Accepted answer
0.3 percentage points, and the direction is the informative half. Medutest reports 97.1 per cent and the certificate 96.8, so the independent figure is higher. As impurity that is 2.9 per cent against 3.2 — 0.91 times as much unassigned area, which is the unusual direction and worth a second look at the integration. Method differences usually make the independent number the lower one, so an independent result above the certificate points at integration limits, a different wavelength, or a different definition of the main peak rather than at a better vial. Ask both parties for the method section before you decide which figure is wrong, because the answer is often neither.
The part that matters: reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.
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.
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.
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.
Compare purity within a single laboratory on the same method, never across laboratories.
edited 21 Feb 2025 by tobias_maartens — fixed an arithmetic slip in the third paragraph
Does this hold for a longer chain length, where the deletion sequences accumulate? – tare_weight 2 months ago Do you have the chromatogram for this, or just the summary figure? – RP_C18 4 months ago add a comment