A Certificate of Analysis is the document that proves what's in the vial. A peptide without a CoA is essentially an unknown.
For laboratory research, you cannot include an undocumented peptide in your method section, cannot reproduce a result, and cannot defend the data against reviewer scrutiny. This article explains every field you'll find on a typical peptide CoA and what to verify.
The header
Every CoA starts with metadata. Three things matter most:
Compound + sequence
Full amino acid sequence in single-letter code. Missing sequence = red flag.
Lot / batch number
Must match the number on the vial label. If they don't match, the document doesn't apply.
Test + report dates
CoAs >24 months old should prompt fresh testing requests.
Identity confirmation: the molecular weight section
Look for two values:
- Theoretical molecular weight — calculated from the amino acid sequence using a standard mass table. For most research peptides this is between 500 and 5000 Daltons.
- Observed molecular weight — measured by mass spectrometry (ESI-MS or MALDI-TOF). Should match the theoretical value within ±1 Da.
If the observed mass is more than 1 Da off, something is wrong. Common discrepancies:
| Discrepancy | Likely cause | Concern |
|---|---|---|
| +16 Da | Oxidation (Met / Trp residues) | Yes |
| −1 / +1 Da | Deamidation (Asn / Gln) | Yes |
| +18 Da | Water adduct | Usually harmless |
| +22 Da | Sodium adduct | Usually harmless |
A reputable CoA will include the actual MS spectrum, not just a number.
Purity: the HPLC section
This is where the headline "≥99%" claim comes from. The HPLC section should contain:
- A chromatogram image — a graph showing peaks plotted against retention time
- The main peak's area percentage — usually the largest peak in the chromatogram, this is the reported purity
- The detection wavelength — typically 214 nm (peptide bond absorbance) or 220 nm
- The column type and gradient — for full reproducibility (e.g. C18 column, water/acetonitrile with 0.1% TFA)
- Main peak area ≥98% for most research peptides
- ≥99% for high-quality sources
- Symmetrical peak shape (Gaussian)
- No major impurity peaks within ±2 minutes of the main peak
- Multiple peaks of similar height
- A "shoulder" on the main peak indicating a closely related impurity
- A reported purity higher than what the visible chromatogram supports
Net peptide content (the often-missing field)
This is the percentage of the gross weight in the vial that is actual peptide. The rest is:
- TFA counter-ions from synthesis (typically 5–25% of mass for highly basic peptides)
- Bound water
- Inorganic salts
A vial labeled "21 mg" may contain only 16–18 mg of actual peptide if net content is 75%. This dramatically affects how you calculate dosing in your research protocols.
Net peptide content is typically measured by:
- Amino acid analysis (AAA) — the gold standard
- Quantitative NMR (qNMR) — increasingly common
- Nitrogen content (Kjeldahl) — older method, less precise
If a vendor shows you a 99% HPLC purity number but no net peptide content figure, you don't actually know how much peptide you have.
TFA salt content
Most peptides are synthesized using TFA-based protocols, and the final product comes out as the trifluoroacetate salt. The TFA contributes mass without contributing peptide. CoAs should specify either:
- The TFA content (as a percentage, typically 5–25%)
- Or that the peptide has been converted to a different salt form (acetate, hydrochloride)
Water content
Lyophilized peptides retain residual water. Anything under 5% is normal. Above 10% suggests poor drying and may affect long-term stability.
What a complete CoA looks like
A high-quality research peptide CoA includes, at minimum:
- ✓Compound name + full sequence
- ✓Lot number + test date + report date
- ✓Theoretical and observed molecular weight (with MS spectrum)
- ✓HPLC chromatogram with purity calculation
- ✓Net peptide content
- ✓Salt form and salt content
- ✓Water content
- ✓Storage and handling recommendations
- ✓Signature or lab identifier
If a vendor's CoA is missing more than two of these, ask for a complete document or look elsewhere.
Independent vs in-house testing
Many manufacturers issue their own CoAs. These are useful but inherently conflicted — the lab testing the product is the same lab that produced it. Independent third-party testing (the supplier sends a sample to an external accredited laboratory) is the higher standard. Where it's available, prefer it.
Why this matters for laboratory work
If you're using research peptides in academic or industrial labs, your method section needs to specify:
- Manufacturer and lot number
- Verified purity by HPLC
- Verified identity by MS
- Net peptide content (for accurate concentration calculations)
Reviewers and replication studies depend on this. Skipping it now creates problems later.
How we handle test data
Test documentation accompanying our products reflects HPLC analysis performed during quality verification. Reference batches across our catalog have been submitted for independent third-party HPLC verification, and documentation is available on request. If you need specific test data for the lot you've received, get in touch and we'll send what we have.