Purity is not one number
Two vials can both be labelled 99% and contain measurably different amounts of peptide. The label is not lying — it is answering a narrower question than you asked.
Ask what a peptide’s purity is and you get a percentage. Ask what the percentage measures and the question turns out to have at least three answers, which routinely disagree by twenty points.
Three different questions
Chromatographic purity asks: of the material that eluted from the column, what fraction was the main peak? This is the number on almost every label. It is a statement about the peptide relative to other peptide-like species.
Net peptide content asks: of the mass in this vial, what fraction is peptide at all? This counts counter-ions, residual solvent and water as what they are — not peptide.
Molar purity asks: of the molecules present, what fraction are the target molecule? Rarely reported, and different again from both.
A vial can honestly be described as 99% pure by the first measure and 78% peptide by the second. Neither figure is wrong. They answer different questions, and only one of them is on the label.
Where the missing mass goes
Synthetic peptides are usually purified by preparative HPLC using trifluoroacetic acid as an ion-pairing agent. Basic residues — arginine, lysine, histidine — retain TFA counter-ions through lyophilization.
The mass cost scales with how basic the peptide is. A sequence rich in arginine and lysine can carry a substantial counter-ion load; a neutral sequence carries very little. So net peptide content is partly a property of the sequence, not only of how carefully the material was made. Comparing net content between two different compounds tells you less than comparing it between two lots of the same one.
Add adsorbed water — lyophilized powder is hygroscopic and picks up moisture whenever a vial is opened warm — and the gap between vial mass and peptide mass is entirely ordinary.
Why it matters for your numbers
If you calculate concentration from vial mass, you are assuming net content is 100%. It is not. At 80% net content, every concentration you derive is 20% high, and the error is systematic — it will not average out across replicates, and it will make your work disagree with a laboratory that corrected for it.
The correction is arithmetic, and it needs one figure the certificate should already give you.
What to ask for
The useful request is not “what is the purity.” It is:
- Chromatographic purity, with the detection wavelength stated — 214 nm for peptide-bond detection.
- Net peptide content, with the method named.
- Residual moisture.
- Counter-ion identity, if your work is sensitive to TFA. Trifluoroacetate is not inert in every assay system, and in cell-based work it is worth knowing about.
A supplier who can answer all four is measuring their material. A supplier who can answer only the first is quoting it.
Every Aminopia lot reports all four. See how we test.