How to read a peptide Certificate of Analysis

A Certificate of Analysis is the only document that connects a specific vial to a specific set of measurements. It is worth reading carefully, because two COAs that both say "99%" can describe materially different material.

Purity and content are two different numbers

The figure most suppliers put on the label is chromatographic purity, usually by RP-HPLC at 214 nm. It answers a narrow question: of everything the detector saw, what fraction was the target peak? It says nothing about how much of the vial is peptide.

Net peptide content answers the other question. A lyophilised vial also contains water, counterions and residual salts. A batch at 99% chromatographic purity can be 75–85% peptide by mass, with the remainder made up of trifluoroacetate counterion and adsorbed water. Both numbers are legitimate; reporting only the first is what makes them misleading.

• HPLC purity — area percent of the target peak. Look for the method: column, gradient, wavelength, run time.
• Net peptide content — typically by amino acid analysis or nitrogen determination. Often absent from budget COAs.
• Water content — Karl Fischer titration. Directly affects how much peptide is actually in the vial.
• Counterion — TFA or acetate, with the residual level stated.

Identity is a separate measurement from purity

Purity tells you the sample is homogeneous. It does not tell you the peak is the compound you ordered. Identity comes from mass spectrometry: the observed monoisotopic or average mass should match the theoretical mass for the sequence, usually within a fraction of a dalton. A COA with a purity figure but no MS trace has confirmed consistency without confirming what the material is.

Checks that take thirty seconds

• Does the batch number on the document match the batch number on the vial label? A COA for a different lot is not evidence about your lot.
• Is the test date present, and is it consistent with the manufacturing date? A report predating the batch it describes is a copy-paste artefact.
• Is the raw chromatogram included, or only a summary table? A purity figure without the trace behind it cannot be checked.
• Is the testing laboratory named, and is it independent of the seller?
• Are the acceptance criteria stated, or only the results? Results without criteria cannot fail.
• Is the document legible at full resolution? Deliberately low-resolution scans are common where the underlying report would not survive inspection.

Storage and reconstitution affect the numbers later

A COA describes the batch at the time of testing. Lyophilised peptides are generally stable at -20 °C in the dark, but repeated freeze-thaw cycles, exposure to humidity during weighing, and time in solution all degrade material in ways the original document cannot account for. If a batch has been open and handled, the COA describes its history, not its present state.

All materials described here are supplied for laboratory research and in-vitro use only. Nothing on this page is guidance for use in humans or animals.