Is 98.8% a realistic purity ceiling for cagrilintide given its chain length?

Setup, so nobody has to ask: 98.8% · cagrilintide.

I have worked this out and I would like someone to find the error, because I suspect there is one.

My working so far, for the record, is below, and I am fairly sure the error is in the unit conversion rather than the algebra.

How many significant figures are actually justified here?

In practice, the single most important distinction is between what purity measures — the fraction of detected material that is your target — and what you actually want to know — how much of the material in the vial is your target.

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.

Reconciling gross mass to label claim

Column pore size affects mass transfer — a 100 Angstrom packing on a 5 kDa peptide restricts diffusion, broadening the peak and potentially hiding small impurities in the shoulders.

The limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."

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

Specifically, reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

Integration of the shoulder — whether you use perpendicular drop or tangent skim — determines what area gets assigned to the main peak versus the impurity table.

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 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.

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

The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.

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

Worth being precise here: 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.

The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.

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

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

Purity is a method-dependent figure, and that is not a limitation of the measurement, it is a property of what the measurement actually answers.

Sample solvent strength affects peak shape — if you inject in strong solvent on a gradient starting in weak solvent, the solvent peak can distort your main peak or create a false shoulder.

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

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

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

edited 18 Apr 2026 by a_lindgren — added the method parameters