How to read a certificate of analysis

What each line on a certificate means, from lot number to chromatogram.

The header

Every certificate starts with the certificate number, the product name and the lot number printed on the vial. The lot number ties the paper to the physical material: check that it matches your label before you rely on any result.

Identity

Identity confirms the vial contains the compound it claims to. Mass spectrometry measures the molecular mass of the main component and compares it with the theoretical mass calculated from the sequence. A match within the instrument's tolerance is reported as Conforms.

Purity

Purity is measured by reverse-phase HPLC. The sample is separated on a column and detected by UV absorbance, usually at 214 nm where peptide bonds absorb. Purity is the main peak's area as a percentage of all detected peaks. A result of 99.4% means 99.4% of the UV-detectable material eluted as the target compound.

The chromatogram

The chromatogram is the trace the purity number comes from. Look for one dominant, sharp peak and small or absent side peaks. Wide or split peaks can point to degradation or closely related impurities.

What a certificate does not tell you

A purity figure is not a measure of net peptide content, which also accounts for water and counter-ions; net peptide content is typically 70 to 85%. A certificate covers the lot tested, under the method listed, on the date shown, and reports only the tests that were run.