Reading a peptide COA: purity, mass, endotoxin

Purity is a percentage of what

HPLC purity is usually reported as area percentage at a specific wavelength. It means the target peak is that share of the total integrated area, not that the vial contains that share of mass.

A 98% figure with a large unassigned peak at a different retention time is worse than a 96% figure with one small shoulder. Ask for the chromatogram, not the number alone.

Mass confirmation rules out the wrong sequence

A peptide can be pure and still be the wrong molecule. Mass spectrometry confirms the observed mass matches the calculated mass for the intended sequence, which is the cheaper of the two failure modes to catch and the more expensive to discover late.

For sequences above ten residues this check should be routine, not optional.

Endotoxin matters if you repackage

Endotoxin is measured in EU per milligram or per vial. A buyer who repackages into smaller vials is multiplying the number of containers any contamination would reach, so a screen before repackaging is cheap insurance.

The test is not free, which is why lines sold for repackaging are the ones where it should be requested explicitly.

What is often missing

Water content, residual solvents and sterility are frequently absent from a commercial peptide COA. Absence is not proof of a problem, but a buyer paying for pharmaceutical-adjacent standards should know which boxes are unticked.

The honest position is to state what was tested. A certificate that tests three things and says so is more trustworthy than one that implies everything was covered.

Research use only. Not for human or veterinary use.