Electrospray ionization (negative ions) and tandem mass spectrometry of monohydroxy arachidonate metabolite isomers. (A) Product ions obtained following collisional activation of 5-hydroxy-6,8,11,14-eicosatetraenoic acid [M − H]− at m/z 319; (B) Product ions obtained following collisional activation of 12-hydroxy-5,8,10,14-eicosatetraenoic acid [M − H]− at m/z 319; (C) Product ions obtained following collisional activation of 8-hydroxy-5,9,11,14-eicosatetraenoic acid [M − H]− at m/z 319; (D) Product ions obtained following collisional activation of 15-hydroxy-5,8,11,13-eicosatetraenoic acid [M − H]− at m/z 319. These MS/MS spectra were obtained using a tandem quadrupole mass spectrometer.