What “≥99% Purity” Actually Means on a Chromatogram

Purity figures are the most quoted and least understood numbers in peptide sourcing. A stated 99% does not mean 99% of the vial mass is peptide.

A relative, not absolute, measurement

Chromatographic purity is the integrated area of the target peak divided by the total integrated area of all detected peaks, expressed as a percentage. It describes the composition of the UV-absorbing, chromatographically resolved material only. Counterions, residual water, and salts generally do not absorb strongly at the detection wavelength and therefore do not appear as peaks at all.

What the number excludes

Because of this, a lyophilised vial reported at 99% chromatographic purity may still be a substantial fraction water and counterion by mass. This is normal and expected for peptides, not evidence of adulteration — but it means purity and net peptide content are separate specifications that should both be documented.

Method dependence

Purity is also method-dependent. Detection wavelength matters: peptides are commonly monitored near 214 nm, where the peptide bond absorbs, rather than 280 nm, which depends on aromatic residues that some sequences barely contain. Gradient steepness and column chemistry affect whether closely related impurities resolve into separate peaks or hide beneath the main one. A shallow, well-resolved gradient is a more demanding test than a fast one.

Reading the number honestly

A purity figure is meaningful when reported alongside its method and paired with identity confirmation and a net peptide content figure. Presented alone, with no method and no supporting document, it is a marketing claim rather than an analytical result.

Disclosure. This article is educational and summarises published scientific literature. It is not medical advice, and it does not describe outcomes you should expect. These statements have not been evaluated by the Food and Drug Administration, and the products discussed are not intended to diagnose, treat, cure, or prevent any disease. The products discussed are supplied for laboratory and research use only and are not for human or veterinary use, administration, or consumption. For adults 21 and over.