What does purity mean for a peptide?

Purity as a peak area percentage in HPLC

The purity of a peptide is expressed as a percentage derived from an HPLC (high performance liquid chromatography) chromatogram. During this analysis, a sample passes through a column, and different substances reach the detector after a specific retention time depending on their physicochemical properties. Each substance produces a peak in the chromatogram. The area under the main peak, divided by the total area of all peaks combined, gives the purity percentage. The higher that value, the larger the share of the detected signal area that originates from the main component.

What the remaining percentage represents

The remaining share of the signal area consists of related impurities. These are substances that can form during synthesis, storage or handling, and that are structurally related to the main component without being identical to it. Examples include:

• Deletion sequences, where an amino acid was skipped during synthesis
• Truncated products, incomplete chains that were terminated prematurely
• Oxidation products, formed through reaction with oxygen during storage
• Other degradation products resulting from hydrolysis or other chemical reactions

These impurities appear in the chromatogram as smaller secondary peaks next to the main peak.

Why high purity matters for reproducible research

Reproducibility is a central requirement in laboratory research. A sample with lower purity contains a larger amount of secondary components that can influence the outcome of an experiment without this being immediately apparent. This complicates comparison between different experiments, batches or laboratories. A higher purity percentage limits the number of unknown variables in a research setup and contributes to consistency between measurements.

Purity versus identity

Purity and identity are two distinct parameters that together provide a complete picture. Purity, determined by HPLC, indicates the amount of main component relative to secondary components within the same sample. Identity is established using mass spectrometry (MS), where the measured molecular weight is compared to the theoretically expected molecular weight of the intended peptide sequence. A sample could, in theory, show a high purity percentage while the main component still does not correspond to the expected substance. For this reason, a complete certificate of analysis (CoA) always includes both an HPLC result for purity and an MS result for identity confirmation.