When a peptide arrives with a Certificate of Analysis reading ≥99% purity by HPLC, most researchers treat that number as a proxy for quality. It isn’t. It’s a useful signal — but purity and potency are two orthogonal measurements, and confusing them is the most common reason your assay results don’t reproduce.

What HPLC purity actually measures

Reversed-phase HPLC separates a sample by hydrophobicity. The integrator reports the area under the main peak as a percentage of the total chromatogram area. At ≥99%, you’re being told that less than 1% of the sample (by mass, approximately) has a retention time different from the target compound.

That is not the same as saying the main peak is what you think it is. HPLC cannot distinguish between a target peptide and a peptide that differs by a single amino acid but happens to co-elute. It also can’t distinguish target peptide from a deamidated or truncated variant that elutes at the same spot.

What mass spec adds

LC-MS (or at minimum a confirmatory MALDI-TOF) closes that gap. Mass spec verifies identity: the observed monoisotopic or average mass has to match the theoretical value for your sequence within a reasonable tolerance (≤10 ppm on high-res instruments, ≤0.1 Da on lower-res ones). A peak at the right HPLC retention time with the wrong mass is not your compound.

A well-scoped CoA reports both purity (HPLC area %) and identity (observed vs. theoretical mass). Vitatide should publish those fields only where uploaded supplier or lab source documents support them.

Where potency comes in

Purity and identity still don’t tell you whether the compound is biologically active. Potency — usually an in-vitro binding, receptor- activation, or functional assay — is the one measurement that closes the loop. It’s also the one nobody can put on a CoA for you, because potency is assay-specific.

What you can do is run a reference standard side-by-side with your received material on your own assay, and check that the dose- response curves superimpose. If they don’t — even when HPLC purity and mass spec look clean — something has gone wrong. Common culprits:

  • Partial deamidation of asparagine / glutamine residues during synthesis or storage. This shifts purity barely but destroys binding.
  • Oxidation of methionine. Same problem; mass shifts by +16 Da, often below the resolution of a routine MALDI check.
  • Wrong disulfide bridge pairing. Intramolecular isomers are identical by mass and usually by HPLC.
  • Impurities that have agonist or antagonist activity. The main peak is fine; the 0.8% minor peak is half your signal.

Practical implications

If you’re starting a new compound or a new supplier, do one potency check against a reference standard. After that, tracking lot- to-lot HPLC purity and mass is useful when the supplier’s documentation and process are consistent. Vitatide’s public product pages should stay evidence-scoped: review available documentation before listing, publish COA-backed fields, and avoid using verified or tested to imply evidence that is not present in the source documents.

Bottom line: purity and identity are necessary, not sufficient. Spend the extra hour on a potency check the first time; after that, trust your CoA.