How to actually read a certificate of analysis
A COA is only useful if you know which numbers matter. Here is what each section on a peptide certificate is telling you — and the three things people most often misread.
Read a certificate in this order
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Batch number
Match it against the number printed on your vial label. No match, no certificate.
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Identity — LC-MS
Measured mass against the mass calculated from the sequence. This is the test that can fail hard.
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Purity — HPLC-UV
Target peak area over total integrated area. Check the wavelength: 214 nm and 280 nm are different measurements.
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Net content — gravimetric
Fill weight against label claim. This is what lets you calculate a real concentration on reconstitution.
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Test date
A certificate from two years ago describes a batch you almost certainly do not have.
The batch number comes first for a reason: every other number on the page is only about your vial if that one matches.
Most buyers glance at the purity percentage and stop there. That single number is the least interesting part of a well-constructed certificate.
## Purity is a relative measurement
An HPLC-UV purity figure is the area of the target peak divided by the total integrated peak area, expressed as a percentage. It tells you what fraction of the UV-absorbing material is your peptide. It does not tell you how much peptide is in the vial — counter-ions, residual solvent and water are largely invisible to this method.
## Identity is the number that can fail hard
LC-MS identity confirmation compares the measured mass against the mass calculated from the sequence. Purity can be 99% and the material still be the wrong compound. A certificate without identity confirmation is describing something pure, not something correct.
## Net content closes the loop
Gravimetric net content verifies the fill weight against the label claim. Together with purity, it is what lets you calculate an actual concentration when you reconstitute.
## Three common misreadings
First, treating a representative COA as a batch COA — always match the batch number on your vial. Second, assuming a wavelength: purity at 214 nm and at 280 nm are different measurements and are not interchangeable. Third, ignoring the test date; a certificate from two years ago describes a batch you almost certainly do not have.
