What an HPLC Purity Figure Does — and Does Not — Tell You About a Research Peptide

Purity values reported for synthetic peptides are usually derived from reversed-phase high-performance liquid chromatography (RP-HPLC) with UV detection. The number is typically the area of the main peak expressed as a percentage of the total integrated peak area. That figure is useful, but it is a summary of one chromatographic experiment rather than a complete description of the material.

Where peptide impurities come from

A comprehensive review of related impurities in peptide medicines by D'Hondt and colleagues (Journal of Pharmaceutical and Biomedical Analysis, 2014) groups peptide-related impurities by origin: those introduced during synthesis and those formed by degradation during storage. Synthesis-related examples include amino acid deletions or insertions, incomplete removal of protecting groups and stereoisomers formed by racemisation. Degradation-related examples include oxidation, deamidation and aggregation.

Why one percentage cannot capture everything

• Co-elution: impurities that are chemically very similar to the target sequence — for example a diastereomer — may elute under or very close to the main peak, so the main-peak area can include them.
• Detection: UV-based area percent assumes comparable response between the main component and its impurities, and it does not see non-chromophoric components.
• Scope: area percent describes peptide-related species. It does not measure water, residual solvents or counter-ion content, which are reported separately when tested.

For this reason, chromatographic purity is normally read alongside an identity test, most commonly mass spectrometry, which confirms that the main peak has the expected molecular mass.

What makes a purity result trustworthy

The ICH Q2(R2) guideline on validation of analytical procedures, adopted in November 2023, describes how a procedure should be shown to be fit for its intended purpose. Its performance characteristics include specificity/selectivity, accuracy and precision across the reportable range. A purity value from a procedure with demonstrated specificity for the relevant impurities carries more information than an unqualified percentage.

Reading a certificate of analysis

When reviewing a COA for laboratory work, it is reasonable to check which analytical method produced the purity value, whether an identity test is reported, the lot or batch number the data refer to, and the test date. These details connect the reported figure to a specific, traceable batch of material.