How to read a Certificate of Analysis (COA) step by step
A Certificate of Analysis (COA) is the document a laboratory issues to describe the identity and purity of a specific batch of research material. Reading one properly means confirming three things: that the compound is what it claims to be, that its purity meets the stated threshold, and that the document corresponds to the batch you actually have in hand.
What information a COA contains
A serious COA is not a pretty label: it is a report of analytical tests. Before you look at the numbers, locate the fields that identify the document. Without them, the results can't be traced to anything.
- Compound name and, where applicable, its sequence or formula.
- Batch/lot number — the link between the paper and the vial.
- Date of analysis and date of issue.
- The laboratory that ran the tests, ideally an independent third party.
- Methods used (HPLC, mass spectrometry, etc.).
If the COA has no batch number or doesn't name the method, treat it as incomplete. A certificate with no traceability could belong to any production run.
Step 1: verify identity by mass
Mass spectrometry (MS, often LC-MS or MALDI-TOF) confirms that the molecule weighs what it should. The COA reports a theoretical mass (calculated from the sequence) and an observed mass (measured). The two should match within a narrow tolerance.
For peptides, the observed mass is usually expressed as the protonated ion [M+H]⁺ or as multiply-charged ions [M+2H]²⁺. Don't be alarmed if the number isn't identical to the theoretical mass: what matters is that the difference is minimal and that the main peak dominates the spectrum.
Step 2: interpret HPLC purity
High-performance liquid chromatography (HPLC) is the core assay. It separates the components of the sample and measures what percentage of the total corresponds to the compound of interest. That percentage is the purity.
In this market the reference standard is ≥99%. When reading the chromatogram, check two things:
- The main peak: it should be tall, symmetrical, and well defined. A value of 99.2% means that 99.2% of the area under the curve belongs to your compound.
- The secondary peaks: these represent impurities. Fewer and smaller peaks indicate cleaner material.
Be wary of a purity figure quoted without the chromatogram to back it up. The number alone is a claim; the graph is the evidence. We go deeper into this in what HPLC purity is.
Step 3: review the complementary tests
Depending on the compound and the laboratory, the COA may include additional tests that give context about handling the material:
| COA field | What it measures | What to look for |
|---|---|---|
| Purity (HPLC) | % of the compound vs. impurities | ≥99%, symmetrical peak |
| Identity (MS) | Observed molecular mass | Matches the theoretical mass |
| Content / net weight | Amount of peptide in the vial | Close to the nominal value |
| Appearance | Physical aspect | Lyophilized powder, no residue |
| Water content | Residual moisture | Low, per specification |
Not every COA reports each row, and that doesn't invalidate the document. HPLC purity and MS identity are the minimum you should expect.
Warning signs
- No batch number, or a batch that doesn't match the vial.
- Purity with no chromatogram attached.
- No laboratory named, or tests run by the seller itself with no outside verification.
- Missing dates, or dates later than the batch you received.
- Low-resolution images where you can't read the axes or the retention times.
Quick checklist before using a batch
When you receive a COA, confirm in order: that the batch number matches the vial, that the MS identity corresponds to the compound, that the HPLC purity meets the threshold with its chromatogram visible, and that the laboratory and the dates are present. If all four points check out, the certificate is usable as a reference for your research.
If you're just getting started, the guide What are peptides? covers the basic concepts, and the rest of the resources live in Learn. Every batch of compounds such as tirzepatide comes with its own per-batch COA. Once the batch is verified, the next step is usually reconstitution: use the reconstitution calculator to work out volumes and concentrations without errors.