Why would VendorInvestigate report 97.6% where the supplier certificate says something higher?

The particulars: VendorInvestigate · 97.6%.

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?

A 97.6 per cent result against a higher certificate figure is usually a method difference, not a disagreement about the vial. Inter-laboratory spread on identical peptide material runs half a per cent to a full per cent by RP-HPLC. Gradient slope, detection wavelength and integration convention each move the number, and a gentler gradient resolves impurities that a steeper one hides under the main peak — so the better method reports the worse figure. Get the column, the gradient and the wavelength from both VendorInvestigate and the supplier before treating 97.6 per cent as a contradiction.

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

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

In practice, 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.

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

edited 27 Mar 2025 by eoin_mcgarry — reworded for clarity after a comment

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

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.

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.

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.

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

Worth being precise here: 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.

More usefully, 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.

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

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

In practice, identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.

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.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

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

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

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.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.