How peptide purity is measured: HPLC and mass spectrometry

Purity is the single most important quality metric for a research peptide. A sequence that is 80% pure behaves very differently from one that is 98% pure, and the nature of the impurities — not just their quantity — can confound an experiment. Two analytical techniques dominate purity assessment: high-performance liquid chromatography (HPLC) and mass spectrometry (MS).

What purity actually means

Purity usually refers to the proportion of the target peptide relative to related impurities: truncated sequences, deletion products, incomplete side-chain deprotection and similar by-products of synthesis. It is most often reported as an area percentage rather than an absolute weight.

HPLC: separating the mixture

Reverse-phase HPLC pushes the sample through a column that separates components by hydrophobicity. A UV detector, typically reading at 214 or 220 nm, produces a chromatogram in which each component appears as a peak. Dividing the target peak's area by the total area of all peaks gives the reported purity. A single, well-resolved dominant peak indicates a clean preparation.

Mass spectrometry: confirming identity

HPLC tells you how much; mass spectrometry tells you what. By ionising the molecules and measuring their mass-to-charge ratio, MS confirms that the measured molecular weight matches the expected value for the sequence. Techniques such as ESI-MS and MALDI-TOF can also reveal deletions or adducts that might co-elute unnoticed on HPLC alone.

Why both are needed

Neither method is sufficient on its own. HPLC quantifies but can miss an impurity that shares the target's retention time; MS confirms identity but is not inherently quantitative. Used together they give a defensible picture, which is why every Vela batch ships with a certificate of analysis combining both. For help interpreting one, see our article on reading a peptide spec sheet.