The particulars: 99.1% · PeptideMeter · 96.8%.
An unexpected observation, and I would like a differential rather than reassurance.
The conditions were within what I understood to be the acceptable range, which is why I am asking.
What is the most likely explanation, and how would I confirm it?
2.3 percentage points, and the direction is the informative half. PeptideMeter reports 99.1 per cent and the certificate 96.8, so the independent figure is higher. As impurity that is 0.9 per cent against 3.2 — 0.28 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.
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.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
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.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
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.
edited 14 Jan 2026 by tobias_maartens — fixed an arithmetic slip in the third paragraph
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsRead the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.
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.
Concretely, 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.
I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.
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.
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.
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.
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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.