Purity

On a peptide certificate, purity nearly always refers to chromatographic purity: the area of the main peak as a percentage of all integrated peak area in a reverse-phase HPLC analysis. A declared ≥98% therefore means that no less than 98% of the UV signal recorded at 214 nm came off the column as one peak with that method.

The rest of the signal

The remainder is usually a by-product of synthesis: deletion sequences lacking a residue, chains whose protecting groups were not fully removed, oxidised methionine, deamidated asparagine, or dimers. Because these impurities closely resemble the target, they elute close to it — so a method with good resolution counts for more than the headline number.

Purity versus content

This is the point that catches people out: purity only reflects what the detector registers. Residual water, the acetate or TFA counter-ion and any excipients that do not absorb have no chromophore and are left out of the calculation. A peptide of 99% purity can still consist of 15–25% salt and water by weight. Net peptide content is measured separately, normally by amino acid analysis.

A sensible threshold

For most comparative studies ≥98% is the practical minimum, and the step from 98% to 99% matters less than whether the lot’s identity was verified by mass spectrometry as well.

Related terms

certificate of analysis. See HPLC purity explained.