Formation of deprotonated molecules [M—H] − of α‐tocopherol under fast‐atom bombardment conditions

The formation of deprotonated molecules, [M—H] − , of α‐tocopherol under fast‐atom bombardment (FAB) conditions has been discussed on the basis of a proton subtraction reaction between analyte and matrix molecules. The use of proton‐accepting matrices such as diethanolamine (DEA) and pentamethyleneglycol (PMG) gave higher intensities of the [M—H] − ion and more definitive spectral data than when other matrices, a 1:2 (v/v) mixture of thioglycerol (TG) and dithiothreitol (DTT), or m ‐nitrobenzylalcohol ( m ‐NBA), were used. The order of proton acceptability evaluated for the matrices used was DEA>PMG>DTT> m ‐NBA. It was concluded that the proton acceptability, which is measured by gas‐phase basicity or proton affinity, is a most important factor for the formation of both [M + H] + and [M—H] − ions under FAB conditions. Further, the formation of molecular ions, M +· , under FAB conditions and the formation of [M—H] − ions by electron impact have been discussed by using α‐tocopherol.