Issue Date: January 15, 2007
Shimming Fields Of Portable Magnets
Shimming may be the way around the inhomogeneous magnetic fields that usually beset single-sided magnets used for portable NMR spectroscopy of large samples. It involves placing small "shim" magnets around the primary magnet to fine-tune the magnetic field. Bernhard Blümich and his colleagues at RWTH Aachen University, in Germany, show that they can obtain a homogeneous field by shimming the magnetic field of a conventional U-shaped single-sided magnet with four pairs of small NdFeB magnets (Science, DOI: 10.1126/science.1135499). The researchers fixed two pairs of the shim magnets at the bottom of the U-shaped magnet and placed two pairs of movable shim magnets at the top. The shimming setup also includes three single-sided radio-frequency coils to help fine-tune the magnetic field. They obtained proton NMR spectra with resolution that they say is improved by a factor of 30 relative to other single-sided NMR measurements. The resolution is sufficient for the researchers to resolve the peaks in toluene and acetic acid spectra. Such homogeneous magnetic fields could make established multidimensional NMR techniques possible with large objects.
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