July 28, 2026 · 5 min read
Why 99% purity, on its own, tells you nothing
Purity is a ratio, not a quantity — and it can be perfectly true about the wrong molecule.
"99% purity" is the most repeated claim in research peptide supply and the least informative. Not because it is usually false, but because of what it structurally cannot tell you.
Purity is relative
An HPLC purity figure is an area ratio: the target peak divided by the total area of all peaks the detector saw. Everything invisible at that wavelength is excluded from both sides of the ratio. Trifluoroacetate counter-ions, residual salts and water do not absorb at 220 nm and do not appear at all.
So a vial can be 99% pure and still be 30% something-that-is-not-peptide by mass. The certificate is not lying; it is answering a narrower question than the one buyers think they asked. This is why water content by Karl Fischer belongs on the same page — it is the line that converts a ratio back into a quantity.
Purity is silent about what the peak is
The purity figure describes the largest peak's share. It does not confirm that peak is the peptide you ordered. A deletion sequence missing one residue, a truncation, an incompletely deprotected product — each of these can elute close enough to the target to be integrated as one peak, or to be reported as a clean sample of a molecule that is not what the label says.
The check is mass spectrometry, and it is cheap. Observed mass against theoretical mass, printed side by side. A certificate with a purity figure and no identity test has left out the test that makes the purity figure meaningful.
The pair, not either half
| Test | Answers | Blind to |
|---|---|---|
| RP-HPLC purity | What share of UV-absorbing material is the main peak | Salts, water, counter-ions; whether the peak is the right molecule |
| ESI mass spec identity | Whether the main component has the expected mass | How much of the sample it is |
| Karl Fischer water | How much of the label mass is moisture | Everything about chemical identity |
Read together, the three answer a question worth asking: how much of what you paid for is in the vial, and is it the right molecule. Read alone, purity answers a question nobody asked.
What to do with this
- Treat a lone purity percentage as a marketing figure until the certificate behind it is in front of you.
- Check that identity is reported as an observed mass, not as the word "conforms".
- Compare price per milligram after reading the water content line, not before.
- Prefer a supplier whose certificates you can pull up yourself from the lot number, without asking.