Two numbers appear on most peptide certificates and are routinely treated as one. Chromatographic purity describes the proportion of detected material represented by the main peak. Peptide content describes how much of the weighed mass is actually peptide. A preparation can be highly pure and still be substantially something other than peptide by mass.
Where the mass goes
Lyophilised peptide is not neat peptide. The remainder is typically water and counter-ions carried through from purification, and both are invisible to a chromatographic purity determination because neither is detected under the conditions used to measure the peak.
- Residual water, which varies with lyophilisation and with subsequent handling.
- Counter-ion, which depends on the purification and salt-exchange history of the batch.
- Residual solvent from synthesis and purification.
Which number a calculation needs
Any calculation that converts a weighed mass into an amount of peptide needs peptide content, not chromatographic purity. Using purity in that calculation systematically overstates the amount present, and the size of the error is not constant between lots.
Why this shows up as irreproducibility
Because peptide content varies between batches while purity often does not, a laboratory that calculates from purity will see a shift when it reorders — and will usually attribute the shift to something else, because the certificate for the new lot looks equivalent to the old one.
Reading the two together
Purity answers whether the material is clean. Peptide content answers how much peptide a given mass contains. Identity, established separately, answers whether the main peak is the intended molecule. A certificate that reports only one of the three has answered only one of three questions, and the two unanswered ones are not safe to assume.
References
- ICH Q2(R2) — Validation of Analytical ProceduresInternational Council for Harmonisation
- USP General Chapter <1225> — Validation of Compendial ProceduresUnited States Pharmacopeia
- ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratoriesInternational Organization for Standardization
References are cited as the standards and guidance this note is written against. Citing a standard is not a claim of conformance to it.
Scope
This note covers documentation, procurement, storage, and analytical practice for laboratory and manufacturing materials. It is not guidance for preparing or using any material in a person or an animal, and nothing in this catalog is supplied for that purpose.