“99% purity” is printed on a lot of labels. But purity is not an opinion — it is a measurement produced by specific analytical instruments. Understanding how that number is generated helps you tell a meaningful figure from a marketing one.
HPLC: separating the mixture
High-Performance Liquid Chromatography pushes a dissolved sample through a column packed with fine particles. Different molecules travel through the column at different speeds depending on how strongly they interact with the packing material. As each component exits, a detector records it as a peak on a chromatogram, plotted against time.
For peptides, reversed-phase HPLC (RP-HPLC) is the workhorse. The purity percentage is the area of the main peak divided by the total area of all peaks, expressed as a percentage. A clean, well-resolved single peak sitting on a flat baseline is what high purity looks like. Shoulders, secondary peaks, or a noisy baseline all indicate impurities — truncated sequences, deletion products, or residual reagents.
Mass spectrometry: confirming identity
HPLC tells you how pure, but not what. Mass spectrometry answers the identity question. The instrument ionizes the molecule and measures its mass-to-charge ratio with high precision. By comparing the measured molecular weight to the theoretical weight of the intended sequence, MS confirms you have the correct peptide and not a similar-mass impostor.
Together, the two are complementary: HPLC quantifies purity, MS verifies identity. A COA that includes both gives you a far more complete picture than either alone.
Why the raw traces matter
A number is easy to type. A chromatogram and a mass spectrum are much harder to fake convincingly, and they let an informed reader judge the result independently. When you can see the actual peak shape and the actual measured mass, “99%” stops being a claim and becomes something you can verify with your own eyes.
The limits of a single test
Even excellent HPLC and MS do not capture everything — they say little about endotoxin levels, residual solvents, or counter-ion content, which is why a full COA reports those separately. Purity is necessary but not sufficient; treat it as one pillar of quality, alongside identity confirmation and the supporting analytical data.
For research use only. Not for human or veterinary use.