Aggregates and why a purity assay may miss them entirely

On aggregates, I would rather understate and be corrected upward than overstate and be quoted. That is a house style here and it is a good one.

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.

Coming back to post #12, because the follow-up matters more than the original answer.

The number people quote for aggregates is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own.

What I would check first on aggregates is whether the thing being measured moved or whether the way of measuring it moved. Those look identical in a graph.

Aggregates would be much easier to settle if anyone reported the denominator. Almost nobody reports the denominator.

Adding the measurement that post #36 says would settle it.

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 this thread reach "we do not know" about aggregates than reach a confident answer that nobody can support when asked.

I changed my mind about aggregates 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.

The arithmetic in post #57 is right; the assumption feeding it is the part to check.

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.

Posted with less confidence than the sentence structure implies.