Numbers first: 98.2% · Janoshik · 97.9%.
I have a result I cannot explain, and I would rather diagnose it than guess.
I have checked the obvious explanations and eliminated the two easiest ones.
What is the differential here, and which test discriminates between the options?
0.3 percentage points, and the direction is the informative half. Janoshik reports 98.2 per cent and the certificate 97.9, so the independent figure is higher. As impurity that is 1.8 per cent against 2.1 — 0.86 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.
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.
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.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemIn practice, 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.
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.
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 6 Aug 2026 by mz_4113 — expanded the table to cover the lower concentration
Specifically, understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
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.
Compare purity within a single laboratory on the same method, never across laboratories.
In practice, the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
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.
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.
Start from what the detector sees, because that tells you what the number means.
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.
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.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.