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

The question in the title: A rising related-substance total over six months: degradation or method drift? I will give what I have already checked below so nobody repeats it.

Posting the method first, because I know what the first three replies will otherwise be.

• Column: C8, 4.6 x 150 mm, 3.5 um
• Mobile phase: 0.1% TFA in water / 0.1% TFA in acetonitrile
• Gradient: 14% to 58% organic over 23 minutes
• Detection: 214 nm
• Injection: 9 uL
• Sample: retatrutide, reconstituted to 1.0 mg/mL, injected within an hour

The main peak integrates at 96.6% of total area. There is a small feature on the trailing edge that I cannot decide is a shoulder or a baseline artefact, and that is what I am actually asking about.

Aggregates may be a multiple of the monomer mass and may not elute at all under a standard method. What does not come off the column does not appear in the area percentage.

This follows post #3 rather than contradicting 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 read post #3 twice before replying, because I had assumed the opposite.

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.

That is what the documentation says. What happens in practice is usually close.

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.

Marking that as an opinion rather than a finding.

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.

I would put the burden of proof on the interesting explanation, not the dull one.

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.

Rising related-substance came up in a thread eighteen months ago and was answered well. I cannot find it, which is itself the problem, so here is the reconstruction.

Post #7 is the version of this I will quote in future. One addition.

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.

I checked the source rather than the summary, and they differ.

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.

That holds under the stated conditions and I have stated them.

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.

I have no financial interest in anything named in this thread and I want to say so before I comment on rising related-substance, because it is the sort of subject where it matters.