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Sample concentration for HMBC

Sample concentration for HMBC

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:

H —-> C
3         16 and 9
5         16, 11 and 10
9          15 and 10
11        13

1-bond correlations are also seen for 9 and 12.

 


Last updated: 01/22/2003