Peptide Purity and Identity Testing: Which Method Answers Which Question

RP-HPLC separates by hydrophobicity on a C18 or C8 stationary phase using an acetonitrile–water gradient, usually with trifluoroacetic acid or formic acid as an ion-pairing modifier. The target peak's area as a proportion of total peak area gives the purity figure almost every certificate quotes.

The weakness is co-elution. Deletion sequences differ from the target by a single residue and are often barely resolved under a single set of conditions. Running an orthogonal method — a different column chemistry, or the same column at a different pH — changes selectivity and can split a peak that looked clean. Where purity genuinely matters, one gradient on one column is not a sufficient answer.

ESI-MS gives an accurate molecular mass and so confirms consistency with the expected structure. High-resolution instruments narrow the tolerance usefully. But mass is not unique: isomers are identical by mass, and a near-isobaric impurity can sit inside a loose window.

Tandem MS is the substantial upgrade. Fragmenting the peptide and reading the resulting ion series reconstructs sequence, which is identity evidence of a different order from a single intact mass. For short peptides this is routine; for larger molecules, enzymatic digestion followed by peptide mapping achieves the same end.

AAA hydrolyses the peptide and quantifies the released amino acids against standards. It answers the question chromatographic purity cannot: what fraction of the vial's mass is actually peptide, as opposed to water and counter-ions.

It also provides a composition check. A composition inconsistent with the claimed sequence is strong evidence that something is wrong, independent of whatever the chromatogram showed.

Karl Fischer titration quantifies water, which in a lyophilised peptide is rarely negligible and is not constant between lots. Ion chromatography quantifies acetate or trifluoroacetate.

Together with AAA these close the mass balance. Without them, a stated vial mass is an upper bound on peptide delivered, and the error is systematic rather than random — it always overstates.

A defensible minimum for a research peptide is: identity by MS with sequence confirmation where the compound has close relatives; purity by RP-HPLC with the chromatogram supplied; peptide content by AAA; water by Karl Fischer; and counter-ion identified. Anything less leaves a question unanswered, and it is worth knowing which one.