Ernesto Danieli

Small Magnets for Portable NMR Spectrometers

High-resolution nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful analytical tools used to probe details of molecular structure and dynamics. The study of large molecules such as proteins requires high sensitivity and high spectral resolution, which are both achieved with strong magnetic fields. These fields are generated by huge superconducting magnets, which are made stronger and bigger each year to tackle larger and larger molecules. The results of this amazing technological effort are bulky and static magnets permanently installed in dedicated NMR laboratories. The size of the superconducting magnets, their sensitivity to harsh environments, and the cost of maintenance and operation keep this technology away from fume hoods and production sites, where simpler devices that provide access to medium-size molecules would be needed. Robust NMR magnets can be made from permanent magnets like those used for NMR spectroscopy in the 1960s and 1970s. But to achieve high resolution for standard sample volumes, the permanent magnets then were as big as superconducting magnets today and weighed several hundred kilograms. Considering that the magnetic field strength remains constant when the volume of the magnet is scaled down to gain portability and that fields of up to 2 T are generated by permanent magnets, small magnets would offer a sensitivity only a factor of three smaller than that achieved in a standard (7 T) superconducting magnet (see the dotteddashed lines in Figure 1). This limitation is an affordable price to be paid for a small and portable system. However, a second factor seriously compromises the signal-to-noise ratio in the miniaturization process: the reduction of sample volume. For each magnet geometry the ratio between the magnet size and the size of the sensitive volume is a constant. When the size of the magnet is reduced, a smaller volume of highfield homogeneity is generated. For example, if the oldfashioned Varian T-60 magnet, with a volume of about 1 m, is reduced to palm-size dimensions, a sensitivity loss of about three orders of magnitude is expected (circle in Figure 1). Although this approach is valid in cases where the amount of sample is limited (capillary NMR), this sensitivity loss is simply unacceptable for most applications. We report herein on the construction of a small permanent magnet with an extraordinarily homogeneous magnetic field B0 suitable for measuring H NMR spectra of solutions in standard 5 mm NMR sample tubes (Figure 2). Weighing only 500 grams, the magnet can be transported along with the spectrometer, and NMR measurements can be performed on demand with this robust device at minimal maintenance cost. To efficiently reduce the sensor volume by three orders of magnitude over that of typical C-magnet designs, individual magnet blocks were compactly arranged in a cylindrical array based on the design by Halbach. This array provides a generous volume for sample positioning (large bore/magnet size ratio), and generates a magnetic field perpendicular to its cylinder axis (Figure 2), which allows the use of sensitive solenoidal radio-frequency (rf) coils to detect the NMR signals. In theory, the magnetic field generated by an infinitely long magnet built from perfect magnet blocks would be highly homogeneous along the length of the sample tube with almost zero stray field. However, in practice, the finite length of the magnet and the statistical imperfections of the sintered magnet blocks deteriorate the predicted homogeneity by several orders of magnitude. The new design presented herein combines three Halbach rings with different geometric proportions optimized to account for the field distortions along the cylinder axis due to the finite magnet length. To tackle the important source of inhomogeneity introduced by the variability of the pieces, each ring is composed of fixed trapezoidal elements with parallel gaps between them that guide the movement of rectangular magnet blocks (Figure 2). These pieces can be moved radially in and out to mechanically shim the magnetic field with highly efficiency and accuracy. By displacing the rectangular blocks in each ring with defined angular modulations and amplitudes, it is possible to independently Figure 1. Signal-to-noise ratio (SNR) for permanent and superconducting magnets as a function of the field strength B0. Squares show the SNR for water in a 5 mm NMR tube. Dashed and dotted lines correspond to solenoidal and birdcage rf coils used with permanent and superconducting magnets, respectively. The circle indicates the SNR value for a reduced sample volume in a capillary with a diameter of 0.3 mm.

