount of adsorbed polymer. While thick films see an overall increase of molecular mobility at higher temperatures, monolayer films are best characterized by the decreasing fraction of a short, i.e. relatively rigid, component. These effects are consistent with the concept of two regions, one in which chain dynamics deviate from bulk and another where chain dynamics are reduced but bulk-like, although chains inside each region may also experience motional heterogeneity.

From LASER physics to the para-hydrogen pumped RASER.

The properties of the LASER with respect to self-organization are compared with the key features of the p-H2 pumped RASER. According to LASER theory the equations of motion for the LASER can be derived from the enslaving principle, i.e. the slowest-changing order parameter (the light field in the resonator) enslaves the rapidly relaxing atomic degrees of freedom. Likewise, it is shown here that the equations of motion for the p-H2 pumped RASER result from a set of order parameters, where the transverse magnetization of the RASER-active spin states enslaves the electromagnetic modes. The consequences are striking for nuclear magnetic resonance (NMR) spectroscopy, since long-lasting multi-mode RASER oscillations enable unprecedented spectroscopic resolution down to the micro-Hertz regime. Based on the theory for multi-mode RASER operation we analyze the conditions that reveal either the collapse of the entire NMR spectrum, the occurrence of self-organized frequency-combs, or RASER spectra which reflect the J-coupled network of the molecule. Certain RASER experiments involving the protons of 15N pyridine or 3-picoline molecules pumped with p-H2 via SABRE (Signal Amplification By Reversible Exchange) show either a single RASER oscillation in the time domain, giant RASER pulses or a complex RASER beat pattern. The corresponding 1H spectra consist of one narrow line, equidistant narrow lines (frequency-comb), or highly resolved lines reporting NMR properties, respectively. Numerous applications in the areas of material sciences, fundamental physics and medicine involving high precision sensors for magnetic fields, rotational motions or molecular structures become feasible.

Surface UV aging of elastomers investigated with microscopic resolution by single-sided NMR.

Depth profiles taken from the surface of UV irradiated natural rubber sheets have been measured with microscopic resolution using a Profile NMR-MOUSE. An NMR observable related to the sum of the spin echoes in the Carr-Purcell-Meiboom-Gill pulse sequence was used to characterize the cross-link density changes produced by the action of UV radiation in each sheet. The aging process was investigated as function of irradiation time and penetration depth. An exponential attenuation law with a space dependent absorption coefficient describes the change in the NMR observable with penetration depth. An Avrami model is used to describe the dependence of the absorption coefficient on the aging time. The method can be applied to investigate the effect of various aging agents on the surfaces of elastomers.

Single-sided sensor for high-resolution NMR spectroscopy.

The unavoidable spatial inhomogeneity of the static magnetic field generated by open sensors has precluded their use for high-resolution NMR spectroscopy. In fact, this application was deemed impossible because these field variations are usually orders of magnitude larger than those created by the microscopic structure of the molecules to be detected. Recently, chemical shift resolved NMR spectra were observed for the first time outside a portable single-sided magnet by implementing a method that exploits inhomogeneities in the rf field designed to reproduce variations of the static magnetic field. In this communication, we describe in detail the magnet system built from permanent magnets as well as the rf coil geometry used to compensate the static field variations.

Self-diffusion measurements by a mobile single-sided NMR sensor with improved magnetic field gradient.

A simple and fast method of measuring self-diffusion coefficients of protonated systems with a mobile single-sided NMR sensor is discussed. The NMR sensor uses a magnet geometry that generates a highly flat sensitive volume where a strong and highly uniform static magnetic field gradient is defined. Self-diffusion coefficients were measured by Hahn- and stimulated echoes detected in the presence of the uniform magnetic field gradient of the static field. To improve the sensitivity of these experiments, a Carr-Purcell-Meiboom-Gill pulse sequence was applied after the main diffusion-encoding period. By adding the echo train the experimental time was strongly shortened, allowing the measurement of complete diffusion curves in less than 1min. This method has been tested by measuring the self-diffusion coefficients D of various organic solvents and poly(dimethylsiloxane) samples with different molar masses. Diffusion coefficients were also measured for n-hexane absorbed at saturation in natural rubber with different cross-link densities. The results show a dependence on the concentration that is in good agreement with the theoretical prediction. Moreover, the stimulated-echo sequence was successfully used to measure the diffusion coefficient as a function of the evolution time in systems with restricted diffusion. This type of experiment proves the pore geometry and gives access to the surface-to-volume ratio. It was applied to measure the diffusion of water in sandstones and sheep Achilles tendon. Thanks to the strong static gradient G(0), all diffusion coefficients could be measured without having to account for relaxation during the pulse sequence.

Multiecho sequence for velocity imaging in inhomogeneous rf fields.

The unambiguous determination of velocities with spatial resolution in a multiecho PFG NMR sequence strongly depends on the homogeneity of the B1 field. This affects, in particular, the use of surface coils that bear considerable potential for on-line flow monitoring where a fast-imaging sequence can become vital. However, even with most rf coils dedicated for imaging applications, B1 inhomogeneities are sufficiently large to generate severe problems in performing velocity-imaging experiments. In this paper, the use of a combination of different phase cycles in Carr-Purcell sequences is discussed. The suggested phase cycling scheme tolerates large flip angle imperfections arising in inhomogeneous B1 fields, and thus allows acquisition of a maximum number of echoes within a pulse train. The performance of the velocity-imaging sequence is proven by using phantom samples developing known laminar flow patterns.

Profiles with microscopic resolution by single-sided NMR.

