Reading a peptide COA: what each line actually proves
A certificate of analysis is a claim sheet, not a guarantee. Here is what HPLC area%, mass spec identity, endotoxin and sterility lines each do and do not establish — and which questions to ask when a line is missing.
Every vendor in this market publishes a certificate of analysis. Very few explain what the certificate can and cannot support. This post walks the four lines that matter on a research peptide COA, in the order a reviewer should read them.
Identity: mass spectrometry
The first question is not purity — it is whether the vial contains the molecule named on the label. Electrospray mass spectrometry answers this: the observed ion envelope is matched against the theoretical isotopic distribution for the stated sequence. A COA that reports purity without an identity method is reporting a property of an unverified substance.
What it does not prove: enantiomeric purity, sequence errors that preserve mass (rare but possible in isobaric substitutions), or the absence of non-ionizing contaminants.
Purity: HPLC area percent
Reversed-phase HPLC with UV detection at 214–220 nm separates the main peak from related substances. Area percent is the ratio of the main peak's integrated area to all detected peak areas. It is a relative measure of what the detector sees under those conditions — not an absolute mass fraction, and blind to anything that co-elutes or does not absorb at the chosen wavelength.
A 99.2% area result is meaningful only alongside the chromatogram's detection threshold and the method's resolution. Ask for the chromatogram, not just the number.
Endotoxin and sterility
For material handled in a laboratory, bacterial endotoxin (LAL or recombinant-factor assays, reported in EU/mL) and sterility (membrane filtration or direct inoculation) describe bioburden control during fill and finish. They are process controls, not potency measures, and they are the lines most often absent from marketplace COAs.
The line that matters most: the batch code
None of the above is checkable unless the certificate is bound to the exact vial in your hand. A batch code printed on the label, repeated on the certificate, and resolvable to a dated document is what turns a PDF into evidence. If the certificate cannot be tied to a batch, it is marketing collateral.