Quantitative analysis of known compounds!

[MS-NMR]

The separation of deuterium labeled compunds by GC works well, but 13C labeled compounds do not separate well from the natives. In our laboratory we analyze native compunds and use internal standards with multiple 13C atoms in them. Whlie the isotopically labeled compounds may not elute quite exactly with the natives - it is not enough separation to notice - and I look for one peak marker through the native and the labeled comounds.

On the inclusion of 13C in a molecule, remember that the native compunds have 13C already in them. Assuming derivitization: The TMS ether of a four carbon diacid will show 10% relative abundance because the natural 13C present. (A total of ten carbon atoms - 3 in each TMS and four in the acid.) Thus, to see inclusion, you need to have sufficient 13C in the glucose source to ensure that the acids you analyze are formed from the added glucose. And if you use a TMS ester, the silicon contributes to the m+1 abundance as well.

A dimethyl ester of a our carbon diacid would show about 6% relative abundance for m+1.

I would suggest running a mixture of acids down the GC and make sure you have then well seprated and if you can get some labeled acids, make some mixtures and shoot them to see what the combined spectra would look like. You will need some monolabeld, dilabeled, etc because the spectra for each will be different.

Dear Don_Hilton,
Thank you very much. I'll make the mixtures and shoot them.

[HW Mueller]

I hesitate to mention this, since this was all a long time ago and memory sometimes plays tricks, but I think it was this project with oxalic acid mentioned above which gave trouble with finding the 13C/12C ratio due to the ratio´s variation according to peak section.
So the U in the glucose indicates that all C-atoms are C-13.