How do I reconcile 98.8% from VendorInvestigate with 98.2% from the supplier?

Conditions: 98.8% · VendorInvestigate · 98.2%.

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.

Should I be treating this as a failure or as noise?

Mechanically, gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.

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.

To be exact about it, 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.

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

edited 3 Jul 2024 by swirl_dont_shake — added a caveat about sampling

The part that matters: understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

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.

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

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

The underlying point is that read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.

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

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.

The relevant detail is that reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

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.

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