Reference compounds

Water (LC-MS grade), acetonitrile (LC-MS grade, ≥99.95 %), hexane (UV/IR grade, ≥99.5 %), methanol (LC-MS grade ≥ 99.95 %), isopropanol (LC-MS grade, ≥99.95 %), acetone (LC-MS grade, ≥99.9 %), butanol (for synthesis, ≥99.5 %), trichloromethane/chloroform (UV/IR grade, ≥99.8 %) and citric acid (≥99.5 % p.a.) were purchased from Carl Roth (Karlsruhe, Germany), ethyl acetate (Reag. Ph. Eur. ≥ 99.9 %) was purchased from Biosolve (Valkenswaard, Netherlands), formic acid (98 %) and acetic acid (100 %) from Fisher Scientific (Schwerte, Germany), hydrochloric acid (Reag. Ph. Eur., 37 %) and disodium hydrogen phosphate (NormaPur) from VWR Chemicals (Darmstadt, Germany), and 2,6-di-tert-butyl-4-methylphenol (BHT, 99 %) from Sigma-Aldrich (Munich, Germany). Reference compounds and isotopically labeled internal standards (IS) were purchased from Cayman Chemicals (Ann Arbor, MI, USA) for endocannabinoids and oxylipins, or Avanti Polar Lipids (Alabaster, AL, USA) for sphingolipids, LPAs, and lipids analyzed by LC-HRMS. For polar compounds, tryptophan and related metabolites, isotopically labeled IS and Reference compounds were purchased from Sigma-Aldrich or Cambridge Isotope Laboratories (Tewksbury, MA, USA). Detailed descriptions can be found in the supplementary material.