Dear All,
I'm being doing DOSY experiments to determine molecular weight of polymers and i was wondering if the peak intensities shown in the spectrum for diffusion coefficients mean anything.
Does it mean a higher peak intensity has more molecules of that certain molecular weight?
Also, is there a formula that the NMR instrument uses to translate the raw data to obtain these diffusion coefficients and peak intensities?
You can look at our paper here - it has a lot of the required background - the references have the formulas.
Article PFG-NMR Studies of Linear and Dendritic Polymers
In the absence of magnetization transfer (as in decoupling or NOE type experiments) the observed peak integrations are in the same ratio as the number of protons giving rise to the peak.
Thank you for the paper.
Instead of the integrations I'm referring to the peaks which show up on the opposite side of the diffusion coefficients in the 2D DOSY NMR.
Do you know what those intensities represent?
OK - you mean the peaks that are plotted on the diffusivity axis. Is the intensity of these peaks in ratio with the proportion of the corresponding components? Not entirely. Imagine that you have two components with different diffusion coefficients. Compound A is 25% of the mixture has formula C9H18, and compound B is 75% of the mixture and has formula C6H6. The intensities of the peaks on the diffusivity axis should be the aggregate total of the intensities of the peaks of the corresponding species, which reflect both the number of hydrogens and the compostion - A then is 18 X 0.25 or 4.5, while B is 6 X 0.75 which is also 4.5!
Thank you for your explanation.
So you are saying that the the intensities on the diffusivity axis are similar to looking the peaks on the proton nmr axis? That's why when I integrate the 2D peaks I am able to determine the percentage of the mixture I ran on the DOSY NMR right?
Do you happen to have any reference for this?
If you can identify peaks that correspond solely to certain components you can do it from the topline (ppm axis) spectrum.
For a simple example, if you have two methyl groups - one at around 2.1 from 2-dodecanone and one around 3.5 from methanol. The ratio of their intensities in the regular (not DOSY) NMR spectrum of the sample gives you the ratio of the molar % of the sample in those components. Each methanol has one CH3, each dodecanone has one CH3. So the integrations are proportional to the amount of CH3 groups present. If you want it to be by weight %, you will have to factor in the greater mass of the dodecanone.
Once you start running the DOSY, the intensity also depends on the diffusion coefficient and the gradient strength in individual spectra that are used to determine the Diffusion coefficients.
The side (diffusion coefficient) spectrum should be the sum of the intensities of the peaks that have the same diffusion coefficient (assumed to be from one type of compound). So while if we had 1:1 methanol and 2-dodecanone, the two methyl peaks at 3.5 and 2.1 are also 1:1, the summation peaks will not be 1:1 but will be either 4:24 (if we see the methanol OH) or 3:24 (if the OH is exchanged out and not seen).
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