Everything you need to know
A purity percentage is not a property of the vial. It is the result of a defined method applied to a sample, and changing the method changes the number without anyone being wrong.
Why do two labs report different purity for the same peptide?
Because purity is method dependent, and two results labelled "HPLC purity" can come from methods that are not interchangeable. The detection wavelength decides which species are visible at all, since 214 nm sees the peptide bond and 280 nm sees aromatic residues, and a related substance that is obvious at one is invisible at the other. Integration rules decide whether small peaks are counted, skimmed into a neighbour, or dropped below a threshold. Column chemistry and gradient decide whether two species separate or co-elute as one clean peak. Sample preparation and diluent affect recovery and can create degradation that was not in the vial. None of this requires bad faith from either laboratory.
What moves a purity percentage the most?
In rough order of impact: whether low-level related substances are integrated or excluded by a reporting threshold, the detection wavelength, the column and gradient, sample preparation and diluent choice, and whether the method reports area percent alone or is tied to a quantitative standard. A difference of a few tenths of a percent between laboratories is unremarkable. A difference of several percent usually points at a threshold or an integration decision rather than at the material.
Does disagreement mean one lab is wrong?
Not by itself, and jumping there usually stops the investigation too early. It can mean two different methods, two different vials from a poorly controlled fill, or different sample handling. It can also mean something more consequential: that identity was confirmed at one laboratory and assumed at the other, so one report is quoting a purity for a peak it assigned differently. A purity number attached to an unconfirmed identity is the weakest result on either certificate, and it is worth checking before concluding anything about the material.
What should I do when two reports conflict?
Stop treating either PDF as final and compare the inputs. Was it the same vial or two vials? What wavelength did each use, and what reporting threshold? How was identity established in each case? Was content measured at all, or only purity? If a commercial decision rests on the answer, a replacement, a release, or a dispute, ask both laboratories for method details and the chromatograms, and consider a third measurement with the method agreed in advance rather than chosen afterwards.
Is a difference in content more or less common than a difference in purity?
Content is where vials fail most often, and it is also where two laboratories can diverge for a reason that has nothing to do with chromatography: the reference standard. A standard characterised only for chromatographic purity, rather than for how much peptide it actually contains, biases every content result calculated against it. If two content figures disagree, the standard is the first thing to ask about. How often vials fail on content rather than purity.
Should I ask for the chromatogram?
Yes, and a laboratory that will not provide one is telling you something. The percentage is a summary of the chromatogram, and the chromatogram shows what the summary omits: how many impurity peaks there are, whether they are early or late eluting, whether the main peak is symmetrical, and whether anything sits on its shoulder. Two vials reported at the same purity can look completely different, and only one of the two documents shows it.