What a percentage actually asserts
An HPLC purity figure says: of what is in this sample, 99% is a single substance. Read that sentence again, because it never names the substance. Purity is a statement about homogeneity, not about identity.
A vial can be 99% pure and 100% wrong, the wrong compound, cleanly synthesised. It can also be the right compound at the right purity in the wrong amount, five milligrams sold as ten. And it can pass all three tests and still arrive degraded, because a certificate describes the batch at analysis, not the vial after transit.
Identity, quantity, condition. Three questions the headline number does not touch, each with its own check.
Being precise about what peptide purity means also clears up a common misreading. The percentage is relative to what a detector saw at one wavelength, in one run, under one method; change the gradient or the wavelength and the same vial can report a different figure without anyone cheating. That is why our quality standard gives the floor as a figure by HPLC with the chromatogram reproduced on the certificate: batch figures are meant to be compared with each other, not with a rival’s differently measured claim.
Four questions, not one
What complete verification actually covers.
When one number does all the talking
Purity became the market’s shorthand because it compresses into a badge. Identity confirmation, net content and cold-chain handling resist compression, so they fell out of the conversation, and sellers learned to compete on the number that survived.
The result is purity inflation: claims drifting toward 99.9% and beyond, printed in fonts larger than the evidence behind them. Meanwhile the failures that actually burn buyers, wrong compound, short fill, cooked in transit, sit precisely in the questions the badge ignores.
Our advice compresses too: treat purity as an entry ticket, not a verdict. Below 99%, walk away; at 99%, start asking the other three questions.
The analytical literature makes the same point from the other side. Papers on HPLC peptide purity methods on PubMed spend much of their length on what one chromatogram can miss: impurities that co-elute with the main peak, sequence variants with near-identical retention, water and salts the detector never sees. None of that argues against measuring purity. It argues for reading the figure together with identity, net content and storage history, which is why the freeze-thaw page belongs to a buying series and not only to a handling one.
- Right purity, wrong compoundClean synthesis of the wrong sequence passes every purity test ever run.
- Right compound, short fillMilligrams are promised on the label and verified almost nowhere. Ask for net content.
- Right vial, dead on arrivalDegradation en route is invisible on paper. Packing standards are part of quality, not logistics.
- Right paper, wrong batchThe recycled-certificate trick, covered in full in the fake-COA guide.
The other two guides
This page closes the buying series that starts with the seller and the paper.
About the number
Purity questions that deserve straight answers.