Purity percentage is not one number

Two laboratories can test the same vial and report different purity figures without either being wrong. The number depends on how it was calculated.

Area percent

The most common method. The target peak's area as a share of total peak area in the chromatogram. Straightforward, and the default on most certificates — but it measures only what the detector responded to. Salts, water, and residual solvents are largely invisible at 214 nanometres and simply do not enter the calculation.

Mass percent

The target compound as a share of the powder's actual weight. This is the stricter figure, and usually the lower one, because everything the chromatogram missed now counts. A material reported at 98% by area might be 85% by mass once water and counter-ions are accounted for.

Why they diverge

Lyophilized peptides are hygroscopic — they draw moisture from air. Synthesis leaves counter-ions behind, commonly trifluoroacetate from purification, which can account for several percent of dry weight. None of that shows up in an area-percent calculation.

What to look for

A certificate that reports area percent alongside water content and residual solvent levels lets you reconstruct the mass figure yourself. One reporting a bare percentage does not.

The practical consequence is comparability. Two suppliers quoting 99% may be quoting different quantities entirely, and the higher number is not necessarily the better material. Method before magnitude.