Bernhard Blümich

The NMR-mouse: construction, excitation, and applications

A mobile nuclear magnetic resonance (NMR) device similar to a bore-hole probe has been developed for applications in materials science and biomedicine. Inhomogeneous polarizing and radio-frequency (rf) magnetic fields are applied to arbitrarily large samples from one side. Different experimental techniques have been tested to measure transverse and longitudinal relaxation times and translational diffusion constants. Good contrast for discrimination of material properties is gained when the residual dipolar coupling is retained in soft matter by avoiding spinlock effects in multi-pulse techniques. Applications to characterization of products from technical elastomers, skin, and coatings on iron sheets are reported.

Deuteron two-dimensional exchange NMR in solids

Abstract The deuteron two-dimensional exchange experiment for the study of slow molecular motions in polycrystalline and amorphous solids is presented. A pair of four-pulse sequences for obtaining pure phase 2D spectra of a spin I = 1 system and phase cycles for minimizing artifacts are derived from spin dynamics calculations. A comprehensive description of the experiment and the 2D data processing is given. The information content of 2D exchange spectra of powders is discussed. For discrete jump motions, 2D powder spectra exhibit characteristic ridge patterns originating from the singularities of the 2D lineshape. From the geometry of such ridge patterns the angles of reorientation of the electric field gradient tensors can be determined directly, thus yielding information about the type of motion. The method is illustrated by two examples: the two-site jump of dimethyl sulfone and the six-site jump of hexamethylbenzene.

DYNAMICS OF MOLECULAR REORIENTATIONS: DIRECT DETERMINATION OF ROTATIONAL ANGLES FROM TWO-DIMENSIONAL NMR OF POWDERS

Abstract Two-dimensional solid-state deuteron NMR spectroscopy has been applied to study rotational motions in powders. In general, tensorial interactions give rise to powder patterns producing 2D exchange signals with characteristic ridges which manifest the motional mechanism in a model-independent fashion. For deuterons in particular, the angles through which the molecules rotate are read directly from elliptical ridges in the 2D spectrum. Experimental data on the two-site exchange in dimethylsulphone are given as well as theoretical spectra for different types of molecular motion.

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.

Ex Situ NMR in Highly Homogeneous Fields: 1H Spectroscopy

Portable single-sided nuclear magnetic resonance (NMR) magnets used for nondestructive studies of large samples are believed to generate inherently inhomogeneous magnetic fields. We demonstrated experimentally that the field of an open magnet can be shimmed to high homogeneity in a large volume external to the sensor. This technique allowed us to measure localized high-resolution proton spectra outside a portable open magnet with a spectral resolution of 0.25 part per million. The generation of these experimental conditions also simplifies the implementation of such powerful methodologies as multidimensional NMR spectroscopy and imaging.

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

Residual dipolar couplings by 1H dipolar-encoded longitudinal magnetization, double- and triple-quantum nuclear magnetic resonance in cross-linked elastomers

The measurements of residual dipolar couplings in elastomer system is desirable, because they reflect the hindrance to molecular motions by the cross-linking, topological constraints and the external factors like mechanical stress. Dipolar-encoded longitudinal magnetization nuclear magnetic resonance (NMR) decay curves, double-quantum and triple-quantum NMR buildup intensities for measuring the residual dipolar couplings, and the associated dynamic order parameters are introduced. It is shown that in the short excitation time regime the effective dipolar network is simplified. In the limit of this model based on localized dipolar couplings, the spin response to two-dimensional pulse sequences used to record multiple-quantum (MQ) NMR coherences was evaluated for longitudinal magnetization, double-, and triple-quantum coherences of methylene, and methyl protons in synthetic 1,4-cis-polyisoprene. The dynamic order parameters can be evaluated from this NMR response using a classical scale-invariant polymer mo...

Degradation of historical paper: nondestructive analysis by the NMR-MOUSE

The NMR-MOUSE is a mobile sensor for single-sided NMR inspection of organic materials which takes advantage of the principles of magnetic resonance and inside-out-NMR. Historical books dating from the 17th century were measured at different points by positioning the NMR-MOUSE on the paper. Different degrees of paper degradation can be discriminated from the regularized inverse Laplace transform of the envelope of the acquired echo signals. For the first time the degradation of historical paper was characterized entirely nondestructively by NMR. As a contribution to current preservation efforts, NMR shows great promise for future use in damage assessment of historical documents.

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.

Noninvasive Testing of Art and Cultural Heritage by Mobile NMR

Nuclear magnetic resonance (NMR) has many applications in science, medicine, and technology. Conventional instrumentation is large and expensive, however, because superconducting magnets offer maximum sensitivity. Yet NMR devices can also be small and inexpensive if permanent magnets are used, and samples need not be placed within the magnet but can be examined externally in the stray magnetic field. Mobile stray-field NMR is a method of growing interest for nondestructive testing of a diverse range of materials and processes. A well-known stray-field sensor is the commercially available NMR-MOUSE, which is small and can readily be carried to an object to be studied. In this Account, we describe mobile stray-field NMR, with particular attention to its use in analyzing objects of cultural heritage. The most common data recorded are relaxation measurements of (1)H because the proton is the most sensitive NMR nucleus, and relaxation can be measured despite the inhomogeneous magnetic field that typically accompanies a simple magnet design. Through NMR relaxation, the state of matter can be analyzed locally, and the signal amplitude gives the proton density. A variety of stray-field sensors have been designed. Small devices weighing less than a kilogram have a shallow penetration depth of just a few millimeters and a resolution of a few micrometers. Access to greater depths requires larger sensors that may weigh 30 kg or more. The use of these sensors is illustrated by selected examples, including examinations of (i) the stratigraphy of master paintings, (ii) binder aging, (iii) the deterioration of paper, (iv) wood density in master violins, (v) the moisture content and moisture profiles in walls covered with paintings and mosaics, and (vi) the evolution of stone conservation treatments. The NMR data provide unique information to the conservator on the state of the object--including past conservation measures. The use of mobile NMR remains relatively new, expanding from field testing of materials such as roads, bridge decks, soil, and the contents of drilled wells to these more recent studies of objects of cultural heritage. As a young field, noninvasive testing of artworks with stray-field NMR thus offers many opportunities for research innovation and further development.