Preparation of standards and mixes. From the endocannabinoid reference standards or solutions, individual stock solutions were prepared of 1 mM in ACN (1-AG and 2-AG), 1 mM in EtOH (LEA, DHEA, AEA, PEA and SEA) or 0.1 mM in EtOH (NADA). A mixed calibration stock solution of unlabeled standards was prepared in ACN with concentrations of 27 µM of PEA, 13.5 µM of LEA, DHEA, AEA, 1-AG, 1-AG, OEA and SEA, and 1.35 µM of NADA. This solution was 18 times diluted to obtain the CAL9 solution, which was serially diluted 1:1 v:v with 0.4 mg/mL BHT in MeOH, to obtain CAL8, CAL7 etc. until CAL2.