Office of Research

Center for NMR Spectroscopy

Line Width

Once you have obtained a spectrum one of the ways you can evaluate the shimming job is to look at the FWHH (full width at half height) or resolution of a peak in the spectrum. This is easily done by expanding around a peak and then setting the cursor directly on the tip of the peak. Typing nl on the command line will position the cursor at the peak maximun. This is shown below:

Typing dres on the command line will then give the FWHH in Hz as well as the digital resolution of the spectrum (total Hz of the spectral width / total real points) as shown below:

For this particular sample (0.1% ethylbenzene in CDCl3) a FWHH of 0.3 Hz is quite good and can be considered well shimmed. A spectrum with a FWHH of ca. 0.5 Hz is perfectly acceptable as long as the peak shape is symmetrical and does not show evidence of any of the common shimming defects.

It should be noted that the above spectrum was taken with the sample spinning at 20 Hz. To get an idea of the utility of spinning the sample the spectrum can be re-acquired by turning the spinner off. The resulting spectrum is shown below:

The resulting FWHH is now 0.86 Hz! Obviously rotating the sample is averaging much of the transverse inhomegeneity. This spectrometer has only a limitted number of non-spin shims and hence all 1D spectra not based on subtractive techniques should be run spinning

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