Wednesday, February 13, 2008
Ultra-high Resolution NMR in the Earth's Magnetic Field
Many NMR spectroscopists take great pride in their ability to shim their very expensive magnets well and acquire NMR spectra with beautiful sharp lines. Despite the skill of the spectroscopist, the line widths are still limited by the homogeneity of the magnetic field. There are NMR instruments designed to measure NMR spectra in the earth's magnetic field. If these instruments are set up outside of any external magnetic interference, then the magnetic field at the sample (the earth's field) is extremely homogeneous and one can obtain very sharp NMR lines - much sharper than those easily obtained in an expensive superconducting magnet. In the figure below, the 1H NMR spectrum of TMS is shown at 9.4T and in the earths magnetic field. The vertical scale of the spectrum has been increased to show the 29Si satellites. Note that the spectrum obtained in the earth's magnetic field (Stephan Appelt, Holger Kühn, F. Wolfgang Häsing and Bernhard Blümich, Nature Physics 2, 105-109 (2006)) has far superior resolution compared to the one acquired in a well shimmed 400 MHz magnet. Of course, this resolution comes at a price - there is essentially no chemical shift dispersion at such low fields and the sample must be polarized by an external magnet before data are collected.