How to Read a Peptide Certificate of Analysis (COA)

A practical walkthrough of a research peptide COA: batch numbers, identity by mass spectrometry, HPLC purity, chromatograms and the red flags that mean a COA cannot be trusted.

After Hours Labs Research Team

 ·  4 min read

Key takeaways

  • The batch number on the COA must match your vial — otherwise the COA says nothing about your material.
  • Identity (mass spectrometry) and purity (HPLC) are separate tests. You need both.
  • HPLC purity does not measure water, salts, endotoxins or heavy metals.
  • Missing lab details, dates or chromatograms are red flags.

A Certificate of Analysis (COA) is the single most important document that comes with a research peptide. It is the lab’s record of what was actually in a specific batch — not what the label says should be there. Reading one takes a few minutes, and it is the fastest way to tell a well-documented supplier from one that is guessing.

The anatomy of a peptide COA

Most COAs follow the same structure. Here is what each section tells you and what a good result looks like.

SectionWhat it tells youWhat good looks like
Product & label claimThe compound and stated amount (e.g. 10 mg)Matches your vial exactly
Batch / lot numberWhich production run was testedSame number as on your vial
Identity (MS)Whether the molecule is the right oneObserved mass matches theoretical; “conforms”
Purity (HPLC)Share of peptide-related signal in the main peakHigh %, with chromatogram attached
ContentHow much peptide is actually presentClose to the label claim
Endotoxin / metalsNon-peptide contaminantsBelow stated limits, with LOQ shown
Lab, date, signatureWho tested it and whenNamed lab, recent date, authorized signature

A sample COA, annotated

Here is what a complete COA looks like in practice. This is a fictional example for illustration only. It is not a real product, lot or laboratory.

Certificate of AnalysisExample Peptide-X · 10 mg
Sample only
Lot / batch
EX-26-0117
Testing lab
Sample Analytical Labs (independent) · Report #SAL-44821
Manufacture date
2026-02-10
Test date
2026-03-02
Appearance
White lyophilized powder Conforms
Purity (HPLC, 220 nm)
99.1% Pass
Identity (ESI-MS)
Observed 1419.6 Da · expected 1419.5 Da Conforms
Net peptide content
84.3%
Quantity per vial
10.2 mg
Endotoxin (LAL)
<0.5 EU/mg
Water content
4.1%

What to notice:

  • The lot number is the anchor. EX-26-0117 has to match the vial, or the rest of the document doesn’t apply to your material.
  • Identity is within 0.1 Da. The observed and expected masses agree, so the main component is the right molecule.
  • Purity and net peptide content are different numbers. 99.1% purity alongside 84.3% net peptide content is normal, because water and counter-ions make up part of the powder’s mass.
  • The quantity is slightly above the label claim (10.2 mg vs 10 mg), which is typical of a well-filled vial.
  • The testing lab and report number are named, so the report can be verified with the lab.

1. Batch number — the critical field

A COA only describes one batch. The lot number printed on it must match the number on your vial or packaging. A COA with no batch number, a different one, or one that claims to cover several batches tells you nothing about the material in your hands.

✓

At After Hours Labs every COA is batch-matched: the lot number on your vial corresponds to the COA you can look up on our COA page.

2. Identity: is it the right molecule?

Identity is confirmed by mass spectrometry (MS or LC-MS). The instrument measures the molecular weight of what is in the sample, and that is compared with the theoretical molecular weight of the target peptide. A good COA shows both values and a clear “conforms” or “pass”.

!

A sample can be “99% pure” of the wrong compound. Purity without an identity result is an incomplete COA.

3. Purity by HPLC

Purity is measured by high-performance liquid chromatography, usually with UV detection around 214–220 nm, where the peptide bond absorbs. The result is the area of the main peak as a percentage of all peptide-related peaks. It captures synthesis by-products and degradation products — but not water, counter-ions such as acetate or TFA, endotoxins or heavy metals. We go deeper in HPLC Purity Explained.

4. Read the chromatogram, not just the number

Good COAs include the chromatogram itself. Look for one tall, sharp main peak at a consistent retention time and a flat baseline. Shoulders on the main peak, split peaks or a scatter of smaller peaks point to impurities or degradation. A clean blank run shows the instrument was clear before the sample.

5. Content, endotoxin and heavy metals

Some COAs also report content — the measured amount of peptide, estimated against a reference standard. Purity tells you how clean the peptide is; content tells you how much is there. Depending on the application, labs may test for bacterial endotoxins (EU/mg) and heavy metals (ppm). When a result says “not detected”, check the limit of quantification — it only means the level was below what the method can see.

COA red flags

  • No batch number, or one that doesn’t match the vial
  • Vendor or product name that doesn’t match what you bought
  • Purity reported with no identity (mass spec) result
  • No chromatogram or raw data
  • No testing lab named, no date, no signature
  • One COA covering several batches
  • Signs of editing — mismatched fonts, altered dates, a document that can’t be traced back to a lab

Check your batch

DocumentationCOA lookupFind the batch-matched COA for your lot.View →GuideHPLC purity explainedWhat the purity percentage includes — and leaves out.View →CatalogResearch peptidesEvery lot third-party HPLC tested.View →

All After Hours Labs products are sold strictly for qualified in-vitro laboratory research. They are not for human or veterinary use, and nothing in this article is medical advice or a recommendation for use in people or animals.