Reading the chromatogram: what a purity percentage hides
The purity figure is the most quoted number in this market and the most misread. It comes from a chromatogram, it measures separation, and it says nothing about identity.
Where the number comes from
In an HPLC run, the sample moves through a column and its components separate over time, drawn as peaks on a chart — the chromatogram. The purity percentage is typically an AREA percentage: the main peak's share of the total peak area the detector saw. It is a statement about proportions inside the chart, not an absolute measure of anything.
What the area percentage can hide
Three things, mainly. Components the detector is blind to at the chosen wavelength never appear in the chart at all. The main peak being large says nothing about WHAT the main peak is. And run conditions — column, gradient, wavelength — shape the picture; the same material can produce different-looking charts in different runs. None of this makes purity useless; it makes purity narrow.
The question purity cannot answer
Is the molecule the one named on the label? That is identity, answered by a different analysis on a different instrument — typically mass spectrometry, where the measured mass either matches the expected mass or does not. A 99-point-something purity for the wrong substance is a precise description of the wrong thing. When a one-page certificate shows purity alone, the identity question is not answered badly — it is not answered.
How to read the pair together
Identity confirms what it is; purity describes how clean it is; the batch reference ties both to the vial in front of you. Any one alone is a fragment. The full field-by-field walk is in how to read a COA.
See both analyses on real certificates
Free account, no payment details. Certificates shown with what was analysed and what was not.