HPLC and Mass Spectrometry: How Peptide Purity and Identity Are Tested

Two questions, two methods

Analytical testing of a research peptide answers two different questions. Is this the right molecule? That is identity, usually confirmed by mass spectrometry. How much of the sample is that molecule rather than something else? That is purity, usually measured by high-performance liquid chromatography (HPLC). A useful COA reports both.

HPLC: separating and measuring

In reversed-phase HPLC, the dissolved sample is pushed through a column packed with a non-polar stationary phase while the solvent mixture gradually changes. Molecules leave the column at different times depending on how strongly they interact with it, and a UV detector — commonly set around 214–220 nm, where peptide bonds absorb — records each as a peak.

Purity is reported as the area of the main peak divided by the total area of all peaks. Related impurities such as truncated or deletion sequences, oxidized forms and synthesis by-products appear as smaller peaks around the main one.

Mass spectrometry: confirming identity

Mass spectrometry ionizes molecules — typically by electrospray (ESI) or MALDI — and measures their mass-to-charge ratio. The observed mass is compared with the theoretical molecular weight calculated from the sequence. A match within the instrument's tolerance confirms the main component is the intended peptide; unexpected masses can reveal modifications or wrong sequences.

What neither method tells you alone

• HPLC cannot confirm identity: a high-purity peak could still be the wrong peptide.
• MS alone cannot give purity: it shows what is present, not reliably how much.
• Neither measures net peptide content by weight; that needs amino acid analysis or elemental nitrogen analysis.
• Neither detects every possible contaminant; endotoxin, residual solvents and heavy metals need their own tests.

Reading the numbers

Look for the chromatogram behind any purity figure, the detection wavelength, and the observed versus theoretical mass. A COA that shows these, for a named batch from a named laboratory, is a document you can check. See our guide to reading a Certificate of Analysis for the full checklist.