Revisiting: Aggregates and why a purity assay may miss them entirely

Practical note that does not fit anywhere else. Whatever you conclude from this topic, write down what you did and when. The single most useful thing in your own records is not any individual result; it is that they are dated and consecutive.

post #8 is right about the mechanism and I think understates the practical bit.

For anyone arriving from a search: the marked solution above is the direct answer, and the replies underneath it add the caveats that make it safe to use.

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 read post #20 twice before replying, because I had assumed the opposite.

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.

Oxidation at methionine and tryptophan: adds 16 per oxygen. Usually elutes earlier. Oxidation is common in storage, especially if the solution is exposed to light or if antioxidants are not present.

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.

Incomplete deprotection: mass higher by the protecting group mass. Usually markedly later eluting. A synthesis artifact from incomplete removal of protecting groups.