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The endocannabinoid 2-arachidonoyl glycerol (2-AG) undergoes spontaneous isomerization to biologically inactive 1-AG. This effect has not been adequately addressed in previous studies that reported 2-AG concentrations in biological samples.Liquid chromatography tandem-mass spectrometry (LC-MS/MS) was used for 1-AG and 2-AG analyses.Identical collision-induced disintegration spectra were found for 1-AG and 2-AG. For specific detection of both compounds, which share a common mass transition, baseline chromatographic separation is mandatory, even when applying MS/MS technology with its generally high detection specificity. When using standard chromatographic conditions with the very short run times typically used in LC-MS/ MS methods, co-elution of 2-AG with 1-AG, which is present in human serum, causes false 2-AG results.Our data highlight that the analytical specificity of MS/MS can be limited by interference from isobaric isomers with identical disintegration patterns. The specificity of this technology must be carefully evaluated for each individual application.