Identity Testing: Why Peptide Sequence Matters

A high purity percentage answers one question: how clean is the dominant HPLC peak? Identity testing answers a different question: is that peak actually the peptide on the label? Without both, a COA tells half the story.

Why identity is non-negotiable

Peptide catalogs contain hundreds of similar sequences. Manufacturing errors, mislabeling, and wrong-vial fulfillment happen. Two peptides can produce nearly identical HPLC purity numbers while being entirely different molecules.

Mass spectrometry is the standard identity method: the measured mass-to-charge ratio should match the expected molecular weight of the target sequence, accounting for salt form (TFA, acetate) and modifications (DAC, PEG, acetylation).

What a good identity report includes

• Observed mass and calculated mass with acceptable tolerance (typically within a few daltons for small peptides).
• Method — ESI-MS, MALDI-TOF, or LC-MS/MS.
• Batch reference tied to the lot you received.
• For modified peptides, confirmation of the full modified structure, not just the core sequence.

When identity data is weak

• "Identity confirmed by HPLC retention time" — retention time is not identity.
• MS reported without the actual spectrum or mass value.
• Generic statement "matches standard" with no numerical data.
• Molecular weight that does not account for your peptide's salt or conjugation form.

The pairing rule

Always read identity alongside HPLC purity and the chromatogram. Our vetting methodology scores identity confirmation and purity as separate criteria for exactly this reason.