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HPLC purity explained — what "≥99%" actually means

Research use only — not for human or animal consumption

HPLC (high-performance liquid chromatography) separates the components of a sample so each can be measured. A purity figure such as "≥99% by HPLC" means that, under the method used, at least 99% of the detected material corresponds to the target compound and less than 1% to other peaks. The method and detection settings determine what the number actually represents.

What HPLC does

HPLC pushes a dissolved sample through a column under high pressure. Different molecules travel through the column at different rates, so they emerge — elute — at different times. A detector records each as a peak on a chromatogram. The target compound shows as the main peak; impurities, fragments, or related substances show as smaller peaks.

Purity is calculated from the relative size of those peaks: the area of the target peak as a percentage of the total detected peak area. "≥99%" means the target accounts for 99% or more of what the method detected.

Why "by HPLC" is not optional

A purity number without its method is incomplete. The figure depends on:

Detection method — UV detection at a given wavelength sees compounds that absorb at that wavelength; it may not see everything (for example, residual solvents or counter-ions). This is why HPLC purity and mass balance are different questions.
Column and conditions — the stationary phase, gradient, and run time affect how well peaks separate. Poor separation can hide an impurity under the main peak.
Integration — how the software draws the boundaries of each peak affects the percentage.

So "≥99% by HPLC at [wavelength], [method]" is a far more useful statement than "99% pure" on its own.

What HPLC does and doesn't tell you

It does tell you the proportion of detected material that is the target, and it flags the presence and rough size of impurity peaks.

It does not, by itself, confirm identity. A peak at the expected retention time is consistent with the target but is not proof of it. That is why purity (HPLC) and identity (mass spectrometry) are reported together — each answers a different question. See Confirming identity by mass spectrometry.

Reading a purity result on a COA

When you see a purity figure on a certificate of analysis:

Check the method line — wavelength, column, gradient or at least a method reference.
Look for the chromatogram or a reference to it; a clean main peak with small, well-separated impurity peaks is what you want to see.
Treat the percentage as method-specific — comparing two suppliers' "99%" only makes sense if the methods are comparable.

How Amino Society reports purity

Each batch is analysed by an independent laboratory, and the HPLC result is recorded on the batch COA alongside the identity confirmation. We list a compound as in stock only once that documentation is in place. Quote your lot code and we will send the full report, including the analytical detail behind the purity figure.