Purity Percentage

Purity percentage is an area ratio, not a mass ratio. A sample is separated by reversed-phase chromatography, the ultraviolet trace is integrated, and the main peak's area is divided by the total area of all integrated peaks. Detection sits near 214 nanometres, where the amide bond itself absorbs, so the figure describes the proportion of ultraviolet-absorbing material that eluted as the target under one particular set of conditions.

Every clause in that definition is a limitation. Absorbance at 214 nanometres scales roughly with the number of peptide bonds, so a short truncated fragment under-reports relative to the full-length chain. Species that fail to elute within the gradient window, that do not absorb, or that co-elute under the main peak contribute nothing to the denominator. Salts, water, residual solvent and endotoxin are invisible to the method entirely.

A purity figure is therefore one of at least four numbers needed to describe a vial: purity by chromatography, identity by mass spectrometry, net peptide content by amino acid analysis, and the contaminant panel. Purity alone supports the conclusion that the chromatographic profile is dominated by a single species, and supports no conclusion about what that species is or how much of it the container holds.

The failure mode is a bare number. A stated ninety-eight percent with no chromatogram, wavelength, column, gradient or integration threshold is an assertion rather than a measurement, and a steep short gradient reports a higher figure than a shallow one on identical material because closely related impurities merge into the main peak. A powder can be ninety-nine percent pure by area and still be mostly salt and water by weight.