Miniaturization of NMR Systems: Desktop Spectrometers, Microcoil Spectroscopy, and “NMR on a Chip” for Chemistry, Biochemistry, and Industry

Spectroscopy, and “NMR on a Chip” for Chemistry, Biochemistry, and Industry Sergey S. Zalesskiy,† Ernesto Danieli,‡ Bernhard Blümich,*,‡ and Valentine P. Ananikov*,†,§ †Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991, Russia ‡Institut für Technische Chemie und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, D-52074 Aachen, Germany Department of Chemistry, Saint Petersburg State University, Stary Petergof, 198504, Russia

Mobile sensor for high resolution NMR spectroscopy and imaging

In this work we describe the construction of a mobile NMR tomograph with a highly homogeneous magnetic field. Fast MRI techniques as well as NMR spectroscopy measurements were carried out. The magnet is based on a Halbach array built from identical permanent magnet blocks generating a magnetic field of 0.22T. To shim the field inhomogeneities inherent to magnet arrays constructed from these materials, a shim strategy based on the use of movable magnet blocks is employed. With this approach a reduction of the line-width from approximately 20kHz to less than 0.1kHz was achieved, that is by more than two orders of magnitude, in a volume of 21cm(3). Implementing a RARE sequence, 3D images of different objects placed in this volume were obtained in short experimental times. Moreover, by reducing the sample size to 1cm(3), sub ppm resolution is obtained in (1)H NMR spectra.

High-resolution NMR spectroscopy under the fume hood.

This work reports the possibility to acquire high-resolution (1)H NMR spectra with a fist-sized NMR magnet directly installed under the fume hood. The small NMR sensor based on permanent magnets was used to monitor the trimerization of propionaldehyde catalyzed by indium trichloride in real time by continuously circulating the reaction mixture through the magnet bore in a closed loop with the help of a peristaltic pump. Thanks to the chemical selectivity of NMR spectroscopy the progress of the reaction can be monitored on-line by determining the concentrations of both reactant and product from the area under their respective lines in the NMR spectra as a function of time. This in situ measurement demonstrates that NMR probes can be used in chemistry laboratories, e.g. for reaction optimization, or installed at specific points of interest along industrial process lines. Therefore, it will open the door for the implementation of feedback control based on spectroscopic NMR data.

Low-gradient single-sided NMR sensor for one-shot profiling of human skin.

This paper describes a shimming approach useful to reduce the gradient strength of the magnetic field generated by single-sided sensors simultaneously maximizing its uniformity along the lateral directions of the magnet. In this way, the thickness of the excited sensitive volume can be increased without compromising the depth resolution of the sensor. By implementing this method on a standard U-shaped magnet, the gradient strength was reduced one order of magnitude. In the presence of a gradient of about 2 T/m, slices of 2mm could be profiled with a resolution that ranges from 25 μm at the center of the slice to 50 μm at the borders. This sensor is of particular advantage for applications, where the scanning range is of the order of the excited slice. In those cases, the full profile is measured in a single excitation experiment, eliminating the need for repositioning the excited slice across the depth range to complete the profile as occurs with standard high gradient sensors. Besides simplifying the experimental setup, the possibility to move from a point-by-point measurement to the simultaneous acquisition of the full profile led to the shortening of the experimental time. A further advantage of performing the experiment under a smaller static gradient is a reduction of the diffusion attenuation affecting the signal decay measured with a CPMG sequence, making it possible to measure the T(2) of samples with high diffusivity (comparable to the water diffusivity). The performance of the sensor in terms of resolution and sensitivity is first evaluated and compared with conventional singled-sided sensors of higher gradient strength using phantoms of known geometry and relaxation times. Then, the device is used to profile the structure of human skin in vivo. To understand the contrast between the different skin layers, the distribution of relaxation times T(2) and diffusion coefficients is spatially resolved along the depth direction.

Environmentally induced quantum dynamical phase transition in the spin swapping operation.

