Our mass spectral library contains seven different compound libraries. The MCEBIO library was prepared with an OT-2 liquid handler (Opentrons Labwork) to pool and dilute ten compounds in each well of three 384-well plates, resulting in a concentration of 20 µM for each compound in a mixture of methanol and water (1:1). For the MCESCAF, MCEDRUG, OTAVAPEP, ENAMMOL, and ENAMDISC libraries, the Echo 650 Liquid Handler (Beckmann Coulter) was used to pool eight compounds in 384-well plates and a CERTUS FLEX liquid dispenser (Fritz Gyger AG) diluted the samples with 80-90 µL of a mixture of methanol and water (1:1), resulting in a concentration between 8-12 µM. The NIHNP library was further processed at the University of California San Diego, California, USA. Up to seven compounds were pooled in 96-well plates, resulting in a concentration of 5 µM. Because the plates showed strong evaporation, they were refilled with 50-100 µL of a mixture of methanol, acetonitrile, and water (4:4:2), which was the previous mixture. Therefore, the end concentration is unknown.