Purity is a ratio
HPLC purity compares the target peptide’s peak with everything else the detector picks up, such as shorter or altered sequences left from synthesis. A “99.7%” purity says the peptide material is 99.7% the target. It says nothing about the total amount.
Net content is an amount
Net peptide content measures how much of the target peptide is present, usually by quantitative HPLC against a reference standard of known concentration. It is reported as a weight, for example “10.49 mg” against a “10 mg” label claim, or as a percentage of the powder’s weight.
Why 99% pure isn’t 99% peptide
A freeze-dried peptide powder holds more than the peptide itself. It also contains:
- Counter-ions: salts from synthesis and purification, such as acetate or trifluoroacetate (TFA), bound to the peptide.
- Residual water that the powder takes up.
HPLC purity at the usual wavelength doesn’t count these, so a powder can be 99% pure by HPLC while the peptide itself makes up noticeably less than 100% of its weight. That is why the label claim should be checked against the measured net content, not the purity figure.
What to look for
- A net content result in mg, compared with the label claim and a stated acceptance range (for example ±10%).
- A purity result with its method and specification.
- Both numbers tied to the same batch number as your vial.