Sample concentration for HMBC – How much is enough?
This isn’t a black-and-white issue. In general, the answer is a few mg, assuming it’s a small molecule. The higher the MW, the more mg we need. With less material, the experiment runs longer.
Below is a comparison of 2 samples of codeine, 3 mg and 18 mg. 3 mg yields a good spectrum in 1/2 hr. The more concentrated sample, run for longer time, shows several additional peaks. Remember that codeine is quite small, MW = 299.
|3 mg of codeine
total acquisition time: 35 min
|18 mg codeine
total acquisition time: 1 hr. 10 min.
The expanded plots below compare the same region of the 2 spectra. (Top: 3 mg sample Bottom: 18 mg sample) The concentrated sample shows eight additional 2- and 3-bond correlations, and two 1-bond correlations.
|The additional correlations seen in this region of the spectrum of the more concentrated sample are:
1-bond correlations are also seen for 9 and 12.
Last updated: 01/22/2003