Home Blog Reading the paperwork
Net peptide content, and the 18% error nobody catches
Why the milligrams on the label are not the milligrams of peptide, and what that does to a concentration you already wrote down.
A 10 mg vial does not contain 10 mg of peptide. It contains 10 mg of lyophilized solid, and somewhere between 75 and 90 per cent of that is peptide. The rest is counter-ion, residual solvent and water.
Where the other 15 per cent goes
Preparative HPLC runs in trifluoroacetic acid. The peptide comes off the column as a TFA salt, and TFA is heavy: a chain with several basic residues can carry enough of it to account for a fifth of the dry weight on its own. Add a few per cent of water that survived the dryer and the arithmetic moves.
What it does to the peptide mass
| Assumed | Actual at 82% net | |
|---|---|---|
| Label mass | 10.0 mg | 10.0 mg |
| Net peptide content | 100% | 82% |
| Peptide in the vial | 10.0 mg | 8.2 mg |
| Difference | (none) | 1.8 mg (18%) |
Eighteen per cent is enough to shift any quantitative figure calculated from the label mass, and not enough to look wrong on paper.
Where the right number comes from
It is on the certificate for the lot in your hand, not on the product page. The product page carries an average across lots, and net content varies between lots of the same peptide by several points, which is enough to matter to anyone working from it.
Common questions
Does a 10 mg peptide vial contain 10 mg of peptide?
No. It contains 10 mg of lyophilized solid, and somewhere between 75 and 90 per cent of that is peptide. The rest is counter-ion, residual solvent and water. At 82 per cent net peptide content, a vial labeled 10 mg holds 8.2 mg of peptide.
Why is part of a lyophilized peptide's weight not peptide?
It is because preparative HPLC runs in trifluoroacetic acid, so the peptide comes off the column as a TFA salt. TFA is heavy, and a chain with several basic residues can carry enough of it to account for a fifth of the dry weight on its own. A few per cent of water that survived the dryer adds to it.
How large is the error from reading the label mass as peptide mass?
At 82 per cent net peptide content it is 1.8 mg in a vial labeled 10 mg, which is eighteen per cent. That is enough to shift any quantitative figure calculated from the label mass, and not enough to look wrong on paper.
Where is the net peptide content for a specific lot published?
It is on the certificate for that lot, not on the product page. The product page carries an average across lots, and net peptide content varies between lots of the same peptide by several points, which is enough to matter to anyone working from the figure.
For laboratory research use only. Not a drug, not a supplement, and nothing here is a claim about what any of this material does in a person or an animal.
