Accepted answer
On a 20 mg vial every percentage point of water and counter-ion is 0.2 mg, so the answer is a multiplication once you have the net peptide content off the certificate. Settle first whether the 20 mg on the label is gross fill or net peptide, because the two differ by exactly the quantity being asked about. If the certificate quotes net peptide content of 85 per cent, a 20 mg gross fill holds 17 mg of peptide; at 78 per cent it holds 15.6 mg. That 1.4 mg gap is 7 per cent of the label, larger than any purity difference anybody argues about, and it is invisible to a purity figure because water and acetate are not impurity peaks. Amino acid analysis or a nitrogen determination gives you the number; an HPLC area per cent never will.
Content assay requires a reference standard of known purity and traceability, which is why it costs more than purity testing does.
The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
Reconciling gross mass to label claim
| Component | Typical share | Counted in purity? | Counted in content? |
|---|
| Target peptide | 88–94 % | Yes, as main peak | Yes |
| Related impurities | 1–3 % | Yes, as other peaks | No |
| Counter-ion (TFA or acetate) | 2–8 % | No | No |
| Residual water | 2–6 % | No | No |
| Bulking agent, if present | 0–40 % | No | No |
Concretely, if a sample shows high purity but low content, the explanation is usually that the standard used for quantitation had a different purity than claimed.
The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
One qualification: a single result from a single vial is a point estimate, and repeating the assay on a second aliquot is worth doing if the first result is surprising.
If a supplier gives you content without the standard's purity, ask them to provide it.