Formation of (M-H)+ Ions under Fast Atom Bombardment Conditions: Trolox and .ALPHA.-Tocopherol.

Formation of the [M-H]+ ions of Trolox and α-tocopherol produced under fast atom bombardment (FAB) conditions has been examined by using collision-induced dissociation (CID) technique with tandem mass spectrometry. The CID spectra of the [M-H]+ ions of these compounds have been compared with those of the M+· and [M+H]+ ions. The CID spectra of the [M-H]+, M+·, and [M+H]+ ions of Trolox showed characteristic ion peaks at m/z 163, 164, and 165, respectively, whereas the CID spectra of the [M-H]+, M+·, and [M+H]+ ions of α-tocopherol showed a common peak at m/z 165 which comes from an intramolecular hydrogen rearrangement reaction, despite a common structure unit corresponding to those fragment ions. In order to explain the results obtained, possible structures of the [M-H]+ ions of Trolox and α-tocopherol would be presented though the site(s) of a hydrogen loss from analytes could not be determined. The structure characteristics of analytes and the mechanism to form [M-H]+ ions under matrix FAB conditions have been discussed, and it has been concluded that the formation of molecular-related ions [M-H]+, M+·, and [M+H]+ occurs competitively according to the nature of analyte and matrix molecules under matrix FAB conditions.