What does a 98.2% purity with a list of related substances attached let me conclude?

Accepted answer

98.2 per cent is a statement about area, and the other 1.8 per cent is everything the detector saw and did not assign to your peak. Read it as 98.2 of every 100 units of peak area at whatever wavelength was used, not as 98.2 per cent of the mass in the vial. With the list of related substances attached you can at least see how the figure was produced, which is the difference between a measurement and a claim. What it still does not tell you is content: how many milligrams are actually there.

Worth being precise here: understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.

Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.

Inter-laboratory studies on identical peptide material routinely find half-a-per-cent to a full-per-cent spreads in reported purity on the same sample.

The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.

If you are ranking vendors, specify a method and have all samples tested at the same place.