NMR integration progress for solubility measurements
In a previous post I reported about issues with using H NMR integrations for quantitative assessment of solute concentration to determine solubility. Using default parameters, a study by David Bulger revealed that integration of the methyl group on 2-phenylbutanoic acid lead to only 8.4% average error while the methine H gave an average of 26% under-integration.(ONSC-EXP102)
We have tried to select methyl or methylene groups whenever possible but obviously this won't work for many compounds, especially our series of aromatic aldehydes and carboxylic acids.
Khalid Mirza has just done a study using longer relaxation times and the results appear very promising (ONSC-EXP103). For 4-pyrenebutanoic acid using default parameters the integration of the aromatic vs. methylene hydrogens was off by 9-18% while extending the relaxation time (d1=50s) reduced the error to less than 1.2%.
Furthermore, Marshall Moritz used the new settings in a measurement of a known concentration of 4-nitrobenzaldehyde in acetonitrile and came within 3.2% using an aromatic hydrogen (ONSC-EXP111). Use of the aldehyde hydrogen was not as good, although much better than what we normally see with the default settings.
So I am cautiously optimistic that we are likely to get better data going forward.
Labels: NMR, relaxation time, solubility
posted by Jean-Claude Bradley @ Thursday, June 25, 2009
3 comments
3 Comments:
I suggest making a very dilute solution of Cr(acac)3 in the solvent of choice that you are determining the solubility in. Then I would use a microsyringe and inject about 1-3microlitres into the solution of interest. Retry the measurement...check what the T1s are now and whether adding the relaxation agent has affected the solubility determination itself.
Tony - thanks. That should be interesting to try. My only concern is that I seem to recall that it broadens the peaks but it can't hurt to see what happens.
If you keep the concentration low enough you will get the T1 relaxation effect and with nominal line broadening. A little line broadening should not be an issue in terms of comparing integrals...as long as they don't broaden so much that they overlap and you get intensity stealing.
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