We have been successful in analysing an amino piperidine compound on a DB-5MS column, but are noticing an unwanted impurity. The chemist purifies the sample as a HCl salt, then the free base would be formed again for analysis. The impurity remains.
We have a standard of the free base which continues to show good chromatography.
Therefore we tried to GC the HCl salt to see if the impurity is carried from there, or introduced in the formation of free base.
The white solid is dissolved in methanol (as with the free base oil) and injected as before. The chromatography now shows a "peak" at the expected RT, but instead of going back to baseline, it shows a raised, messy signal for a couple of minutes.
How are these observations explained?
By Anonymous on Friday, November 1, 2002 - 03:58 am:
You have discovered why chromatographers do not try to do GC on salts.
Depending upon the availability of Lewis bases in the solvent and in the injection port, your conversion of a HCl salt to a free base by heating under an inert gas is not quanitative nor timely.
You need to try to find the impurity by another method of chromatography or spectroscopy.
By Carlos Teixeira on Friday, November 1, 2002 - 07:30 am:
To analise amine bases by GC is very difficult, I will try other chromatographic method , LC is better.
By gcboy on Friday, November 1, 2002 - 12:58 pm:
thank you for the replies.
it's a shame that we cant stick with the GC analysis, we had a nice method for this compound.
i wasn't sure what would happen if we injected the HCl salt onto the GC.. i mean, in methanolic solution the compound would dissociate so we have the free base in there, but when passed into the injection port would the sample rapidly volatilise to leave the free base plus HCl?.. or would the salt precipitate out, mucking up the liner?? (we have it packed with glass wool).
the impurity is visible by NMR, but remains unidentified. we dont feel this technique is sensitive enough to quantify our product. the damn impurity seems to pop up when the HCl salt is free based (dissolve in aq. base and extract with EtOAc).
maybe simple reversed-phase LC would work with RI detector?, compound is fairly simple N-alkyl substituted piperidine with short chain primary alkylamine off the ring.
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