How to Read an HPLC Chromatogram on a Peptide COA

Learning how to read an HPLC chromatogram is the fastest way to tell whether a purity claim on a peptide certificate of analysis is backed by real data. The trace looks technical, but it answers a simple question: how many components did the method detect, and how much of the signal belongs to the main one?

Quick answer

How do you read an HPLC chromatogram on a COA? The x-axis is retention time; the y-axis is detector signal, usually UV absorbance near 214–220 nm for peptides. Each peak is a detected component. Purity is normally the main peak’s area as a percentage of all integrated peak area. Check the method, the integration table, and a mass spectrometry identity result.

The two axes

The horizontal axis shows retention time, the minutes after the sample enters the column at which each component reaches the detector. In reversed-phase HPLC, the mode used for most peptide purity work, more hydrophobic molecules are held longer and elute later. The vertical axis shows detector response, typically UV absorbance in milli-absorbance units (mAU).

Peptide methods usually detect near 214–220 nm because the peptide bond itself absorbs strongly there, so almost every peptide species in the sample shows up. Detection at 280 nm alone is weaker evidence: it sees mainly tryptophan and tyrosine, and a peptide without those residues barely registers. The review by Mant and colleagues listed below covers the HPLC modes used for peptides in detail.

Peaks, retention time and area

Each peak is one detected component, or several that the method did not separate. The tall peak should be the target peptide; smaller peaks are related impurities such as the deletion and truncated sequences described in peptide impurities. Purity is calculated from peak area, not height, because a broad short peak can contain as much material as a narrow tall one.

The integration table

Below the trace, a good certificate lists every integrated peak. The example is illustrative, not data from any real lot:

Here the reported purity would be 99.13%: the main peak’s share of the total integrated area. Peak 3 sits close to the main peak, which is where a shorter or steeper gradient could merge the two and inflate the purity figure. What 99% purity actually means explains why a percentage is only as good as the method behind it.

Method details that change the number

• Column. Most peptide methods use a C18 reversed-phase column; chemistry and particle size affect separation.
• Mobile phases and gradient. Water and acetonitrile, each with a small amount of acid such as TFA, run as a gradient. A long shallow gradient separates close impurities; a short steep one can hide them.
• Detection wavelength. 214–220 nm sees nearly all peptide species; other wavelengths can under-report impurities.
• Integration settings. Thresholds decide which small peaks are counted. Very small peaks can be dropped entirely.

Because of these choices, two laboratories can report different purity values for the same lot, and neither is necessarily wrong. That is why a named method matters more than any single decimal place.

Red flags on a chromatogram

• A purity number with no trace or integration table.
• A main peak with a flat, clipped top, a sign of detector saturation that makes the area unreliable.
• A trace cropped so the early or late part of the run is missing.
• No lot number, date or method on the page.
• Chromatograms that look identical across different lots, which suggests a recycled document.

More supplier-level checks are in how to evaluate a research peptide supplier.

What a chromatogram cannot tell you

HPLC does not prove the main peak is the correct molecule; mass spectrometry supplies identity evidence, as covered in HPLC vs mass spectrometry. It does not measure water, salts or counter-ions, which is why net peptide content is a separate figure. And it says nothing about endotoxin, which needs its own test, described in endotoxin testing for research peptides. Revoryn Labs publishes available batch documentation in its public COA ledger.

Key takeaways

• Retention time runs along the x-axis; UV signal, usually at 214–220 nm, runs up the y-axis.
• Purity is the main peak’s share of total integrated area, not its height.
• The integration table is where the purity number comes from; it should be shown.
• Gradient, wavelength and integration settings can all move the result.
• HPLC answers purity, not identity, net content or endotoxin.

Frequently asked questions

What does area percent mean on a chromatogram?

It is each peak’s area divided by the total area of all integrated peaks. The main peak’s area percent is usually reported as chromatographic purity.

Why is peptide HPLC run at 214 or 220 nm?

The peptide bond absorbs strongly at these wavelengths, so nearly every peptide species in the sample is detected, including impurities without aromatic residues.

What is the small peak at the very start of the run?

It is often the solvent front: material that passes through the column without being retained. Methods usually exclude it from integration, and the certificate should make clear which peaks were counted.

Can two labs report different purity for the same lot?

Yes. Different columns, gradients, wavelengths and integration settings can shift the result, which is why the method should always accompany the number.

Sources

• Mant CT, Chen Y, Yan Z, et al. HPLC analysis and purification of peptides. Methods Mol Biol. 2007;386:3-55. doi:10.1007/978-1-59745-430-8_1
• D’Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30. doi:10.1016/j.jpba.2014.06.012

For laboratory and in-vitro research use only. Not for human or veterinary use.