Identity testing: proving what the peptide is.

Why identity is a separate question from purity

Purity measures composition within a sample. Identity answers a prior question: is the main substance actually the labeled peptide at all? Mislabeling, substitution, and synthesis errors are real failure modes in unregulated markets — and only identity methods can catch them.

The tools of identity confirmation

• Molecular weight by mass spectrometry — the workhorse. Measured mass must match the theoretical mass of the sequence within a tight tolerance. See LC-MS.
• MS/MS sequencing — the peptide is fragmented and the fragment masses map onto the expected amino acid sequence. This confirms the order of residues, not just total mass.
• Amino acid analysis (AAA) — hydrolysis quantifies each amino acid; the composition profile should match the sequence. Used for both identity support and content.
• Retention time vs a reference standard — in HPLC, the authentic peptide and the sample should co-elute under identical conditions.
• Melting point / optical rotation — classical identity tools, weaker on their own but still reported on some COAs.

What identity testing looks like on a COA

• A measured mass plus theoretical mass, with an explicit tolerance or pass/fail.
• Optionally an MS/MS match statement or AAA composition.
• The method and instrument conditions named — an unexplained "identity: pass" is weak evidence. See how to read a peptide COA.

The strongest identity workflows

Research-grade suppliers aiming at credibility typically stack at least two orthogonal methods: molecular weight by MS plus either MS/MS sequencing or AAA composition, run against a reference standard. When a vendor reports only one method — or none — treat identity as unverified.

Identity data is also the layer that catches counterfeit substitution (a different, cheaper peptide in the vial), which purity numbers alone would never reveal. For documented counterfeiting patterns, see our article fake COAs are real.