[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 rest scattered.
So: correct identity, mediocre purity, one dominant impurity. How do people weigh that? Genuinely asking, because I keep going back and forth.
best — the order this archive was captured in
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.
for anything outside the GLP-1s i would put identity first and purity second every time. the naming mess in the research-peptide supply chain means "is this the right thing" is a live question.
and neither number tells you how much peptide is in the vial. three questions, three assays, one document.
research use only reminder since this is not a GLP-1: handling and analysis discussion is fine, protocols are not.
94% is 94%. all this analysis is a long way of avoiding the word bad.
a 94% that is one identified oxidation product on an old vial and a 94% that is six unknown peaks are not the same material. that is not avoidance, it is the actual difference.
- 1The 3.2% single impurity is the whole question and MS can probably tell you…6 comments in this branch · started by u/sig_figs_sam