HPLC purity explained: what the percentage means

Almost every peptide supplier quotes an HPLC purity figure. It is the most quoted number in the category and the least understood. This note explains what the number measures and where it stops being informative.

What HPLC actually does

High-performance liquid chromatography (HPLC) pushes a sample through a packed column under pressure. Different components of the mixture travel at different speeds and elute at different times. A detector records how much material elutes at each moment, and the result is the chromatogram: a series of peaks against time.

For a research peptide, the target compound should be the main peak.

Where the purity percentage comes from

Purity is normally reported as the area of the target peak divided by the total area of all peaks, expressed as a percentage. It is a relative measure: it compares the target to everything else the detector saw in that run. It is not an absolute quantity of substance, and it does not sum to truth about the whole vial.

What the number does not tell you

• Identity. A high purity peak can still be the wrong molecule if something co-elutes at the same retention time. Purity without identity confirmation is incomplete.
• The method behind it. Detection wavelength, column chemistry and gradient all change the answer. A purity figure is only meaningful next to the method that produced it.
• Resolved impurities. Two substances that elute very close together can be reported as one shallow impurity. The percentage is silent about what is hidden inside that peak.

Reading the chromatogram yourself

• Retention time — when the target elutes, and whether it is consistent with a reference standard.
• Resolution — are the neighbouring peaks clearly separated, or merged into a shoulder?
• Baseline — a flat, stable baseline makes the area calculation trustworthy.
• Peak symmetry — an obviously distorted main peak usually indicates a problem worth investigating.

Why the method matters as much as the number

Two laboratories can report different purities for the same material by changing the method. That is not fraud; it is a property of the technique. The useful question is not "what percentage?" but "by what method, against what standard, and can I see the trace?" A verifiable certificate answers all three.

We publish the analytical method and the chromatogram with each batch, and list them on the lab test results page. Purity without identity confirmation is only half the picture, which mass spectrometry completes. For the document itself, see how to read a certificate of analysis.