[Lab] LC-MS confirmed identity but the UV purity is 94%. now what
Research peptide, not a GLP-1, from a vendor I will not name because this is a methodology question rather than a vendor post. MS confirms the expected molecular mass and the fragmentation is consistent with the sequence. So it is the right molecule. UV purity integrates to 94.1%, with a single impurity at 3.2% and…
The 3.2% single impurity is the whole question and MS can probably tell you what it is. If it is a related substance — a truncation, an oxidation, a des-amido — that is a synthesis and purification story and it is disappointing rather than alarming.
If it is unrelated, that is a different conversation. Can you get a mass on that peak?
i can and i should have. edit next day: it is +16 on the parent mass. so an oxidation product, almost certainly methionine, and almost certainly a storage or handling artefact rather than a synthesis failure.
+16 with a met in the sequence is about as unambiguous as this gets. which means the material was made properly and then sat somewhere with air.
and oxidation is one of the routes that does not need much water, so it is one of the few things that does progress in a dry vial over long enough.
so the practical read is: correct molecule, honest vendor, old vial. that is a much more actionable conclusion than "94% bad".
and this is why identity plus profile beats a single purity figure. 94% tells you nothing about which of six stories you are in.