What HPLC purity actually measures

High-performance liquid chromatography (HPLC) separates the components of a sample. A small volume of the dissolved sample is pushed through a column packed with a stationary phase. Different molecules interact with that phase differently, so they leave the column at different times. A detector at the outlet records a signal as each one passes, and the result is a chromatogram: a trace of signal against time, with a peak for each separated component.

The main compound gives the largest peak. Anything else in the sample that the detector can see — related substances, residual reagents, degradation products — gives a smaller peak somewhere else on the trace.

The purity figure most certificates report is an area percent: the area under the main peak divided by the total area of all peaks, expressed as a percentage. If the main peak accounts for 99 units of area and every other peak together accounts for 1, the reported purity is 99%.

This is a relative measurement. It compares the main component with the other components the detector could see under the conditions used. It is the standard way of reporting chromatographic purity, and it is what "purity by HPLC" means on our certificates.

Because area percent compares peaks, anything that does not produce a peak is not part of the calculation. That has practical consequences worth knowing:

• Water and residual solvent are generally not seen by a UV detector at the wavelengths used for peptides and small molecules, so they do not appear in the chromatogram and are not counted against purity.
• Salts and counter-ions that do not absorb at the detection wavelength are likewise invisible to it.
• Inorganic material is outside the scope of the method entirely.

None of this makes the figure wrong. It makes it specific. "99% pure by HPLC" means 99% of the detector-visible material was the main compound. It is not a statement that 99% of the mass in the vial is the compound. Those are different quantities, measured by different methods.

Every certificate we publish names the analytical method alongside the result. That is not decoration. A reader who sees "HPLC" next to the purity knows to interpret it as area percent; a reader who sees "mass spectrometry" next to the identity result knows it confirms molecular mass rather than quantifying anything. Reading the method first is the habit that keeps the number honest — we walk through the whole document in how to read a certificate of analysis.

A chromatogram with a single clean peak tells you the sample is homogeneous. It does not, on its own, tell you what the peak is. That is what the mass spectrometry result is for: it confirms that the material has the molecular mass expected of the named compound. Purity says how clean; identity says what. A certificate that reports both, as ours do, answers both questions. See mass spectrometry for identity confirmation.

Every listing on this site is published with its purity, its full specification and its lot certificate. The purity figure on the listing is the measured HPLC result for the lot being sold, not a target or a typical value, and it changes when the lot changes. The quality page puts it directly: a lot that does not meet specification is never listed. For the researcher, that means the number on the listing, the number on the certificate and the number for the vial that arrives are the same number, because they are all the same lot. If a certificate for a lot already on the shelf is needed, support sends it the same business day against the lot number.

Because area percent depends on the method — column, mobile phase, gradient, wavelength — two laboratories can report slightly different figures for the same material without either being wrong. The figure that matters is the one measured on your lot by a laboratory with no commercial interest in the result. If you commission your own independent testing and the result falls below the stated purity, send us the report: our returns policy covers replacement or a full refund in that case.