Would you accept 96.8% on cagrilintide from QST without a content assay?

What I am working with: 96.8% · cagrilintide · QST.

I am trying to build something sustainable rather than something thorough that I will abandon.

I have already decided the broad direction; this is about the specifics.

What should I decide now, and what should I defer?

Mechanically, start from what the detector sees, because that tells you what the number means.

Temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.

In practice, buffer versus acid in the mobile phase changes the ionisation state of basic and acidic residues, shifting retention and selectivity — same vial, potentially different separation.

I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.

The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.

Tailing factor measures peak shape, and a badly tailing peak spreads into the region where small impurities live, forcing tangent-skim integration that assigns tail area to the main peak.

Concretely, gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.

Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.

If you are ranking vendors, specify a method and have all samples tested at the same place.

Worth being precise here: read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.

Retention time is sequence-specific and method-specific, so comparing your result to a supplier value using a different method is meaningless without method documentation.

The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.

Compare purity within a single laboratory on the same method, never across laboratories.

Gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.

Detection wavelength matters because 214 nm sees the peptide backbone while 280 nm sees only aromatic side chains — so truncation impurities lacking a tryptophan are invisible at 280 nm.

The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.