How to read a peptide HPLC report

What the chart actually shows

An HPLC chromatogram plots detector response on the vertical axis against time on the horizontal axis. The sample is pushed through a column; different components leave the column at different times and produce a peak as they pass the detector.

For peptides the detector is usually a UV detector monitoring a wavelength at which the peptide bond absorbs. The height and area of a peak reflect how much absorbing material passed the detector at that moment.

Retention time

Retention time is how long a component took to travel through the column under the stated method. It is characteristic of a compound under a specific method, not an absolute property — change the column, solvent gradient or temperature and the retention time changes.

This is why a retention time is only meaningful alongside the method conditions printed on the report, and why comparing retention times between two laboratories running different methods is not informative.

The integration table

Below or beside the chromatogram most reports include an integration table listing each detected peak, its retention time, its area, and its area as a percentage of total peak area.

The main peak's area percent is what usually gets quoted as the purity figure. Smaller peaks are related substances: synthesis by-products, deletion sequences, oxidation products, residual reagents or degradation products.

Method conditions worth checking

• Column type and dimensions — reversed-phase C18 is common for peptides.
• Mobile phase and gradient — the solvent mixture and how it changes over the run.
• Detection wavelength — commonly in the low-UV region for peptide bonds.
• Injection volume and sample concentration.
• Run time — a peak eluting after the run ends is never seen.

Common things that look wrong but are not

A large early peak near the start of the run is often the solvent front — unretained material passing straight through the column. It is normally excluded from integration.

A sloping baseline is typical of gradient runs, because the absorbance of the mobile phase itself changes as its composition changes.

Small peaks are expected. Synthesised peptides are not single-substance materials, and a report showing zero minor peaks is more unusual than one showing several.

What a chromatogram cannot tell you

HPLC separates and quantifies; it does not identify. A peak at the expected retention time is consistent with the expected compound but does not confirm it. Confirming identity requires a method that measures a structural property, which in practice means mass spectrometry.