This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Incomplete deprotection: mass higher by the protecting group mass. Usually markedly later eluting. A synthesis artifact from incomplete removal of protecting groups.
Nothing above should be read as advice about what anyone else should do.
Adding the measurement that post #31 says would settle it.
Adding what did not work for me on oxidation pathways for methionine, since the failures never get written up and they are half the useful information.
Where I part company with post #31, and it is a narrow parting.
Oxidation pathways for methionine has a well-known answer and a correct answer, and the interesting work is establishing that they are the same. Nobody has done that here yet.
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.
The disagreement above is smaller than it looks once the terms are fixed.
Picking up post #34: that is the part I would want checked first.
Residual solvents: traces of solvents used in purification. These are usually tested by gas chromatography, not by HPLC. A specification for residual solvents should be stated separately from the purity.
Caveat: everything above assumes the paperwork is what it says it is.
An observation about oxidation pathways for methionine that I cannot explain and am posting anyway, on the principle that unexplained observations are more useful public than private.
My understanding of oxidation pathways for methionine is a few years old and may have been superseded. If it has been, I would genuinely like to know rather than keep repeating it.
Worth separating two things that post #36 runs together.
Relative response factors mean impurities are not detected in proportion to how much of them is present. A one per cent peak is not one per cent by mass unless the response factors happen to match.
I would put a moderate confidence on that and no more.
Offering a way to settle oxidation pathways for methionine rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.
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.
Where I would look next, rather than where I would stop.
This follows post #40 rather than contradicting it.
The arithmetic on oxidation pathways for methionine is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it.
Worth separating two things that post #42 runs together.
Where the oxidation pathways for methionine discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.
Acetate content: counter-ion content. Trifluoroacetate or acetate from the salt form of the peptide. Affects mass calculations and should be stated on a complete certificate.
The conclusion is tentative; the arithmetic underneath it is not.
Related substances: compounds chemically related to the target peptide but not the target peptide itself. The standard method separates them and reports them as area percent. How related they can be before they exceed specification is a regulatory question.
I would rather say I do not know than round it up to an answer.
Taking post #44 at face value and following it one step further.
Second-hand on oxidation pathways for methionine, so weight it accordingly — someone whose method I trust told me this and I have not verified it myself.
Grateful for the specificity. Vague answers to this question are what sent me looking.
The arithmetic in post #47 is right; the assumption feeding it is the part to check.
The honest answer on oxidation pathways for methionine is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter.
Most people get the first two right and then argue about the fourth.
Answering the question post #46 raises rather than the one it answers.
Disulfide formation: if a peptide contains cysteine, it can form disulfide bonds with itself or with other molecules. Under oxidising conditions multiple species appear. Reducing conditions (like DTT) convert them back.
What I would want before treating oxidation pathways for methionine as settled: the method, the sample, and whether anyone tried to find the opposite result. Two of the three are usually missing.
My experience of oxidation pathways for methionine contradicts the reply above. I am posting it as a data point rather than as a refutation, because one person's experience is exactly that.
Where I part company with post #49, and it is a narrow parting.
Peptide impurities that differ by a single residue are the hardest to resolve and the most likely to be biologically relevant, which is an unfortunate combination.
Not a strong opinion, just a consistent one.
On oxidation pathways for methionine: the maintained page in the documentation commons covers the general case with citations and a review date, which is more reliable than any reply here including this one.
Narrowing post #53, because the general version has more than one answer.
Reporting thresholds matter: below a stated threshold, peaks are usually not reported at all. A clean-looking table may reflect a high threshold rather than a clean synthesis.
None of the above is medical advice and I am not qualified to give any.
Oxidation at methionine or tryptophan adds sixteen per oxygen and typically elutes earlier. In aged material it is the modification that grows.
On post #57 — agreed on the reasoning, with one qualification.
Worth separating oxidation pathways for methionine as a question about the compound from oxidation pathways for methionine as a question about the documentation. They get answered by different people and only one of them is answerable here.
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.
The claim is narrower than it sounds, and deliberately so.