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.
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5 comments:
Glenn, this is a neat figure. Students interested in measuring small J's often have the misconception that they are better off at higher field. The next time I hear that, I'll point them to this post!
It would be interesting to compare the linewidths at 1/2 height for these two spectra as a measure of the inhomogeneity (assuming the widths are dominated by field inhomogeneity). In terms of ppm of Bo, I would expect your "high field" spectrum to be more homogeneous. Do you agree?
I do agree that the homogeneity in terms of ppm of Bo is better for the high field spectrum.
Glenn,
we are installing one of those Terranova Earth's Field NMR machines here at M-40.
http://www.magritek.com/terranova.html
Bring your students for a tour when it's up and running.
Victor
www.nmr900.ca
Hi,
Great site! So much to learn and some great examples.
I just started in this area and just wanted to know the advantage of using high versus low field for nmr measurements. I know SNR improves at higher field, but, for example, why can one only do a chemical shift study at high field rather but not at low field (say well under 1 Tesla)?
thanks,
Vinny
Vinny,
Thank you for the question. Aside from the SNR advantage with higher fields, there is also more chemical shift dispersion at higher fields. Peaks are further apart from one another (in Hz) at higher field. For 1H NMR on a 300 MHz spectrometer, one ppm on the chemical shift scale equals 300 Hz. For a 600 MHz spectrometer, one ppm equals 600 Hz. At extremely low fields (like the earths magnetic field) chemical shift resolution between different types of protons is virtually nonexistent.
Glenn
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