HPLC purity testing explained: what the percentage really means

What HPLC actually measures

Reverse-phase high-performance liquid chromatography pushes a dissolved sample through a column packed with a hydrophobic stationary phase. Components separate by how strongly they interact with that phase, and a UV detector — usually at 214 nm, where the peptide bond absorbs — records each one as it elutes.

The output is a chromatogram: a baseline with peaks. Purity is reported as the area of the main peak divided by the total area of all peaks, expressed as a percentage. That is why the figure is properly called area-under-curve purity, not absolute purity.

What the percentage does not include

• Water and residual solvent — a lyophilised powder carries both, and neither shows on a UV chromatogram.
• Counter-ions such as acetate or trifluoroacetate, which can be several percent of the vial mass.
• Anything that does not absorb at the detection wavelength, including many salts.
• Impurities that co-elute with the main peak, hidden underneath it rather than beside it.

Why mass spectrometry is run alongside it

HPLC tells you how much of one thing is present relative to everything else. It does not confirm that the main peak is the compound you ordered. Mass spectrometry supplies that half: the measured molecular mass should match the theoretical mass of the sequence, within the instrument's tolerance.

A report with a purity figure but no mass confirmation establishes that the material is homogeneous — not that it is the right peptide. Ask for both.

Assayed content versus purity

A quantitative report also states assayed content — how many milligrams of peptide the vial actually holds. A 10 mg vial that assays at 9.6 mg of 99% pure material is a very different purchase from one that assays at 7.1 mg, even though both reports show a high purity figure.

Purity is about the quality of what is there. Content is about how much of it there is. Always read the two together.

Reading the chromatogram itself

• Look for a flat baseline before and after the main peak — a drifting baseline suggests a poor method or a dirty column.
• Check peak symmetry. Heavy tailing can hide co-eluting impurities.
• Count the secondary peaks. Several small ones near the main peak often indicate deletion or truncation sequences from synthesis.
• Confirm the sample identifier and date on the report match the lot number on the vial in front of you.

Verify the report independently

Reputable analytical laboratories issue reports with a verification key that resolves on their own website. That check confirms the document came from the lab and has not been edited. Any report without a verifiable key should be treated as an unverified claim.

Our published reports are listed with their verification keys in the COA batch lookup so you can check them against the laboratory directly.