Purity against peptide content
The arithmetic of delivered peptide mass, and whether a certificate permits a mass balance to be closed.
§1Calculator
§2Why the mass balance matters
Three independent terms separate a labelled mass from a delivered target-sequence mass, and most certificates report only one of them.
- Peptide content is the mass fraction of the lyophilisate that is peptide. The remainder is water, counter-ion and residual solvent. Content is determined against a named reference standard; without one, a certificate reports an area percentage rather than a content.
- Chromatographic purity is the relative area of the main peak among the species that absorb at the detection wavelength. It says nothing about how much peptide is in the vial.
- Water and counter-ion are mass that is not peptide. For a basic peptide purified with trifluoroacetic acid, the counter-ion fraction can be substantial.
If the three declared fractions do not sum to approximately one hundred per cent, the certificate is internally inconsistent or incomplete. If they cannot be summed because two are absent, the certificate does not permit a delivered mass to be established at all. That is the most common state of a certificate in this channel, and it is the reason the Institute treats counter-ion and water as certificate fields rather than as analytical refinements.
The standard is CEI-MS-14; the review is counter-ion content and delivered peptide mass.