Reference · Updated 11/4/2024
What HPLC purity means
How reverse-phase HPLC measures peptide purity, what a chromatogram peak actually represents, and the limits of a single purity percentage.
"HPLC purity" is the single most cited number on a peptide Certificate of Analysis, yet it is often reported without any explanation of what the underlying test measures or where its limits lie. High-performance liquid chromatography separates the components of a sample by physical and chemical properties, and the purity figure is a calculation performed on the resulting chromatogram — not a direct measurement of chemical composition.
How reverse-phase separation works
In reverse-phase HPLC, the workhorse method for peptide analysis, a liquid sample is pumped under high pressure through a column packed with silica particles bonded to hydrophobic carbon chains (commonly C4, C8 or C18). Components in the sample partition between the mobile phase (a water/acetonitrile gradient, typically with a trifluoroacetic acid or formic acid modifier) and the stationary hydrophobic phase.
More hydrophobic molecules interact more strongly with the column packing and elute later; more polar or smaller fragments elute earlier. As each component exits the column, it passes through a UV detector, most often set at 210–220 nm where the amide bond of the peptide backbone absorbs strongly, and the instrument records absorbance against time as a chromatogram.
What the purity percentage actually calculates
The reported purity is an area-percent calculation: the area under the main peak divided by the summed area of every peak (including minor ones) detected in the run, multiplied by one hundred. This is a relative measure among detected species, not an absolute measure of chemical purity against every possible impurity in existence.
Because it is a UV-absorbance-based calculation, it will not detect compounds that do not absorb at the chosen wavelength, and it says nothing about identity — a perfectly sharp, 99% pure peak could, in principle, be a single-component impurity that happens to co-elute cleanly if it were not cross-checked against mass spectrometry.
Reading peak shape, not just the number
A published chromatogram lets you evaluate more than the number in isolation. A narrow, symmetric peak with a flat baseline before and after indicates a well-resolved run and a homogeneous eluting species. Peak fronting or tailing, shoulder peaks close to the main peak, or an elevated, noisy baseline all reduce confidence in the reported figure even when the calculated percentage looks high.
Truncated sequences (peptides missing one or more residues from incomplete coupling during synthesis) and deletion or oxidation products typically appear as separate, smaller peaks near the main peak, distinguishable from the target compound by retention time.
Gradient method and column matter
The exact gradient (the rate at which acetonitrile concentration increases over the run) and the column chemistry used will shift retention times and resolution. A well-documented COA lists the column type and gradient conditions, which allows a comparison between two labs' results on the same batch — an important cross-check when third-party verification is being performed independently of the manufacturer's own report.