Why would VendorInvestigate report 99.1% where the supplier certificate says something higher?

For reference: VendorInvestigate · 99.1%.

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 is the most likely explanation, and how would I confirm it?

Specifically, 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.

What each test answers

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.

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

edited 22 Jan 2026 by vialroom — corrected a unit error in the worked example

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

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.

Put another way, 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.

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.

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

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

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

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.

Stated carefully, 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.

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

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

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

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.

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

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

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.

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