How do I reconcile 98.4% from VendorInvestigate with 95.2% from the supplier?

The case in front of me: 98.4% · VendorInvestigate · 95.2%.

I think I have a problem. I am not yet sure whether it is a real problem or a measurement artefact.

I want to know whether this is recoverable or whether the honest answer is to write it off.

Is this recoverable, and how would I tell?

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

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.

Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.

Inter-laboratory studies on identical peptide material routinely find half-a-per-cent to a full-per-cent spreads in reported purity on the same sample.

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

Stated carefully, 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.

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.

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.

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

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

The underlying point is that 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.

One qualification: achieving purity above roughly 98 per cent on a 30-residue peptide is fighting the chemistry of synthesis, not the quality of the purification.

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

Stated carefully, the short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.

The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.

Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.

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."

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