How to Read a Peptide Certificate of Analysis (COA)

A certificate of analysis is the single document that tells you what is actually in a vial. Not the marketing copy, not the label claim, not the vendor's reputation. The COA. If you handle research peptides in a lab setting, reading one properly is a skill worth ten minutes to learn, because a COA you can't interpret is a COA you can't trust.

Here is how to work through one, top to bottom.

Start with the lot number

The most important line on a COA is not the purity figure. It's the lot or batch number near the top. That number must match the number printed on the vial in front of you. Purity varies between production runs, so a certificate tied to a different lot describes different material. A "representative" or "sample" COA that isn't lot-matched tells you nothing about what you're holding.

Every lot Stacy & Chad ships carries its own certificate, and the number on the paper matches the number on the vial. When you can line up those two numbers, the document starts meaning something. When you can't, treat the purity claim as unverified.

Read the HPLC purity figure and the chromatogram

High-performance liquid chromatography (HPLC) separates the compounds in a sample as they pass through a column at different speeds. A detector, usually reading ultraviolet absorbance at 214 nm (the wavelength of the peptide bond), records each component as a peak. The area under the main peak, as a percentage of total peak area, is the reported purity.

Two things to check:

• The number. For research use, ≥99% is excellent; 95–98% is standard research-grade; below 95% deserves a closer look for precision work.
• The chromatogram itself. A clean chromatogram shows one dominant peak and a flat baseline. Significant secondary peaks or a raised baseline tell a different story than the headline percentage. Always look at the trace, not just the digit.

HPLC purity reflects UV-absorbing organic impurities. It does not measure water, salts, or non-chromophore contaminants, which is why one test is never the whole picture.

Confirm identity with mass spectrometry

HPLC tells you how pure the material is. It does not tell you the material is the right compound. That job belongs to mass spectrometry. The COA lists a theoretical (expected) molecular mass and an observed mass. Those two values should match within a small tolerance. If they don't, you may have a pure sample of the wrong molecule, which purity alone would never reveal.

Read HPLC and mass spec together. Each is blind to what the other measures.

Check the endotoxin result

A thorough COA reports a bacterial endotoxin result, typically from the LAL (Limulus Amebocyte Lysate) assay, often referenced against USP <85>. This is a documentation of bacterial-endotoxin load in the lot, reported as a pass against a stated specification. It is a separate line item from purity and identity, and its presence signals a more complete analytical workup.

Verify who signed it

Finally, look at the lab. A credible COA names the testing laboratory, carries a date, and comes from a third party rather than the seller's own bench. An undated, unsigned, or in-house-only certificate carries less weight. Where possible, the named lab should be independently findable.

Read in this order (lot, HPLC number, chromatogram, mass spec, endotoxin, lab) and a COA stops being a formality and becomes what it's meant to be: evidence.