I have 20% (w/w) formaldehyde-d2 in water. I want to calculate the isotopic purity using NMR. But I am getting 3 to 4 peaks corresponding to different polymers of methylene glycol. How to find the isotopic purity of formaldehyde-d2? NMR is a suitable method or is there any other method which I can use?
As you have found in water formaldehyde will hydrate and form oligomers. Not only that but the deuterium will even distribute itself with the water and the oligomers. So use a solvent other than water.
From practical point of view, fornaldehyde itself at room temperature is a gas; therefore if you only have an aqueous solution (in H2O or D2O), I think it is impossible to correctly determine its isotopic purity due to the reasons mentioned by David.
If, for example, you have a cylinder with formaldehyde-D2 gas, chances are that you might be able to dissolve it in a neutral solvent like CDCl3 and use NMR for your purposes. Another option would be mass-spectrometry using the instrument equipped with with gas injection auxiliary.
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