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.