What “≥99% Purity” Means — and What It Does Not

“≥99% purity” appears on almost every research peptide sold. It is a real measurement and a weaker claim than it looks.

Relative, not absolute

An HPLC purity figure is a ratio of peak areas: the main peak divided by the total detected area. It is relative to what the detector saw, under one gradient, on one column, at one wavelength. It is not a statement that 99% of the vial’s mass is the target compound.

What is excluded from the figure

• Water. A lyophilised cake typically carries several percent residual moisture. That water is invisible to the chromatogram and is not subtracted from the purity figure.
• Counterions and salts. Peptides purified by RP-HPLC are usually isolated as trifluoroacetate or acetate salts. The counterion contributes real mass and does not appear as an impurity peak.
• Non-absorbing species. Anything that does not absorb meaningfully at the detection wavelength is not counted.

The consequence is that a vial labelled 10 mg at 99% purity may contain appreciably less than 9.9 mg of peptide. For qualitative work this is irrelevant. For concentration-sensitive quantitative work it is not, and the correction requires the water content, the salt form, and ideally an amino acid analysis for absolute peptide content.

Peptide content versus chromatographic purity

These are separate specifications and a rigorous certificate reports both. Chromatographic purity answers “how homogeneous is this?” Peptide content answers “what fraction of this powder’s mass is actually peptide?” A material can be 99.5% pure by HPLC and 78% peptide by content, and both figures can be honest.

Reading the method, not the number

Two suppliers reporting 99.2% may have run methods of very different discriminating power. A shallow gradient over a long run resolves closely related impurities that a fast steep gradient hides beneath the main peak. When comparing suppliers, compare the chromatograms — peak symmetry, baseline noise, and whether the method plausibly separates the species it needs to.