Accurate mass measurements were subject to database searches in the public databases METLIN (Smith et al. 2005) and Human Metabolome Database (Wishart et al. 2013) as well as an in-house library comprising 384 synthetic standards. Database hits were then confirmed by retention time match (in-house library only) and MS/MS spectral match from pooled samples (Supplementary Fig. 2). In a few cases additional synthetic standard compounds were acquired and analyzed on the same platform to confirm compound identity.