How to Read an HPLC Chromatogram, Step by Step

High-performance liquid chromatography (HPLC) is the workhorse method for measuring peptide purity, and the chromatogram it produces is the graph behind the purity number on a certificate of analysis. Learning to read it takes about ten minutes and gives you a way to judge quality for yourself instead of trusting a printed percentage. This guide breaks the chromatogram down piece by piece.

What a chromatogram actually shows

An HPLC chromatogram is a plot of detector signal over time. The horizontal axis is time, usually in minutes. The vertical axis is the detector response, often UV absorbance measured in milli-absorbance units (mAU). As the sample moves through the column, its components separate and each one reaches the detector at a different moment, producing a peak. A pure sample gives one dominant peak; impurities show up as smaller extra peaks.

Retention time: where a peak lands

The time at which a peak appears is its retention time. It reflects how strongly that component interacts with the column under the run conditions. Retention time is characteristic for a given compound, method, and column, so the main peak of your peptide should land in a consistent position across lots run the same way. A shifting main peak or an unexpected large peak at a new retention time is worth questioning.

Peak area and the purity calculation

Purity is not read from peak height. It is calculated from area under the curve. The software integrates the area of every peak, then expresses the main peak as a percentage of the total peak area. This is called area percent. If the main peak accounts for 98.5% of the combined area of all peaks, purity is reported as 98.5%. This is why a tall, sharp main peak with only tiny neighbours signals high purity.

• Height tells you signal intensity, not purity.
• Area is what purity is computed from.
• Area percent = main peak area / total peak area.

Reading peak shape

Shape matters. A clean peptide peak is narrow and symmetrical. Watch for:

• Tailing: the peak drags out on the right side, sometimes hiding co-eluting impurities.
• Fronting: a lean to the left, often a loading or method issue.
• Shoulders: a bump on the side of the main peak, which can indicate a closely related impurity not fully separated.

Sharp, symmetrical peaks with a flat, quiet baseline between them are the sign of a well-resolved, clean run.

The baseline and the gradient

The baseline is the detector signal when nothing is eluting. It should be flat and low. A drifting or noisy baseline can make small impurity peaks hard to see or integrate reliably. Many peptide methods use a gradient, where the solvent mixture changes during the run to separate compounds of different polarity, which is why later peaks are still crisp rather than smeared out.

A quick reading routine

Why this matters when you buy

HPLC tells you how pure a peptide is, but it works best alongside mass spectrometry, which confirms the identity of the peptide by its mass. Together they answer both questions that matter: is it the right molecule, and how clean is it. At LYFE Science, both are part of the standard lot workflow, and the COA is available per lot so you can read the chromatogram yourself.

Once you can read a chromatogram, comparing suppliers becomes simple: the data either supports the purity claim or it does not.

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