"≥ 99 %" is the number every peptide seller wants on the front of the report, because it is the number every buyer scans for first. It is also the number that hides the most. Purity is not a property the molecule simply has — it is the output of a specific measurement, run a specific way, and two labs can hand you two different numbers for the same vial.
This is a short guide to what that number is actually measuring, and why "99 %" on one report is not the same claim as "99 %" on another.
Purity is a measurement, not a fact
HPLC purity is the integrated area of the main peak as a fraction of all peaks the detector sees. Change the column, the gradient, the run time, or the detection wavelength and you change which impurities are resolved from the main peak — and therefore the number. A report that states ≥ 99 % without stating the method is quoting a result you cannot reproduce or compare.
Why the wavelength matters
For peptides the convention is 220 nm, because the peptide bond itself absorbs strongly there. Read at 280 nm — where only aromatic residues absorb — and impurities that lack those residues can vanish from the trace, inflating the apparent purity. If a COA does not name the wavelength, treat the number as decorative. Our reports are run at 220 nm; you can see the wavelength on every trace on the lab reports page.
What a single peak proves
A single clean peak proves the material is one thing. It does not prove that thing is the molecule you ordered. Purity and identity are separate questions: HPLC answers purity, mass spectrometry answers identity. A 99.9 % pure peak of the wrong compound is still the wrong compound. This is why we publish the ESI-MS result next to the HPLC number on every lot.
A purity number is only meaningful with three things attached: the method, the wavelength, and a mass-spec identity beside it. Missing any one of them, the percentage is a marketing figure, not an analytical one.
Comparing across sellers
When you compare two sellers, do not compare the two percentages. Compare the two methods, then the two retention times, then the two mass-spec deltas. If both report 220 nm HPLC with a matching retention time and a mass within half a Dalton of expected, the purity numbers are finally comparable. If they don't, you are comparing a measurement to a slogan.