Peptide purity test: what HPLC purity means

How HPLC separates a peptide sample

In reversed-phase HPLC, the most common method for peptides, a dissolved sample is pumped through a column packed with a water-repelling material. Molecules that stick to it more leave the column later. A UV detector at the outlet, often set around 214 or 220 nm where peptide bonds absorb, draws a peak each time something passes.

The plot of detector signal against time is the chromatogram. A clean sample shows one tall main peak and a few small ones. Each small peak is something else: a peptide missing an amino acid, a modified version, or leftover material from synthesis.

What the purity percentage means

The purity figure is the main peak's area divided by the total area of all peaks, times 100. A report reading 99.2% says that, under that method and wavelength, 99.2% of what the detector saw came out at the target peptide's position.

It is a relative number. Things the detector can't see at that wavelength, such as water, salts or counter-ions left from synthesis, are not in it. That is why purity and net content are separate results.

What HPLC purity does not tell you

HPLC alone does not prove identity. A peak at the expected time is consistent with the right peptide but doesn't confirm it; mass spectrometry does that. It also doesn't say how much peptide is in the vial, and it says nothing about endotoxin, sterility or heavy metals, which are separate tests.

What to look for on the report

A number, not a word: a percentage with the method named (for example RP-HPLC) and ideally the chromatogram itself. A lot number that matches the vial. A test date. If the report says only "high purity" or "pharmaceutical grade", there is no result to check.

Common questions