The accurate determination of compound purity is crucial for characterizing library purity, monitoring the stability of storage compounds, and obtaining meaningful high-throughput screening results. However, current high-throughput techniques for the determination of compound purity are inadequate. We evaluated on-line chromatography detectors, including UV(TWC), UV(214), and ELSD detectors, in a series of studies of 233 compound mixtures prepared with known compositions. Results indicate that both UV(TWC) and UV(214) overestimate the minor component in a mixture whereas ELSD underestimates the minor component. An average of UV(TWC) and ELSD purities gives a more accurate measure of the relative purity for a wide range of compounds in various purity ranges. This technique was applied to 959 compounds from our compound collection to more accurately determine their relative purity.