[2026 update] A rising related-substance total over six months: degradation or method drift? posts 61–79

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.

Thank you for the correction. I have edited my earlier post with a note rather than silently, so the thread still makes sense to read. The error was mine and it was the kind that comes from remembering a figure instead of looking it up.

Worth separating two things that post #59 runs together.

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.

post #63 is right about the mechanism and I think understates the practical bit.

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.

Coming back to post #63, because the follow-up matters more than the original answer.

Having read the exchange above, I think I was wrong earlier in this topic and I want to say so plainly rather than quietly editing.

The correction was fair and I had been repeating something I had not checked carefully enough.

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.

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.

post #67 answers the question as asked. The question underneath it is different.

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

Deamidation at asparagine and glutamine: adds 1 approximately. Frequently appears as a close-eluting pair. It is a chemical modification that occurs during storage.

This follows post #67 rather than contradicting it.

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.

post #70 answers the question as asked. The question underneath it is different.

Thank you for the correction. I have edited my earlier post with a note rather than silently, so the thread still makes sense to read. The error was mine and it was the kind that comes from remembering a figure instead of looking it up.

On post #68 — agreed on the reasoning, with one qualification.

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.

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.

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.