Peptide Content vs Purity: Why a 5 mg Vial Is Not 5 mg of Peptide
Purity is a ratio and content is a mass. Counter-ions and residual water take up the difference, and for quantitative work it matters.
A vial labelled 5 mg does not contain 5 mg of peptide. It contains 5 mg of lyophilised material, of which the peptide is a fraction — typically 70 to 90 per cent. For qualitative work this rarely matters. For anything quantitative it is the difference between a result and an artefact.
What else is in the vial
Three things share the mass with the peptide:
- Counter-ions. Peptides are purified by reversed-phase HPLC using trifluoroacetic acid, and they come off that column as TFA salts. Every basic residue — lysine, arginine, histidine — can carry a counter-ion. On a sequence rich in those, TFA alone can account for 10 to 20 per cent of the mass.
- Residual water. Lyophilisation removes most water, not all. A few per cent typically remains bound, and it increases if the vial has been opened cold and drawn moisture in.
- Residual solvent and salts from the purification, usually a smaller contribution.
Why TFA specifically
Trifluoroacetate is the standard ion-pairing agent in peptide purification because it gives sharp peaks and evaporates readily. The trade-off is that it stays bound to the peptide, and it is not inert: TFA has documented effects in cell culture at concentrations that can be reached simply by dissolving a TFA-salt peptide.
This is why acetate-exchanged material exists and costs more. If a protocol involves cells and is sensitive, the counter-ion is worth knowing rather than assuming.
How peptide content is measured
Two methods appear on certificates:
- Amino acid analysis (AAA) — the material is hydrolysed to free amino acids and quantified. This is the reference method and gives an absolute figure.
- Nitrogen determination — total nitrogen is measured and back-calculated from the sequence. Cheaper, slightly less specific.
Neither is the same as HPLC purity, and this is the distinction that causes most of the confusion. Purity is a ratio; content is a mass. A sample can be 99 per cent pure and 75 per cent peptide by mass at the same time, with no contradiction — the 99 per cent describes what fraction of the peptide material is the target sequence, and the 75 per cent describes what fraction of the powder is peptide at all.
What it means in practice
| Label | 5 mg lyophilisate |
|---|---|
| HPLC purity | 99% |
| Peptide content | 80% |
| Actual peptide | ~4 mg |
| Concentration in 2 ml | ~2 mg/ml, not 2.5 |
A 20 per cent error in concentration is more than enough to move a dose-response curve. If a study is quantitative, ask for peptide content and calculate from that figure rather than from the label.
What to ask a supplier
- Does the certificate report peptide content as well as HPLC purity?
- Is the material a TFA salt or acetate salt?
- Was content measured by amino acid analysis or by nitrogen?
A supplier who cannot answer the first question is quoting a number they have not measured. Our certificate guide covers what else belongs on the document, and the reconstitution calculator handles the arithmetic once you have the real figure.
Every batch we supply is analysed by an independent laboratory for HPLC purity and mass-spectrometric identity, with batch-matched certificates available on request.
All products and information referenced are for in-vitro research and laboratory use only. Nothing here is medical advice, and no therapeutic claim is made or implied.