How to Read an HPLC Chromatogram for Peptide Purity

What a peptide HPLC trace shows

Reversed-phase HPLC separates peptides mainly by hydrophobicity. A sample is injected into a stream of aqueous solvent and flows through a C18 or similar column. An organic solvent, usually acetonitrile with 0.1 % trifluoroacetic acid, is raised gradually, and peptides leave the column in order of increasing hydrophobicity.

A detector records the signal as a function of time, and the result is a chromatogram: signal on the vertical axis, retention time on the horizontal axis. Each peak is a population of molecules that left the column together.

How to find the main peak

The main peak is normally the tallest and largest, but that is only an assumption. Note its retention time and compare it with a reference lot or with an earlier batch of the same compound. A shift of a few tenths of a minute between runs can be normal when the system differs, but the elution order of the main peak and its neighbours should be stable.

A main peak at a very different retention time from earlier batches deserves a question, because a different molecule may elute elsewhere.

Reading secondary peaks

Small peaks near the main peak are usually related substances: truncated or deleted sequences from synthesis, oxidized forms, deamidated forms or peptides with a protecting group still attached. Peaks that appear before the main peak are more polar, and peaks after it are more hydrophobic.

A peak at the very start of the run is typically unretained material such as salts, and a peak at the end can come from the gradient washout.

• Compare the chromatogram with a blank run, which shows system peaks that are not from the sample.
• A broad, humped peak suggests a mixture of closely related molecules.
• A shoulder on the main peak suggests co-eluting impurity that the integration may have merged.

How the percentage is calculated

Area normalization takes the area of the main peak and divides it by the sum of all integrated peak areas. The result depends on three choices: the wavelength, the integration threshold and the baseline. Because it uses detector response and not mass, it assumes every component responds equally at the chosen wavelength.

Components with a low response, such as peptides without aromatic residues or compounds without a chromophore, are under-represented. Components that do not elute during the run, or that co-elute with the main peak, are not counted separately.

Method conditions that matter

The same sample can give different purity values under different methods. Ask for or look for the column chemistry and dimensions, the mobile phases, the gradient, the flow rate, the column temperature, the detection wavelength and the injection amount. A steeper gradient compresses peaks and can hide neighbours, and a shallower one resolves them.

A method used for release testing should be described in a way that allows another laboratory to reproduce it, as the validation guidelines for analytical procedures expect.

What HPLC does not tell you

A single HPLC trace does not establish identity, because many different peptides can give a single clean peak. It does not measure how much peptide is in the vial, how much water or counter-ion the powder holds, or whether the amino acids are the L-isomers. It also cannot show impurities that do not absorb at the detection wavelength.

For these questions, mass spectrometry, amino acid analysis, Karl Fischer titration and chiral methods are used; the guide to comparing analytical methods lists them side by side.

A short review routine

Look at the trace first, then the table. Find the main peak and its retention time. Read the baseline for noise and drift. Count the secondary peaks and note any shoulders. Check the integration marks. Read the stated purity and see whether the trace supports it. Finally, link the HPLC result to a mass result for the same batch. If the report offers only the percentage, the review cannot be completed.