Purity, by chromatography
High-performance liquid chromatography separates a sample by how strongly each component interacts with a column as solvent carries it through. Components emerge at different times and a detector records each as a peak; the target peak's share of the total area is the purity figure.
For a synthetic peptide the hardest impurities to separate are the ones most like the product — deletion sequences missing a single residue. Resolving those is where a careful method differs from a fast one.
Identity, by mass
Chromatography shows a sample is largely one thing. It does not show what that thing is. Mass spectrometry measures the molecule's mass and compares it against the value calculated from the sequence; agreement within instrument tolerance is the identity confirmation.
Mass differences are diagnostic in themselves. Sixteen daltons above expectation suggests oxidation; one dalton suggests deamidation; a deficit matching a residue suggests a deletion sequence.
What analysis does not claim
Neither method measures biological behaviour and neither is a safety assessment. A chromatographically pure peptide of confirmed mass is a well-characterised chemical. That is the entire claim, and it is worth being precise about it.
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