Mobile single-sided NMR

Magnets of the Halbach layout are interesting for the use in mobile NMR/MRI devices. Therefore, the ideal Halbach magnet was iterated using identical bar magnets, which are positioned and oriented based on analytical equations. These configurations were simulated with two-dimensional finite-element methods. Performance factors were defined to relate field strength with homogeneity and mass. A geometry of 16 magnets provided a good compromise between performance and special requirements for the desired use to store hyperpolarized xenon. The construction of such a magnet required the design of dedicated support frames and procedures to mount them. The field strength and homogeneity in the finished magnet were measured by a Hall probe and agreed well with the simulations. The calculation and construction are described in detail and together with the tabled field values for n magnets with n ≤ 80 may prove helpful for building similar devices, which were named NMR Mandhalas (Magnet Arrangements for Novel Discrete Halbach Layout). © 2004 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 23B: 16–25, 2004

Design and construction of a dipolar Halbach array with a homogeneous field from identical bar magnets: NMR Mandhalas

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

Miniaturization of NMR systems: desktop spectrometers, microcoil spectroscopy, and "NMR on a chip" for chemistry, biochemistry, and industry.

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.

http://www.sciencemadness.org/talk/files.php?pid=187132&aid=11494

Small Magnets for Portable NMR Spectrometers

Time-domain NMR is being used throughout all areas of food science and technology. A wide range of one- and two-dimensional relaxometric and diffusometric applications have been implemented on cost-effective, robust and easy-to-use benchtop NMR equipment. Time-domain NMR applications do not only cover research and development but also quality and process control in the food supply chain. Here the opportunity to further downsize and tailor equipment has allowed for “mobile” sensor applications as well as online quality inspection. The structural and compositional information produced by time-domain NMR experiments requires adequate data-analysis techniques. Here one can distinguish model-driven approaches for hypothesis testing, as well as explorative multi-variate approaches for hypothesis generation. Developments in hardware and software will further enhance measurement speed and reveal more detailed structural features in complex food systems.

Time domain NMR applied to food products

The multi-infinity dipole magnet is a new design capable of generating a relatively high and homogeneous permanent magnetic field (0.1–3 Tesla and homogeneity 20 ppm for 3 mm) that is suitable for nuclear magnetic resonance (NMR) or electron spin resonance (ESR) studies of small volume chemical or biological samples. The theoretical design and practical aspects involved in the construction of a prototype tabletop NMR system (0.6 T and homogeneity of 10 ppm in a ellipsoidal region of 3 × 3 × 5 mm3) are described. A simple NMR relaxation time experiment was performed on a water sample doped with 40 mM of CuSO4. This system in its current configuration is suitable for relaxation time studies of materials or for educational use. The introduction of shim coils would increase the homogeneity of the magnet by a factor of 100, allowing the system to be used as a portable NMR spectrometer. In addition the introduction of gradient coils between the poles would transform the spectrometer to a MRI portable system for tissue analysis or simple MR microimaging at fields up to 3 T. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 19B: 35–43, 2003.

Giorgio Moresi

Papers

Miniature permanent magnet for table-top NMR