Electrospray ionization (negative ions) and tandem mass spectrometry of three regioisomeric hydroperoxides and corresponding ketones of arachidonic acid. (A) Product ions obtained following collisional activation of 5-hydroperoxy-6,8,11,14-eicosatetraenoic acid [M − H]− at m/z 335; (B) product ions obtained following collisional activation of 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid [M − H]− at m/z 335; (C) product ions obtained following collisional activation of 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid [M − H]− at m/z 335; (D) product ions obtained following collisional activation of 5-oxo-6,8,11,14-eicosatetraenoic acid [M − H]− at m/z 317; (E) Product ions obtained following collisional activation of 12-oxo-5,8,10,14-eicosatetraenoic acid [M − H]− at m/z 317; (F) Product ions obtained following collisional activation of 15-oxo-5,8,11,13-eicosatetraenoic acid [M − H]− at m/z 317. These MS/MS spectra were obtained using a tandem quadrupole mass spectrometer.