[Question from NMRWiki Q&A forum] Relative quantification (residual solvents)

[nmrlearner]

Relative quantification (residual solvents)

Hi All,

I've been trying to get a clear answer to this question for a while now, but can't seem to get a definitive answer. My approach is following the method described in "Practical NMR for organic chemists" by John Hollerton and Steve Richards. I don't take any issue with this method, by only problem is that it gives a WILDLY different answer to the one I obtain when I do NMR quantification in TopSpin... this is a big problem, since often we are interested in the % of unwanted molecules in our samples.

Let me give you a simplified example of my problem:

Say I have a proton NMR spectrum infront of me. It contains my compound, plus a nominal amount of residual solvent such as dichloromethane.

If I integrate a single proton first, and set the integral of this to 1.00, and then integrate the dichloromethane peak afterwards, and this comes out as 0.02, then I would be correct ( I think) in saying that there is 2mol% of dichloromethane relative to my sample, since integrals are directly proportional to concentration (so for every 100 mol of compound, I'd have 2 mol of dichloromethane)... so far so good, right?

In Hollerton's book, he then says this can be converted to a relative % weight by multiplying this molar percentage by (Mwt Residual solvent / Mwt of compound). Again, so far so good.

The problem is that, as I say, this gives a very different answer to the one obtained in topspin, and I would value anyones understanding of why this may be... Do I need to scale my answer by some factor?

As a side question: say I added an exact quantity of some other reference, say 0.1 mL of benzene, would I easily be able to calculate the exact weight of my compound, knowing the relative integrals, and molecular weights??

Kind regards, and thankyou in advance!!



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