Reading an HPLC result
The output is a chromatogram: a baseline with peaks. Each peak is a component of the sample, and the area under a peak is proportional to how much of it is present. Purity is calculated as the target peak’s area divided by the total area of all peaks.
This is why the trace matters more than the number. A stated purity of 99% is a claim; a chromatogram showing one tall peak and a flat baseline is evidence. A chromatogram with several unlabeled peaks near the target tells a different story than the summary figure suggests.
Why purity figures cluster
Nearly every supplier in this category advertises a similar purity range, which makes the number close to useless as a comparison tool. What differentiates suppliers is whether the certificate is per lot, whether the laboratory is independent, and whether the trace is included at all.