Reading a Certificate of Analysis Properly
Which fields on a COA carry information, which are decoration, and the three that most people never check.
The batch number is the whole document
A certificate describes one production run. If the batch number on the certificate does not match the batch printed on the vial in your hand, the document tells you about someone else's product. This is the most common failure and the easiest to check. A certificate with no batch number at all is not a certificate; it is marketing.
Identity comes before purity
Purity describes how much of what is present is the main component. Identity confirms that the main component is the sequence you ordered. Mass spectrometry is what establishes identity, by measuring molecular weight against the theoretical value for the sequence. A high purity figure with no identity confirmation means the vial contains one thing in high proportion, without establishing what that thing is.
The chromatogram carries more than the number
The purity percentage is a summary of the trace. The trace itself shows the shape. One dominant, symmetrical peak is what you want. Shoulders on the main peak suggest closely-related impurities such as deletion sequences. Multiple significant peaks indicate a mixture. A flat baseline with a single spike and no scale is not a real chromatogram, and reproduced low-resolution images of traces are a known shortcut.
Method and date are not filler
The method line states how the figure was produced - typically RP-HPLC with a stated column, gradient and detection wavelength. Detection at 214nm picks up the peptide backbone; 280nm only sees aromatic residues and will read differently. The test date matters because stability data is time-dependent: a certificate from two years ago describes the material as it was then.
The three fields most people skip
Water content, residual solvent and counter-ion. Together these account for the difference between label weight and actual peptide, and all three are routinely present on a complete certificate and routinely ignored. If you have read the purity figure and stopped, you have read roughly half the document.
What a certificate cannot tell you
It describes the batch at the point of testing. It cannot tell you how the material was stored or shipped afterwards, and a perfect certificate does not survive a temperature excursion in transit. This is why arrival condition matters independently of paperwork, and why the two checks are not substitutes for one another.