In order to detect and quantify each positional isomer without ambiguity, an MRM method was established. Signature daughter ions for each standard HEPE (parent m/z, 317) were as follows: 18-HEPE, 259; 15-HEPE, 219; 12-HEPE, 179; 5-HEPE, 115. For each enantiomer pair, the R isomer was eluted before S isomers. Each signature ion for each species was unique; only two (R and S isomers) peaks are present on each extracted ion chromatogram.