HPLC purity explained
Written and maintained by the AVERON Research Desk · Last reviewed September 2026
Almost every peptide lab report includes a purity figure, and almost every seller's advertising quotes one. It is worth understanding what that number actually represents, because it is narrower than it sounds.
What HPLC is
HPLC stands for high-performance liquid chromatography. A small amount of the sample is dissolved and pushed under pressure through a narrow column packed with fine particles. Different molecules interact with the column differently, so they travel through it at different speeds and come out at different times. A detector at the end records each molecule as it emerges, producing a graph called a chromatogram.
Reading a chromatogram
On a chromatogram, each distinct substance appears as a peak. The largest peak should be the target compound. Smaller peaks are related impurities: shorter or damaged versions of the peptide, leftovers from synthesis, or breakdown products.
How the purity percentage is calculated
Purity is usually reported as the area of the main peak divided by the total area of all peaks, expressed as a percentage. A result of 98% means that, of everything the detector saw, 98% of the signal came from the target compound.
What that number does not include
Material the detector does not see. HPLC detectors usually measure light absorbed at a particular wavelength. Water and many salts produce no signal, so they are not counted at all.
Net peptide content. Lyophilized peptide powders always contain some water and counter-ions such as acetate or trifluoroacetate left from manufacturing. A powder can therefore be highly pure by HPLC while only a portion of its total weight is actually peptide. That is why a separate content or amount measurement matters.
Identity. HPLC tells you how much of the sample is one substance. It does not, by itself, prove that substance is the right one. That is the job of mass spectrometry.
Sterility and contamination. A purity figure says nothing about bacteria, endotoxin or heavy metals.
Why methods matter
The purity figure depends on how the test was run: the column, the solvent conditions, the detection wavelength, and how the peaks were integrated. Two laboratories can report slightly different figures for the same sample. This is one reason an independent laboratory's result, produced with a consistent method across many samples, carries more weight than a figure a seller has produced for its own product.
Why blends usually have no purity figure
When several peptides are combined in one vial, their impurities overlap on the chromatogram and cannot be cleanly attributed to one compound or another. Laboratories therefore usually report the identity and amount of each component, and not a purity figure.