Every research peptide you buy should come with a Certificate of Analysis (COA). It is the single most important document for confirming that the compound in the vial is what the label says, at the purity your experiment needs. Yet many researchers skim it or never open it at all. This guide walks through each section of a typical peptide COA so you can read one with confidence and spot the warning signs of a low-quality supplier.
In this guide: what a COA is, how HPLC purity is measured, what mass spectrometry confirms, which other tests matter, and the red flags that should make you look elsewhere.
What Is a Certificate of Analysis?
A COA is a lab report tied to a specific production batch (lot). It records the analytical tests run on that batch and the results. A trustworthy COA always lists the compound name and sequence, the lot number (which should match the number on your vial), the test date, the testing laboratory, and the methods and results for each test. If any of these are missing, treat the document with caution.
HPLC: How Purity Is Measured
High-Performance Liquid Chromatography (HPLC) separates the components of a sample as they pass through a column. Each component exits at a characteristic time and shows up as a peak on a chromatogram. Purity is calculated as the area of the main peak divided by the total area of all peaks.
What to look for:
- A dominant single peak. One tall, sharp peak with only small minor peaks is what you want.
- The purity percentage. For most research applications, look for 98% or higher. Values of 99%+ indicate excellent synthesis and purification.
- The actual chromatogram. A COA that states a purity number without showing the trace is far less trustworthy.
Mass Spectrometry: Confirming Identity
HPLC tells you how pure a sample is, but not what it is. Mass spectrometry (MS, often LC-MS or MALDI-TOF) measures molecular weight. Compare the observed mass on the report to the theoretical molecular weight of the peptide sequence. They should match closely, typically within about 1 Dalton. A mismatch suggests a wrong sequence, truncation, or a different compound entirely.
Other Tests Worth Checking
- Peptide content (net peptide). Lyophilised powder contains counter-ions and residual water as well as peptide. Net content tells you how much of the weight is the peptide itself.
- Endotoxin testing (LAL). Important for cell-culture and in-vivo research models where bacterial endotoxins can skew results.
- Appearance and solubility. Lyophilised peptides should be a white to off-white powder or cake.
- Counter-ion. Acetate vs. TFA salt forms can matter for some assays.
Red Flags on a Peptide COA
- No lot number, or a lot number that does not match your vial.
- A purity figure with no chromatogram or mass spectrum attached.
- The same COA reused across many products or many months.
- No named laboratory, or test dates that are years old.
- Observed mass that does not match the theoretical weight.
Why Third-Party Testing Matters
In-house testing is useful, but independent third-party verification removes any conflict of interest. When a supplier sends batches to an outside lab and publishes the results, you get an objective check on identity and purity. That transparency is the clearest signal that a supplier stands behind its product. For more on handling your compounds once they arrive, see our peptide storage guide, or start with our complete guide to research peptides.
Buy Research Peptides With Verified Purity
Every compound in our catalogue, from Tirzepatide and Retatrutide to Kisspeptin-10, is supplied with batch-specific analytical data so you can verify what you are working with before your research begins.
Shop Research PeptidesAll products are sold strictly for laboratory research use only. They are not intended for human or animal consumption, or for diagnostic or therapeutic use. Nothing in this article is medical advice.
