This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Aggregates: multiples of the monomer mass. May not elute at all under a standard reversed-phase method. A species that does not come off the column does not appear in the area percentage.
Trifluoroacetate is a counter-ion rather than an impurity, and it appears in the mass balance rather than in the chromatogram. Conflating the two accounts for several confused threads here.
I would want a second opinion before relying on that.
Noted, and I have changed what I was going to do on the strength of it.
Where an impurity is identified rather than merely counted, the certificate is telling you the manufacturer has characterised its own process. That is a meaningful difference in documentation quality.
Happy to expand any of that if it is the useful part.
On post #32 — agreed on the reasoning, with one qualification.
Summarising the trifluoroacetate content thread so far, since it is long and the answer is buried: the first reply has the method, the fourth has the correction to it, and the rest is people agreeing at length.
Scavengers and cleavage-cocktail residues can persist and appear as small early-eluting peaks. They are process-related rather than sequence-related, which is a useful distinction when reading a profile.
Post #36 describes the usual case. This is about the unusual one.
Dimer and higher-order multimers: two or more peptide molecules bonded together. They appear at double the mass and higher. They may or may not separate from the monomer on HPLC depending on the method.
I have left out the parts I could not verify.
That last point is the ceiling on what any purity figure can claim. A method that cannot see a species cannot exclude it, and no certificate says which species its method cannot see.
What I would check first on trifluoroacetate content is whether the thing being measured moved or whether the way of measuring it moved. Those look identical in a graph.
The number people quote for trifluoroacetate content is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own.
Narrowing post #41, because the general version has more than one answer.
Off-target structures: if the sequence synthesis goes wrong, a completely different amino acid can be incorporated. The resulting off-target peptide is a structural isomer with the same mass but a different sequence. No chromatographic purity method detects this without a reference standard.
None of the above is medical advice and I am not qualified to give any.
Practical experience of trifluoroacetate content, offered as one case with the conditions stated, not as a general finding. Conditions first, because they are what make it interpretable.
That last point is the ceiling on what any purity figure can claim. A method that cannot see a species cannot exclude it, and no certificate says which species its method cannot see.
I would rather post the uncertainty than round it away.
I read post #45 twice before replying, because I had assumed the opposite.
A definition problem is doing most of the work in this trifluoroacetate content discussion. Once the term is pinned down I suspect the disagreement mostly goes away and what is left is small.
This settles it for me, at least until somebody posts a reason it should not.
Confirming post #47 from a second method, which matters more than confirming it from a second person.
Truncation products are substantially lower in mass and usually well separated chromatographically, which makes them the easiest class to see and the least likely to be missed.
Happy to be corrected if someone holds better data than mine.
An update on my earlier trifluoroacetate content post: the pattern held for another six weeks and then stopped, which I did not predict and cannot explain.
Post #51 answers the question as asked. The question underneath it is different.
Comparing impurity profiles across suppliers is much more informative than comparing purity figures, and almost nobody has the documents to do it.
That is what I would do. It may not be what is correct.
I read post #50 twice before replying, because I had assumed the opposite.
Trifluoroacetate content sits at the boundary between what this community can usefully discuss and what it cannot, and I think it falls on the discussable side, narrowly.
Trifluoroacetate content would be much easier to settle if anyone reported the denominator. Almost nobody reports the denominator.
A supplier that can tell you what its principal impurity is has answered a harder question than one that can tell you its purity, and the answer is more useful.
Coming back to post #54, because the follow-up matters more than the original answer.
What I would tell a new member reading about trifluoroacetate content for the first time: the confident posts are not the reliable ones, and the reliable ones are longer.
Marking my place. If it changes for me I will come back and say so.
Post #58 put the caveat in the right place and I want to underline it.
Trifluoroacetate content has been discussed here with more heat than it deserves, mostly because two definitions have been in play the whole time.