[2026 update] Reading a chromatogram someone posted without axes

Posting this under the heading it deserves: Reading a chromatogram someone posted without axes Everything below is what sits behind that.

Reading a chromatogram, from the point of view of someone who has read the documentation and is still stuck.

Saying which parts I have understood so that nobody wastes time re-explaining them, and where exactly the understanding stops.

The opening post put the caveat in the right place and I want to underline it.

This is why a purity figure without the underlying chromatogram is weaker evidence than it appears. It is also why two competent laboratories can report different numbers on the same vial without either being wrong.

Old habit: I write down the expected answer before I calculate it.

Narrowing the opening post, because the general version has more than one answer.

On integration: where the baseline is drawn matters more than most people realise. On a clean chromatogram with well-resolved peaks the choice is inconsequential. On a chromatogram with a trailing shoulder or a rising baseline it matters. Differences of one to two percentage points between defensible integrations are ordinary.

The short version is the first sentence; the rest is why.