Quantitative (purity) analysis reflects mass purity, that is the proportion of mass of a sample that is the analyte, however this is difficult to measure directly. For that reason proxies like chromatographic purity are used. If you see a purity report on our products it is most likely an HPLC (High Performance Liquid Chromatography) report. HPLC is the most popular quantitative technique and it is an industry standard for quantitative analysis.
Chromatographic purity is nuanced and depends on the extinction coefficients of analytes, impurities and wavelength of light used. So it is not equivalent to mass purity. However, with a good reference standard, good methods and a relatively pure sample, chromatographic purity is a reliable approximation for mass purity and hence is used for most purposes.
An HPLC report is not a single precise measurement of chromatographic purity, because for many reasons the measurements will vary between injection runs. Instead reports are usually the mean of several (≥3) injection runs with an acceptable measurement error and variance. The more runs, the more accuracy, and the more expense. Our labs ensure measurement error is less than ±2% and confidence is greater than ≥95%. But there is still error and that’s why it’s possible for a ≥98% pure sample to report as ≥100% pure.
For a more thorough explanation of potential sources of error please see the following StackExchange thread: