A certificate of analysis (COA) is the document that accompanies a research-peptide lot and summarises the tests performed on that specific batch. This guide describes the sections typical of research-peptide COAs and what each number is telling you about the material.

The identification section

The top of a COA identifies the peptide and the specific lot:

  • Peptide name and sequence — the human-readable name and the amino-acid sequence, typically written in single-letter code with modifications noted (e.g. an N-terminal acetyl group, a C-terminal amide, or a fatty-acid conjugate).
  • Lot number — a batch identifier that links the COA to the physical material.
  • Molecular formula and monoisotopic mass — the theoretical values, used to interpret the mass-spec identification section below.
  • Appearance — usually reported as “white lyophilised powder” or similar.
  • Test date — the date the lot was analysed, not the shipment date.

Purity by HPLC

The reported purity is almost always determined by reverse-phase high-performance liquid chromatography (RP-HPLC) with UV detection at 214 nm or 220 nm — wavelengths at which the peptide bond itself absorbs. The value reported is the integrated area under the main peptide peak, expressed as a percentage of the total integrated peak area on the chromatogram.

Research-peptide COAs typically report purity in the 95%–99%+ range. The remainder is peptide-related impurities (truncated sequences, oxidised residues, incomplete deprotections) rather than solvent or salt, both of which are quantified separately.

A COA that reports “99% purity” without stating the HPLC method, wavelength, and gradient conditions is less informative than one that includes those parameters, because purity is defined against those specific conditions.

Mass-spectrometry identification

The mass-spectrometry section reports the observed mass of the peptide, typically by electrospray-ionisation mass spectrometry (ESI-MS) or matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF). The observed mass is compared against the theoretical monoisotopic mass or average mass.

A close match (within a few daltons for larger peptides, within tenths of a dalton for smaller ones) confirms that the primary sequence identity of the material corresponds to the labelled peptide. A large discrepancy indicates a synthesis or identification error.

Mass spec is an identity check; it is not a purity check. Two distinct peptides of similar mass could pass an identity check while a purity issue is invisible; that is why HPLC purity and mass-spec identity are always reported together.

Endotoxin

The endotoxin section quantifies lipopolysaccharide contamination — endotoxins from Gram-negative bacterial cell walls — typically measured by the Limulus amebocyte lysate (LAL) assay and reported in endotoxin units (EU) per milligram of peptide. Common thresholds are under 5 EU/mg for research-grade material and much lower values (often under 0.5 EU/mg) for material intended for cell-culture use where endotoxin activity would confound biological readouts.

Endotoxin is not the same as sterility. Endotoxin can be present in the absence of live bacteria (it is a stable cell-wall component), and a low endotoxin number does not itself certify sterility.

Residual solvents and moisture

  • Residual solvents. COAs from higher-tier suppliers report residual synthesis-solvent content (typically acetonitrile, trifluoroacetic acid, DMF, or DCM), often measured by gas chromatography, and compared against ICH Q3C limits.
  • Moisture content. Karl Fischer titration quantifies bound water in the lyophilised material. Higher moisture content correlates with reduced shelf life for many peptides.

Not all research-peptide COAs include these sections; when present, they add depth to the material characterisation.

What a COA does not tell you

  • It does not certify sterility. Purity, identity, and endotoxin content are separate from microbial sterility, which requires membrane filtration or a validated sterilisation step.
  • It does not certify pharmacological activity. Purity and identity say nothing about whether the material is biologically active in a functional assay.
  • It applies only to the lot tested. A COA from lot A does not carry to lot B.

Handling in the research literature

The convention across peptide research is to record the peptide name, lot number, supplier, and reported purity in the study methods section. When a COA is available, storing it alongside the reconstitution log is standard.

Reminder

Nothing in this guide constitutes medical advice or a recommendation to administer any peptide. This is a scientific-reference description of documentation practices described in the research literature. See our disclaimer.