Would you accept 96.4% on liraglutide from WXT without a content assay?

The case in front of me: 96.4% · liraglutide · WXT.

The failure mode I am trying to avoid is making this decision emotionally.

I have twelve months in view and I would like the plan to survive that long.

How would you structure this, and what thresholds would you set in advance?

Area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

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.

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.

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

Understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

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.

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

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

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.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

To be exact about it, 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.

Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.

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

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.