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How much peptide is really in the vial?
A 10 mg vial at 99% purity doesn't hold 9.9 mg of peptide. Part of the powder is salt left from synthesis and bound water. Enter the peptide, salt form and purity to see the actual peptide mass, or the powder you need for a target amount.
10.0 mg × 81.9% content × 98.5% purity = 8.06 mg. Content = 1,419.55 ÷ (1,419.55 + 2 × 114.02) × (1 − 5.0% water).
Purity and net peptide content measure different things
- HPLC purity is the target peptide's share of the peptide-related peaks on the chromatogram. Truncated and deleted sequences count against it. Water, salts and counterions don't show up in it at all.
- Net peptide content is the share of the powder's total weight that is peptide. The rest is counterions and water, and sometimes an added excipient such as mannitol.
A peptide can be 100% pure and still be well under 100% peptide by weight. To get the mass of the target peptide, you need both numbers: powder mass × net peptide content × purity.
Where the missing mass goes
Counterions
Peptides are cleaved from the synthesis resin and purified by reverse-phase HPLC using trifluoroacetic acid (TFA). The acid stays bound to every positively charged site: the free N-terminal amine and the side chains of arginine, lysine and histidine. The peptide is sold as that salt. Some suppliers exchange TFA for acetate or hydrochloride, which are lighter and often preferred for cell work.
The more basic sites a peptide has relative to its size, the more of its weight is salt. Short peptides rich in Lys, Arg and His are hit hardest.
Water
Lyophilized peptides are hygroscopic and hold several percent water even when dry. A COA may report it as a Karl Fischer result.
Excipients
Mannitol or trehalose are sometimes added as bulking agents, which is why one vial shows a solid cake and another an almost invisible film. They add weight but no peptide, so ask whether the labeled mg refers to the peptide or to the whole fill.
Theoretical peptide share by salt form
Peptide share of the dry salt, before water, assuming every basic site carries one counterion. Measured content is usually lower because of water.
| Peptide | MW | Basic sites | TFA salt | Acetate salt |
|---|---|---|---|---|
| BPC-157 | 1419.55 | 2 | 86.2% | 92.2% |
| Ipamorelin | 711.87 | 3 | 67.5% | 79.8% |
| GHK (tripeptide) | 340.38 | 3 | 49.9% | 65.4% |
| Angiotensin II | 1046.19 | 3 | 75.4% | 85.3% |
| Oxytocin | 1007.19 | 1 | 89.8% | 94.4% |
Worked example: BPC-157, TFA salt
BPC-157 (GEPPPGKPADDAGLV, MW 1419.55) has two basic sites: the N-terminus and one lysine. As a TFA salt it weighs 1419.55 + 2 × 114.02 = 1647.59 g/mol, so the dry salt is 86.2% peptide. With 5% water, net peptide content is 81.9%. Ten mg of powder at 98.5% purity holds 8.06 mg of BPC-157, and you'd need 12.4 mg of powder to get 10 mg of peptide.
How content is measured
- Amino acid analysis (AAA): the peptide is hydrolyzed and each amino acid quantified, giving the peptide mass directly.
- Elemental (nitrogen) analysis: counterions and water contain no nitrogen, so measured nitrogen against the sequence's theoretical nitrogen gives the peptide fraction.
- Quantitative HPLC (assay): the sample is measured against a reference standard of known content.
A measured result beats this calculator's estimate. If your COA lists net peptide content or an assay value, enter it in the “measured” field.
What to ask a supplier
- Is the price per mg of powder (gross) or per mg of peptide (net)?
- What salt form is it: TFA, acetate or HCl?
- Is net peptide content measured, and by which method?
- Does the labeled amount include any excipient?
Two quotes at the same price per mg can differ by 10–20% in actual peptide if one is gross weight and the other net. To check the rest of a certificate, see how to verify a peptide COA and peptide testing labs compared, or compare a COA against your own limits with the COA spec checker.
Limits of the estimate
- It assumes one counterion per basic site. Histidine is only partly protonated in some conditions, and acidic residues can lower the real count.
- Modified peptides (lipidated, PEGylated, cyclic or with unusual residues) need the supplier's own salt and content data.
- This is arithmetic on numbers you enter. It is not dosing, medical or safety guidance.