Peptide Purity and HPLC, Explained

Purity is the headline number on almost every peptide listing, and it is routinely misread. It is a relative area measurement from a chromatogram, not an absolute statement about the contents of the vial.

What the instrument actually measures

In reverse-phase HPLC the sample is pushed through a hydrophobic column with a changing solvent gradient. Components leave the column at different times and pass a UV detector, usually set to 214 nm where the peptide bond absorbs. The software integrates the area under each peak, and purity is reported as the target peak area divided by total peak area.

Why the method changes the number

• A shallow gradient resolves close-eluting impurities; a steep one hides them under the main peak.
• Detection at 280 nm only sees aromatic residues and can flatter a sample.
• A short run can leave late-eluting impurities on the column entirely.

This is why a chromatogram with its method parameters is worth more than a number on a letterhead.

Purity versus peptide content

Purity says nothing about mass. A vial labelled 10 mg at 99% purity typically contains less than 10 mg of peptide, because residual water and counter-ions carry mass. Peptide content, measured by amino acid analysis or nitrogen determination, is the figure that tells you how much peptide is present.

Is 99% always better than 98%?

Not necessarily. The identity of the impurities matters more than the total. One per cent of a closely related deletion sequence behaves differently from one per cent of residual solvent. A supplier that publishes the impurity profile lets you make that judgement instead of guessing.