Mass spectrometry of stanozolol and its analogues using electrospray ionization and collision-induced dissociation with quadrupole-linear ion trap and linear ion trap-orbitrap hybrid mass analyzers

Mass spectrometric identification and characterization of growth-promoting anabolic-androgenic steroids in biological matrices has been a major task for doping control as well as food safety laboratories. The fragmentation behavior of stanozolol, its metabolites 17-epistanozolol, 3'-OH-stanozolol, 4alpha-OH-stanozolol, 4beta-OH-stanozolol, 17-epi-16alpha-OH-stanozolol, 16alpha-OH-stanozolol, 16beta-OH-stanozolol, as well as the synthetic analogues 4-dehydrostanozolol, 17-ketostanozolol, and N-methyl-3'-OH-stanozolol, was investigated after positive electrospray ionization and subsequent collision-induced dissociation utilizing a quadrupole-linear ion trap and a novel linear ion trap-orbitrap hybrid mass spectrometer. Stable isotope labeling, H/D-exchange experiments, MS3 analyses and high-resolution/high mass accuracy measurements of fragment ions were employed to allow proposals for charge-driven as well as charge-remote fragmentation pathways generating characteristic product ions of stanozolol at m/z 81, 91, 95, 105, 119, 135 and 297 and 4-hydroxylated stanozolol at m/z 145. Fragment ions were generated by dissociation of the steroidal A- and B-ring retaining the introduced charge within the pyrazole function of stanozolol and by elimination of A- and B-ring fractions including the pyrazole residue. In addition, a charge-remote fragmentation causing the neutral loss of methanol was observed, which was suggested to be composed by the methyl residue at C-18 and the hydroxyl function located at C-17.