What a COA is
A Certificate of Analysis is the analytical record for a single manufactured lot. It is produced by a laboratory after the batch is synthesized and lyophilized, and it travels with that lot for its entire life. Reading one well is the difference between trusting a number and verifying it.
Every COA opens with the sample identity and the lot number. Before anything else, confirm that lot number matches the one printed on the vial — a COA for a different lot tells you nothing about the material in front of you.
Identity, purity, and the chromatogram
Identity confirms the molecule is what the label says. It is typically established by mass spectrometry: the measured mass is compared to the calculated mass for the sequence, and a match within tolerance is the evidence the sequence is correct.
Purity is the proportion of the target compound relative to related impurities, measured by reverse-phase HPLC and reported at a fixed detection wavelength. A single dominant, well-resolved peak with small impurities is what a high-purity result should look like. A purity percentage without a chromatogram is a claim; with one, it is a measurement.
Water content and endotoxin
Water content (often by Karl Fischer) tells you how much residual moisture is in the lyophilizate — relevant to stability. Endotoxin (by LAL, USP <85>) is a bacterial-contamination measure some vendors report. Not every COA includes every test; read which parameters were actually measured rather than assuming.