How do I reconcile 99.1% from PeptideMeter with 97.9% from the supplier?

The specifics, since they change the answer: 99.1% · PeptideMeter · 97.9%.

Something has gone wrong and I would like to know how badly before I decide what to do.

Nothing else in the setup changed, which is what makes this puzzling.

What would you check first, and what would you conclude from each outcome?

Start from what the detector sees, because that tells you what the number means.

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.

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.

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 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.

On the detail: read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.

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

Put another way, 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.

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.

Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.

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

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.

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.

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.

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

edited 9 Jun 2026 by Dr_Jonas_Halvorsen — tightened the wording; no substantive change

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

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.

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

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.

edited 3 Jul 2026 by coring_risk — updated for the 2026 guidance change