Isobaric compounds that also share fragmentation patterns as well as retention times were distinguished via specific selected reaction monitoring (SRMs). LC–MS/MS chromatograms of various hydroxyeicosatetraenoic acids (HETEs) after the injection of a 5-HETE standard at 500 ng/mL. Isobaric HETEs including 5-HETE (13), 8-HETE (12), 11-HETE (11) and 12-HETE (10) were not resolved chromatographically, and they produced both common and selective fragments in MS/MS. The 319/301 transition was the most sensitive but was shared by all HETEs (a, c, e, g). In contrast, the less sensitive but more selective transitions of HETEs were used, including 319/114.7 for 5-HETE (b), 319/154.8 for 8-HETE (d), 319/167.2 for 11-HETE (f) and 319/179 for 12-HETE (h)