What HPLC Purity Testing Measures (and What It Does Not)
"99% pure" is the most quoted number on a peptide listing. Here is how that number is produced by HPLC, and what it can and cannot tell a researcher.
How HPLC works
High-performance liquid chromatography pushes a dissolved sample through a packed column under pressure. Different molecules interact with the column material differently, so they exit (elute) at different times. A detector, commonly UV absorbance around 214 to 220 nm for peptides, records a signal as each component elutes.
The output is a chromatogram: a plot of detector signal against time, with a peak for each detected component.
How purity is calculated
Purity is typically reported as the area of the main peak divided by the total area of all integrated peaks, expressed as a percentage. A result of 99% means the main component accounts for 99% of the detected signal under those conditions.
What a purity number does not tell you
HPLC purity does not confirm identity on its own. A different molecule could produce a single clean peak, which is why purity is paired with mass spectrometry.
It also does not measure net peptide content. Lyophilized peptides contain counter-ions such as acetate or TFA and residual water, so the mass of peptide in a vial is less than the total mass of powder.
Impurities that do not absorb at the detection wavelength, or that co-elute with the main peak, may not be captured.
Reading a chromatogram
Look for one dominant peak, a stable baseline, and a peak table listing retention times and areas. Note the column type, mobile phase and detection wavelength if they are provided, since they affect the result.
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