A peptide certificate of analysis (COA) packs a lot of decisions into one page. Read top to bottom without a map and it is easy to fixate on the purity percentage and miss the numbers that actually govern whether the material is fit for your use. This walkthrough goes line by line through a typical peptide COA and explains what each entry is really telling you.
The header block
The top of the COA identifies the material: peptide name, sequence (often in one-letter or three-letter code), molecular formula, theoretical molecular weight, lot or batch number, and appearance. Check the sequence and molecular weight against what you ordered before reading anything else; everything below assumes this is the right molecule.
Purity by HPLC
The purity line almost always reports reversed-phase HPLC area purity at a stated wavelength (commonly 214 nm). It tells you the target peak’s share of total peak area, a measure of how clean the chromatogram is. Look for the method and wavelength beside the number; a purity figure with neither is not verifiable.
Net peptide content
This is the entry most people skip and the one dosing depends on. Net peptide content (by amino acid analysis) is the fraction of the powder that is actually peptide by mass, typically well below the HPLC purity figure, because water and counter-ions occupy real mass. If the COA has purity but no net peptide content, you cannot accurately calculate a dose.
Water and counter-ion
Water content (Karl Fischer) and counter-ion content (usually trifluoroacetate or acetate) explain the gap between HPLC purity and net peptide content. A complete COA reports both, so the mass balance closes.
Identity by mass spectrometry
The COA should confirm identity by mass spectrometry: the measured mass matched against the theoretical mass. This is what proves the peak on the chromatogram is your peptide and not an impurity of similar retention time.
Putting it together
Read the COA as a set of linked answers: identity (right molecule?), purity (clean chromatogram?), net content (how much peptide by mass?), and water/counter-ion (what makes up the rest?). A COA that answers all four, each with a named method, is one you can act on with confidence.