Quantum information processing relies on coherent quantum dynamics for a precise control of its basic operations. A swapping gate in a two-spin system exchanges the degenerate states |(up arrow, down arrow)> and |(down arrow, up arrow)>. In NMR, this is achieved turning on and off the spin-spin interaction b=DeltaE that splits the energy levels and induces an oscillation with a natural frequency DeltaE/Plancks. Interaction of strength Plancks/tau(SE), with an environment of neighboring spins, degrades this oscillation within a decoherence time scale tau(phi). While the experimental frequency omega and decoherence time tau(phi) were expected to be roughly proportional to b/Plancks and tau(SE), respectively, we present here experiments that show drastic deviations in both omega and tau(phi). By solving the many spin dynamics, we prove that the swapping regime is restricted to DeltaEtau(SE) similar or greater than Plancks. Beyond a critical interaction with the environment the swapping freezes and the decoherence rate drops as 1/tau(phi) proportional to (b/Plancks)2tau(SE). The transition between quantum dynamical phases occurs when omega proportional to sqrt (b/Plancks)2-(k/tau(SE)2 becomes imaginary, resembling an overdamped classical oscillator. Here, 0< or =k2< or =1 depends only on the anisotropy of the system-environment interaction, being 0 for isotropic and 1 for XY interactions. This critical onset of a phase dominated by the quantum Zeno effect opens up new opportunities for controlling quantum dynamics.

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.

Small-scale instrumentation for nuclear magnetic resonance of porous media

The investigation of fluids confined to porous media is the oldest topic of investigation with small-scale nuclear magnetic resonance (NMR) instruments, as such instruments are mobile and can be moved to the site of the object, such as the borehole of an oil well. While the analysis was originally restricted by the inferior homogeneity of the employed magnets to relaxation measurements, today, portable magnets are available for all types of NMR measurements concerning relaxometry, imaging and spectroscopy in two types of geometries. These geometries refer to closed magnets that surround the sample and open magnets, which are brought close to the object for measurement. The current state of the art of portable, small-scale NMR instruments is reviewed and recent applications of such instruments are featured. These include the porosity analysis and description of diesel particulate filters, the determination of the moisture content in walls from gray concrete, new approaches to analyze the pore space and moisture migration in soil, and the constitutional analysis of the mortar base of ancient wall paintings.

Optimized slim-line logging NMR tool to measure soil moisture in situ

We report the optimization of a slim-line logging NMR tool carried out by maximizing the signal-to-noise ratio of the NMR measurements. The tool, based on cylindrical permanent magnets of 20 cm length and 5 cm diameter, has a penetration depth of about 2 cm measured from its surface. This is obtained thanks to a large radio frequency coil whose dimensions are comparable to the sensor size. An analytical expression of the SNR as a function of parameters which take into account the interaction between the radio frequency coil and the magnet shielding is developed. In view of the external constrains such as the one imposed by the excavation hole, a proper tool size is determined in the optimization process. Due to its size and properties, the sensor is suitable to measure water content in the vadose zone, which is the zone comprised within the first meters of the Earth surface and whose study is important for improving water management in agriculture and for refining climate models.

Determining object boundaries from MR images with sub-pixel resolution: Towards in-line inspection with a mobile tomograph

This work evaluates the performance of edge-detection algorithms to determine the sample geometry with high spatial accuracy from low-resolution MR images. In particular, we show that by applying such numerical methods it is possible to reconstruct the internal and external contours of the object with a spatial precision that surpasses the nominal spatial resolution of the image by more than one order of magnitude. Special attention is paid to find the spatial resolution and signal-to-noise ratio required by the described numerical methodology to achieve a desired spatial accuracy. Finally, we discuss the potential application of this image processing approach for in-line quality control of extruded rubber materials, where micrometer spatial precision has to be achieved from images measured in short experimental times. The results presented here prove that the sensitivity of mobile MRI sensors is enough to achieve the spatial accuracy required to proof check the production of extruded rubber fittings in acceptable experimental times.