A certificate of analysis typically reports a single purity percentage, generated from an underlying mass spectrometry run that produces far more detailed data than that one number conveys. Understanding roughly what that underlying report shows is useful even for a buyer who will never run the test themselves.
What Mass Spec Is Actually Measuring
Mass spectrometry identifies molecules by the mass-to-charge ratio of their ionized fragments, producing a spectrum of peaks; for a peptide, the dominant peak should correspond to the expected molecular weight of the intended compound.
“The purity number is the headline. The spectrum underneath it is the actual evidence — and most COAs never show it.”
— an analytical chemist who reviews third-party lab reports for this newsletterWhat the Purity Percentage Summarizes
The reported purity figure is typically derived from the relative size of the main peak against other peaks in the spectrum (impurities, degradation products, or synthesis byproducts) — a single number standing in for a more complex underlying picture.
Why the Underlying Spectrum Can Matter More Than the Number
A COA that provides only the final percentage, with no access to the underlying spectrum or the method used to generate it, gives a buyer no way to verify how that number was actually calculated — and no way to catch whether the dominant peak is even the expected molecule.