A single-sided NMR sensor to produce depth profiles with microscopic spatial resolution is presented. It uses a novel permanent magnet geometry that generates a highly flat sensitive volume parallel to the scanner surface. By repositioning the sensitive slice across the object one-dimensional profiles of the sample structure can be produced with a space resolution better than 5 microm. The open geometry of the sensor results in a powerful testing tool to characterize arbitrarily sized objects in a non-destructive way.

Velocity imaging by ex situ NMR.

A pulsed field gradient stimulated spin-echo NMR sequence is combined with imaging methods to spatially resolve velocity distributions and to measure 2D velocity maps ex situ. The implementation of these techniques in open sensors provides a powerful non-invasive tool to measure molecular displacement in a large number of applications inaccessible to conventional closed magnets. The method is implemented on an open tomograph that provides 3D spatial localization by combining slice selection in the presence of a uniform static magnetic field gradient along the depth direction with pulsed field gradients along the two lateral directions. Different pipe geometries are used to demonstrate that the sequence performs well even in the extremely inhomogeneous B0 and B1 fields of these sensors.

Visualizing flow vortices inside a single levitated drop.

The internal flow dynamics in single liquid drops, kept in place through levitation by a counterflowing continuous fluid phase in a suitably designed glass cell, is investigated by PFG NMR techniques. The positional stability of the drops was confirmed from series of one-dimensional profiles and was found to be below the spatial resolution of the experiment. Velocity distribution functions (propagators) along all three coordinates were obtained and demonstrated the long-time stability of the internal dynamics in terms of the velocity magnitudes occurring in the systems. Finally, velocity imaging was applied to visualize the internal vortex patterns in the drops either as projections onto different planes or within thin slices of selected orientations. Two different fluid systems were investigated in order to cover the principal cases of rigid and mobile interfaces. Different fast velocity imaging techniques were employed for monitoring the vastly differing velocity ranges of both cases, and the high sensitivity of the internal three-dimensional motion to the cell geometry is demonstrated.

Multispin moments edited by multiple-quantum NMR: application to elastomers.

The spin system response to the five-pulse sequence used for measurements of double-quantum and triple-quantum buildup curves is evaluated in the initial excitation/reconversion regime. The multispin dipolar network that is present also in many soft solids like elastomers was considered. It is proved rigorously that the relevant quantity for analysis of double-quantum build-up curves in the initial regime is the second van Vleck moment. The higher-order moments edited by double-quantum as well as higher-order coherences in the multiple-quantum build-up experiments are different from van Vleck moments. These results can be applied to compare (1)H residual moments edited by double-quantum and triple-quantum experiments with those measured by other NMR methods. The sensitivity of multiple-quantum coherences to the changes in the values of residual dipolar couplings for cross-linked natural rubber under uniaxial elongation is also discussed. Under such conditions (1)H second van Vleck moments were measured for different elongation ratios of a cross-linked natural rubber. Moreover, (1)H triple-quantum edited moments were also measured for the same sample under uniaxial compression. The dependence of the second van Vleck moment and the time of the maximum of the double-quantum buildup curve on the cross-link density of natural rubber measured at low magnetic field was also investigated.

Internal fluid dynamics in levitated drops by fast magnetic resonance velocimetry.

Fluid motion inside a levitated drop is determined by the interface properties. Momentum transfer through a highly mobile interface results in stationary vortex patterns inside the drop that dramatically enhance mass transfer between both phases, while immobile interfaces suppress internal dynamics. The presence of small amounts of surface-active substances can result in a partial reduction of interface mobility, the so-called rigid cap. The time dependence of internal flow patterns is presented by means of NMR velocity images of levitated drops, and is compared to fast measurements of the velocity distribution along the three orthogonal coordinates.

[Measuring the contrast-enhancement in the skin and subcutaneous fatty tissue with the NMR-MOUSE: a feasibility study]. / Messung der Kontrastmittelanreicherung in der Haut und im subkutanen Fettgewebe mit der NMR-MOUSE -- eine Machbarkeitsstudie.

PURPOSE: The NMR-MOUSE is an open and mobile sensor for measuring NMR relaxation parameters in organic matters. T1-measurements of the subcutaneous fatty tissue and the skin are reported. MATERIAL AND METHOD: For the first time, the NMR-MOUSE was employed to measure the signal recovery following saturation of the skin and the subcutaneous fatty tissue of three patients, before and after administering a contrast agent. RESULTS: Despite a low signal-to-noise ratio, changes in the relaxation behaviour of the skin could be detected. Malignant tissue exhibits faster signal recovery than scar tissue and healthy tissue, which only show a small difference. CONCLUSIONS: Changes in the relaxation behavior can be monitored with the NMR-MOUSE. Before the clinical use of the NMR-MOUSE, sensitivity, sensor mounting device, and sensor tuning must be improved. Further investigations need to be performed on a statistically relevant number of patients.

Two-dimensional imaging with a single-sided NMR probe.

A new low field unilateral NMR sensor equipped with a two-dimensional gradient coil system was built. A new NMR-MOUSE concept using a simple bar magnet instead of the classical U-shaped geometry was used to produce magnetic field profiles comparatively homogeneous in extended lateral planes defining a suitable field of view for 2D spatial localization. Slice selection along the depth direction is obtained by means of the highly constant static magnetic field gradient produced by this magnet geometry. Implementing a two-dimensional phase-encoding imaging method 2D cross sections of objects were obtained with high spatial resolution. By retuning the probe it was possible to change the depth of the selected slice obtaining a 3D imaging method. The details of the construction of the new device are presented together with imaging tests to show the quality of space encoding.