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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 a stock solution
| Assumed | Actual at 82% net | |
|---|---|---|
| Peptide in the vial | 10.0 mg | 8.2 mg |
| Solvent added | 2.00 mL | 2.00 mL |
| Concentration | 5.00 mg/mL | 4.10 mg/mL |
| Error | — | 18% low |
Eighteen per cent is comfortably enough to move a dose-response curve and not nearly enough to look wrong. Nothing in the experiment reports an error. The number in the notebook is simply not the number in the tube.
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.
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.