How to Read an HPLC Chromatogram on a Peptide Certificate

The purity figure on a peptide certificate comes from a single picture: an HPLC chromatogram. Most people skip straight to the percentage and never look at the trace it came from. The trace carries more information than the number, and it takes about five minutes to learn to read.

What the instrument is doing

High-performance liquid chromatography pushes a dissolved sample through a narrow column packed with a coated material. For peptides this is almost always reversed-phase chromatography: the packing is water-repellent (typically a C18 coating), and the liquid flowing through it starts mostly water and gradually becomes mostly organic solvent, usually acetonitrile. Each component in the sample lets go of the packing at a slightly different point in that gradient, so they leave the column one after another.

As each component leaves, it passes a UV detector. Peptides are usually monitored at around 214–220 nm, where the peptide bond itself absorbs light, so every peptide-like molecule in the sample shows up whether or not it contains aromatic residues.

The two axes

• Horizontal: retention time, in minutes. When a component came off the column. It identifies peaks within one run, but it is not an identity test on its own — different molecules can elute at the same time.
• Vertical: detector response, usually in milli-absorbance units (mAU). How much UV light was absorbed at that moment.

Each component appears as a peak. The quantity that matters is the area under each peak, not its height.

How the purity number is calculated

The software draws a baseline under the trace, marks the start and end of every peak (this is called integration), and measures each area. Purity is the main peak’s area divided by the total area of all integrated peaks, expressed as a percentage. A result of 99.2% means the main peak accounts for 99.2% of the UV-absorbing material the method could see.

Two consequences follow directly from that definition:

• It is area percent, not weight percent. Different molecules absorb UV to different degrees, so area share and mass share are close for closely related peptide impurities but not identical.
• Anything the detector cannot see is not counted. Water, salts, counter-ions and most non-peptide material do not appear at 214 nm. That is why a sample can be 99% pure by HPLC and still contain less peptide by weight than the label states — the powder also holds water and counter-ions. Content is a separate measurement.

What to look at on the trace

• One dominant peak. For a single-sequence peptide there should be one tall, reasonably narrow peak with everything else small by comparison.
• Small peaks either side. Minor peaks eluting just before or just after the main one are typical synthesis by-products — truncated sequences, deletions, oxidised forms. Their presence is normal. Their size is what the purity figure summarises.
• Shoulders. A bump fused onto the side of the main peak, rather than separated from it, is a component the method did not fully resolve. Depending on how the integration was drawn, a shoulder may have been counted as part of the main peak, which flatters the result.
• A flat, stable baseline. A drifting or noisy baseline makes small impurity peaks hard to integrate consistently.
• Blends show several large peaks. A product that combines two or more peptides should show one major peak per component. Purity for a blend is normally reported per component, not as one number for the whole vial.

What the trace cannot prove

• Identity. A clean single peak says the sample is homogeneous, not that it is the right molecule. Identity comes from mass spectrometry — see our guide on how mass spectrometry confirms a peptide’s identity.
• Co-eluting impurities. Two molecules that leave the column at the same moment appear as one peak. A second method with a different gradient or column is how laboratories check for this.
• Quantity. The area percentage says nothing about how many milligrams are in the vial.

What a trustworthy report includes

1. The chromatogram itself, not only the percentage.
2. An integration table listing each peak’s retention time and area.
3. The detection wavelength and, ideally, a summary of the method.
4. A lot code matching the vial, and a report number you can verify with the laboratory.

A purity figure presented without the trace behind it is a claim rather than a measurement. Every lot we sell links to its full laboratory report on our lab results page, where the result can be checked directly with the testing laboratory. For the other fields on a certificate, see reading a certificate of analysis.

Materials supplied by BMS Labs are for laboratory research use only and are not for human or veterinary use.