How to Read a Peptide HPLC Chromatogram

The purity figure on a Certificate of Analysis comes from a chart called a chromatogram. This guide explains what the chart shows, how the percentage is worked out, and what to check before relying on it. It is general analytical background for laboratory researchers.

What the chromatogram is

High-performance liquid chromatography (HPLC) separates the components of a sample as it passes through a column. A detector records a signal over time, and the result is a line graph: time runs along the bottom axis, and detector response runs up the side. Each component that leaves the column produces a peak.

For peptides the usual setup is reversed-phase HPLC. The sample is pushed through a column with a gradient of water and an organic solvent such as acetonitrile, usually with a small amount of an acid modifier. Components that interact more strongly with the column come off later, so they appear further to the right.

The parts of the chart

• Retention time. The time at which a peak reaches its maximum. It identifies a peak only within one method. A different column, gradient or instrument gives a different retention time for the same peptide, so retention times from two reports cannot be compared directly.
• The solvent front. The first signal, very early in the run, comes from solvent and unretained material. It is not part of the peptide.
• The main peak. The tall peak that should correspond to the peptide of interest. On a clean report it is by far the largest.
• Smaller peaks. Minor peaks are other components: related peptide impurities, truncated or modified forms, or leftover reagents. A good report lists them rather than hiding them.
• The baseline. The flat line between peaks. Under a gradient it often drifts slightly. A noisy or steeply rising baseline can make small peaks hard to see.
• The detection wavelength. Peptide bonds absorb strongly at roughly 214 to 220 nm, so that range is the usual choice for purity work. Some reports also show 280 nm, which is sensitive to certain aromatic residues. Purity values taken at different wavelengths are not directly comparable.

How the purity percentage is calculated

Purity by HPLC is normally an area percentage. The software integrates each peak, and purity is the area of the main peak divided by the total area of all integrated peaks, multiplied by 100.

Two details matter. First, anything the integration ignores is left out of the total, which is why reports state an integration threshold or list the peaks that were counted. Second, area percent is not mass percent. It describes how much of the detector signal came from the main peak, not how many milligrams of the vial are the peptide.

What to look for on a real chromatogram

1. Is the chromatogram included at all? A purity figure with no chart behind it cannot be checked.
2. Does the main peak sit where the method says it should? The report should state the method conditions, or at least the column, gradient and wavelength.
3. Is the main peak symmetrical? A shoulder or a split top can hide a second component eluting almost at the same time.
4. Are small peaks listed with their areas? The sum of the listed areas should match the stated purity.
5. Is there a blank or baseline reference? A blank run shows which signals come from the system rather than the sample.
6. Does the date, lot number and sample name match your vial? See how to read a peptide COA.

What HPLC purity does not tell you

• Identity. A clean main peak shows that one component dominates. It does not show that the component is the right molecule. That needs mass spectrometry, covered in how purity is tested.
• Co-eluting impurities. Two different molecules can leave the column together and be counted as one peak.
• Invisible material. Salts, water and counter-ions have little or no absorbance at the detection wavelength, so a high area percent can sit alongside a lower peptide content. See purity versus potency.
• Anything outside the method. Endotoxin, sterility and residual solvents are separate tests.

Reading two reports side by side

If you compare reports from different sources, compare like with like: the same wavelength, similar gradient length, and whether small peaks were integrated. A higher percentage from a method that ignores small peaks is not better evidence than a lower percentage from a method that counts them.

Where to go next

The glossary explains the terms used above in the peptide research glossary. Real chromatograms from our certificates are published on the COA page.

This page is general analytical background for laboratory researchers. It does not describe uses, doses or effects of any compound in humans or animals. For laboratory research use only.