High-resolution spectroscopy

Single-sided nuclear magnetic resonance (NMR) sensors have been used for over two decades to characterize arbitrarily large samples (1). In contrast to conventional NMR apparatus, where the sample must be adapted to fit into the bore of large superconducting magnets, single-sided NMR experiments use portable open magnets, which are placed from 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. Given these detrimental conditions, the standard techniques of conventional NMR do not work, and new strategies need to be developed in order to extract valuable information from the NMR signal (2-8). Starting from simple relaxation-time measurements, more sophisticated methods of ex situ NMR have been developed, such as Fourier imaging (5), velocity imaging (6), and multi-dimensional relaxation and diffusion correlation / exchange maps (7). 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. Proposed in 2001 (8), this approach was recently implemented on using a portable single-sided sensor (9).    Magnetic fields generated by open magnets are believed to be inherently inhomogeneous, precluding acquisition of chemical-shift resolved NMR spectra. This perception is at the root of designing such ingenious tricks as nutation echoes (8), and shim pulses (10,11) to measure the chemical shift ex situ. Here we break with that assumption, and demonstrate experimentally that highly homogenous magnetic fields (a few parts in 107) can be generated external to the magnet in a very simple way (12).

References

1. B. Blümich, “NMR imaging of materials”, Oxford Univ. Press, Oxford (2000).
2. Y-Q. Song, S. Ryu, P.N. Sen, Nature 406, 178 (2000).
3. T.M. Brill, et al , Science 297, 369 (2002).
4. S. Appelt, H. Kühn, F. Wolfgang, B. Blümich, Nat. Phys. 2, 105 (2006).
5. J. Perlo, F. Casanova, B. Blümich, J. Magn. Reson. 166, 228 (2004).
6. J. Perlo, F. Casanova, B. Blümich, J. Magn. Reson. 173, 254 (2005).
7. M.D. Hürlimann, L. Venkataramanan, J. Magn. Reson. 157, 31 (2002).
8. C.A. Meriles, D. Sakellariou, H. Heise, A.J. Moulé, A. Pines, Science 293, 82 (2001).
9. J. Perlo, V. Demas, F. Casanova, C.A. Meriles, A. Pines, B. Blümich, Science 308, 1279 (2005).
10. B. Shapira, and L. Frydman, J. Am. Chem. Soc. 126, 7184 (2004).
11. D. Topgard, R. Martin, D. Sakellariou, C.A. Meriles, and A. Pines, Proc. Natl. Acad. Sci. USA 101, 17576 (2004).
12. J. Perlo, F. Casanova, B. Blümich, Science 315, 1110-1112 (2007).  

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