How do I reconcile 98.4% from VendorInvestigate with 96.4% from the supplier?

Concretely: 98.4% · VendorInvestigate · 96.4%.

I have two candidate explanations and no way to distinguish them.

The same procedure has worked without incident several times previously, which argues against technique.

How do I distinguish the benign explanation from the one that matters?

2 percentage points, and the direction is the informative half. VendorInvestigate reports 98.4 per cent and the certificate 96.4, so the independent figure is higher. As impurity that is 1.6 per cent against 3.6 — 0.44 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 single most important distinction is between what purity measures — the fraction of detected material that is your target — and what you actually want to know — how much of the material in the vial is your target.

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

Mass shifts and what they usually mean

Put another way, 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.

Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room 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.

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

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

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.

The relevant detail is that 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.

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.

I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.

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

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

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

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

The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.

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.

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.

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 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.

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

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.

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

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.