The single most useful skill when buying research peptides is knowing how to read a Certificate of Analysis. A COA is the lab report that tells you what is actually in the vial, and learning to read one turns you from a hopeful buyer into an informed one. The good news: it is simpler than it looks, and a few key numbers tell you almost everything.
What a COA is
A Certificate of Analysis is a document produced by an analytical laboratory that tests a specific batch of a peptide and reports the results. Crucially, a good COA is tied to a specific batch or lot number, not just to the product in general. It confirms two things above all: that the vial contains the right molecule (identity), and that the molecule is pure (purity). Everything else on the page supports those two answers.
The two numbers that matter most
Purity (from HPLC). The purity figure, usually expressed as a percentage from high-performance liquid chromatography (HPLC), is the headline. It tells you what fraction of the material is the intended peptide versus everything else. For research peptides, you want to see 98% or higher, and the best suppliers report 99%+. A purity figure noticeably below that means a meaningful portion of the vial is something other than what you came for.
Identity (from mass spectrometry). Purity tells you how clean the material is, but identity tells you whether it is the right material at all. Mass spectrometry (often shown as MS or LC-MS) measures the molecular weight of the peptide and compares it to the known, expected weight. When the measured mass matches the theoretical mass, you have confirmation that the vial holds the actual compound named on the label, not a cheaper substitute.
The rest of the page
A complete COA usually includes a few more useful items: the product name and sequence, the batch or lot number, the test date, the appearance (typically a white lyophilized powder), and sometimes net peptide content and water or acetate content. You may also see the actual HPLC chromatogram and the mass spec trace, the visual graphs behind the numbers. Seeing those graphs is a good sign, because it means the supplier is showing its work rather than just asserting a figure.
How to spot a weak or fake COA
A few red flags separate a trustworthy COA from a meaningless one. Watch for a missing or mismatched batch number, since a COA that does not match the specific lot you are buying tells you nothing about your vial. Be cautious of suspiciously round or vague numbers with no chromatogram to back them up. And value third-party testing: a report from an independent lab carries more weight than one a seller could have produced in-house. The strongest setup is a recent, batch-specific COA from a third-party lab, complete with the HPLC and MS data.
Quick facts
| What to check | What it tells you | What to look for |
|---|---|---|
| Purity (HPLC) | How clean the material is | 98% or higher, ideally 99%+ |
| Identity (MS) | Whether it is the right molecule | Measured mass matches expected mass |
| Batch number | That the COA matches your vial | Present and matching the lot |
| Third-party lab | Independence of the testing | Named external laboratory |
| Chromatogram | The data behind the numbers | Actual HPLC and MS graphs shown |
The bottom line
A COA is your proof of what you are buying, and reading one comes down to two questions: is it the right molecule (identity, confirmed by mass spec), and is it clean (purity, from HPLC at 98% or higher)? Every batch at Marek Pharma ships with a Certificate of Analysis so you can check both before you ever open the vial. To understand the testing methods behind these numbers in more detail, see the companion guide on peptide purity testing.
For laboratory and research use only.