This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1 · go to the accepted answer.
I changed my mind about rising related-substance after someone here asked me for the source and I could not produce one. That is worth saying out loud because it is the ordinary way it happens.
Two claims get bundled together under rising related-substance and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.
Almost every disagreement in threads like this one dissolves once you say which of the two you are making.
Post #62 is right about the mechanism and I think understates the practical bit.
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.
This has been discussed before and I could not find the thread, so, again.
Post #64 describes the usual case. This is about the unusual one.
The most useful thing anyone has posted about rising related-substance in this category was a table of what had been measured and by whom. That is what I would want again.
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.
Somebody will have a better source than mine, and I hope they post it.
An impurity profile that changes between lots is more informative than the total. A stable profile suggests a controlled process; a shifting one suggests something is varying.
Confirming post #67 from a second method, which matters more than confirming it from a second person.
Storage-related degradation and synthesis-related impurity look different on a chromatogram. A growing oxidation peak over time is not a manufacturing finding.
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.
The arithmetic in post #67 is right; the assumption feeding it is the part to check.
Truncation products: fragments from incomplete synthesis or from degradation. They elute quite differently from the intact peptide because they are much smaller and have different hydrophobicity. They are usually well separated.
Not the answer, but possibly the question that gets there.
Seconded. It reads as careful rather than confident, which is the right register.
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.
Racemisation produces a diastereomer that is mass-identical and chromatographically resolvable only on a method chosen for the purpose. Standard reversed-phase frequently will not separate it.
Agreed on rising related-substance, with one qualification that I think matters. The reasoning holds for the case as described. Change the starting assumption and it does not, and the starting assumption is the part nobody states.
Deletion sequences (incomplete coupling during synthesis): lower in mass by one residue. Chromatographically they usually elute earlier or later depending on the residue's hydrophobicity. They are the most common impurity in solid-phase synthesis.
A qualification I should have led with rather than closed on.
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.
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