HPLC purity: what the percentage on a COA means
The number people look at first on a certificate of analysis (COA) is the purity percentage. It almost always comes from a test called HPLC, or high-performance liquid chromatography. This guide explains what that test measures and what it does not.
What HPLC does
The lab dissolves a small amount of the sample and pushes it under pressure through a column packed with a material that holds each molecule with a different strength. The molecules leave the column at different times, and an ultraviolet detector records each one as a peak. The resulting graph is called a chromatogram.
How the percentage is calculated
Every peak has an area. Purity is the area of the main peak, the expected compound, divided by the total area of all peaks. If the main peak is 99.1% of the total area, the COA reports 99.1% purity.
What to check on the chromatogram
- One dominant peak. A clean lot shows one tall, narrow peak with very small secondary peaks.
- Retention time. The minute at which the main peak appears. It should be consistent between lots of the same compound run with the same method.
- The peak table. It lists every peak with its area and percentage. The total should be 100%.
- The method. Column, mobile phase and detector wavelength. Without them the result cannot be reproduced.
What HPLC does not tell you
HPLC purity shows what share of the sample is the main compound. It does not confirm on its own which compound that is, or how much is in the vial. Identity is confirmed by mass spectrometry and quantity by a content test. That is why a complete COA has several panels, not one.
Purity and quantity are different things
A vial can be 99% pure and still hold less material than the label states. Check that the COA also reports the measured content of the lot.
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