Ask a supplier for a peptide’s purity and you will get a number, usually something like “99.1%.” Ask what that number means and you enter the single most misunderstood corner of peptide analysis. Two figures on a peptide certificate of analysis (COA) sound like they measure the same thing but do not: HPLC area purity and peptide content by mass. Confusing them leads to under-dosing, failed reproducibility, and mass-balance errors. This guide separates the two for good.
What HPLC area purity actually measures
The headline purity figure almost always comes from reversed-phase HPLC with UV detection. The instrument separates everything in the sample by how strongly it interacts with the column, and a detector records what elutes. Area purity is the target peptide’s peak area expressed as a percentage of the total area of all peaks.
In other words, it answers: of everything the detector saw, what fraction was the target? That is a genuinely useful measure of how clean the synthesis and purification were, but it is a measure of the chromatogram, not of the physical material.
The wavelength caveat
UV detection only “sees” species that absorb at the chosen wavelength (commonly 214 nm for the peptide bond). An impurity that absorbs weakly, or not at all, is under-represented or invisible. Change the wavelength and the purity number can change, which is one reason two labs can report different “purity” for the same vial.
The co-elution blind spot
If two species leave the column at the same time, they merge into one peak, and the software counts them as pure target. This is why identity by mass spectrometry (LC-MS) is a necessary companion: it resolves what UV alone cannot.
What peptide content by mass measures
Peptide content by mass answers a completely different question: of the powder you weighed out, what fraction is actually peptide? This is the number that governs dosing, because it reflects the real physical composition of the vial, not just the relative heights of chromatographic peaks.
The reference method is amino acid analysis (AAA): the peptide is hydrolyzed into its constituent amino acids, which are quantified against standards to give an absolute peptide mass. Quantitative NMR (qNMR) provides an orthogonal route for suitable peptides.
Where the missing mass goes
A peptide that is 99% pure by HPLC area is routinely only 75–90% peptide by mass. The balance is real material that HPLC area purity simply does not account for:
| Component | Typical share | Measured by |
|---|---|---|
| Water (hygroscopic, bound) | 4–12% | Karl Fischer titration |
| Counter-ions (TFA or acetate) | 5–20% | Ion chromatography / HPLC |
| Residual salts | Variable | Residue / elemental analysis |
None of these show up in an area-purity figure, yet all of them occupy mass in the vial. That is the whole gap between the two numbers.
Why dosing depends on content, not purity
Suppose a vial is labeled 10 mg and reports 99% purity. If peptide content by mass is 80%, the vial actually contains about 8 mg of peptide, not 10 mg. Dose from the purity figure and you are systematically 20% low, a gap that has nothing to do with impurities and everything to do with water and counter-ions.
Area purity tells you how clean the peptide is. Content by mass tells you how much peptide you actually have. Dosing needs the second number.
Reading both correctly on a COA
A complete peptide COA reports both, and labels each clearly: an HPLC area purity (with the method and wavelength named) and a peptide content by mass (by amino acid analysis), alongside water and counter-ion content so the mass balance closes. When a COA shows only a single “purity” percentage, it is describing the chromatogram, and leaving the number your dose depends on unstated.
