Understanding peptide purity testing

Chromatographic purity, defined

Chromatographic purity is the area of the main peak expressed as a percentage of the total integrated peak area in a chromatogram, under stated conditions. Change the column chemistry, the gradient, the flow rate or the detection wavelength and the number can move, because those variables determine which species resolve into distinct peaks and which co-elute.

This is why any purity figure quoted without its method is not comparable across suppliers. Two laboratories reporting 98% and 99% on the same material may simply be running different gradients.

Chromatographic purity versus net peptide content

Lyophilized peptide is not pure peptide by mass. The cake also contains counter-ions from the purification step — most commonly trifluoroacetate — plus residual water and, occasionally, residual solvent. A vial can be 99% pure chromatographically and still contain appreciably less than the nominal fill weight of peptide.

Impurities a purity assay will not show

Any species that does not absorb meaningfully at the detection wavelength, or that does not elute within the run window, is invisible to the assay. Inorganic salts are the classic example: they contribute mass to the vial and nothing to the chromatogram.

Laboratories that need exact molar concentrations should therefore work from a measured content value rather than assuming that the label fill weight is peptide mass.