A clear guide to reading the quantity value on a COA and applying it to bench calculations
A Certificate of Analysis (COA) is the official record of a peptide’s measured properties for a specific lot. The quantity result is the measured amount of material that passed the analytical test, usually expressed in milligrams or as a percentage of the declared weight. This figure tells you how much of the peptide you actually have in the vial, taking into account any impurities or moisture that the assay detected.
The quantity is reported with a unit and a tolerance range, for example 98.5 % ±0.2 % or 9.8 mg ±0.05 mg. The percentage relates to the theoretical weight of the peptide based on its sequence, while the absolute milligram value reflects the mass you will weigh out. Use the unit that matches your downstream calculations; if the COA gives a percentage, convert it to mass using the nominal lot weight.
When planning an experiment, base your weighing calculations on the COA quantity rather than the label claim. For instance, if you need 1 µmol of a peptide that is listed as 100 mg but the COA shows 98 % purity, the actual usable mass is 98 mg. Adjust your measured weight accordingly to achieve the correct molar amount.
Common mistakes include ignoring the tolerance range, overlooking a moisture correction, or mixing up percentage and absolute values. Always double-check the unit, confirm the lot number matches your vial, and, if high precision is required, re-weigh the material before use.
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