Development of methodologies and portable sensors for high-resolution NMR spectroscopy in inhomogeneous fields
Final Report Abstract
Single-sided nuclear magnetic resonance (NMR) sensors have been used for over two decades to characterize arbitrarily large samples. In contrast to conventional NMR systems where the sample must pass into the bore of large superconducting magnets, single-sided NMR experiments use portable open magnets placed on one side of an object to detect NMR signals ex situ. This configuration is convenient for nondestructive inspection of valuable objects, from which fragmentary samples cannot be drawn, but the convenience is bought at the expense of high and homogeneous magnetic fields that afford spectral resolution in conventional NMR studies. Although many attempts have been presented to reduce the inhomogeneity of the magnets, it was always explored as a chance to increase the size of the sensitive volume of the sensor, and by no means a strategy to generate a homogeneous magnetic field, which was believed to be inherently inhomogeneous precluding acquisition of chemical-shift resolved NMR spectra. These detrimental experimental conditions complicate the implementation of standard techniques of conventional NMR, and for many years the community has focus on the development of new strategies in order to extract valuable information from the NMR signal. Starting from simple relaxation-time measurements, more sophisticated methods of ex situ NMR have been proposed, such as Fourier imaging, velocity imaging, and multi-dimensional relaxation and diffusion correlation / exchange. A remarkable achievement is the use of nutation echoes generated by a combination of static and radio-frequency (rf) magnetic fields with matched inhomogeneities to resolve the chemical shift in inhomogeneous fields. Although at the beginning of this project we shared the perception that generating inhomogeneous fields is the price to be paid for gaining access to study large samples, during the last two years we have broken this assumption demonstrating experimentally that highly homogenous magnetic fields (a few parts in 10^) can be generated external to the magnet by including in the main magnet a set of movable magnets forming what we call a shim unit. The possibility to remove the obstacle (field inhomogeneity) instead of having to dodge it is having a tremendous impact in the field of mobile NMR busting a number of new applications completely discarded in the past. Moreover, the shim unit concept introduced to correct the field inhomogeneities has also been applied to shim closed magnet geometries demonstrating that desktop NMR systems can be used to achieve high-resolution spectroscopy and imaging. In contrast with previous works, where the strategy to obtain high resolution in portable magnets consisted of reducing the sample volume using micro coils, the shimming technique developed in this project allowed us to achieve relatively high homogeneity in a magnet volume fraction (sample volume/magnet volume) several orders of magnitude larger than those previously reported. By including variables of control in other magnet geometries, like the one described is expected to help correcting the field inhomogeneities (mainly of first and second order) allowing to use larger samples or to achieve better homogeneity, factors that would lead to important SNR improvements. In the next sections the working principle of the shim unit is described and the results obtained for open and closed magnets are discussed.
