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Tuesday, October 23, 2007

Measuring 1H Line Shape - The Line Shape Sample

It is important for NMR spectroscopists to know how well their magnets are shimmed and how well their probes are performing as far as line shape and width are concerned. A quantitative specification is needed so one magnet-spectrometer-probe configuration can be compared meaningfully to another. We use a standard sample of 1% chloroform in acetone-d6. The resonance of CHCl3 is examined. The width of the line (in Hz) is measured at 50% of the line height, 0.55% of the line height and 0.11% of the line height. 0.55% of the line height was chosen as it is the height of the 13C satellites. The specification is usually stated as x/y/z where x = the width at 50%, y = the width at 0.55% and z = the width at 0.11%. Often just y/z is given. These numbers are often quoted with and without spinning the sample. In the example below from our Bruker AVANCE 400 spectrometer on a non-spinning sample, the line shape was 0.27 Hz / 5.28 Hz / 8.72 Hz.

3 comments:

Anonymous said...

The lineshape, if you go to such low line widths as your very nice example spectrum, can also be very heavily dependent on the actual tube! Try repeating a "good" run with another sample or, better yet, try comparing the various 1% chloroform in deuterated acetone samples that you have in the lab: you will be amazed at the variations!

Anonymous said...

12CHCl3 and 13CHCl3 are isotopologs not isotopomers since isotopomers differ only by position of isotopes not by content. Isn´t it?

Glenn Facey said...

Anonymous,
Thank you for the comment. Yes, I think you are correct. You have increased my vocabulart by one word.
Glenn