Field-induced Ion Chemistry Leading to the Formation of (M-2nH)+· and (2M-2mH)+· Ions in Field Desorption Mass Spectrometry of Saturated Hydrocarbons

Abstract

The formation of [M-2H] +. ions has been reported in the field desorption mass spectrometry of saturated hydrocarbons. It is shown that these ions predominantly have an alkene structure and that a field-induced ion chemistry in multimolecular or condensed layers produce [M-2nH] +. and [2M-2mH] +. ions with n and m = 1, 2 from saturated hydrocarbons. For the primary reaction of the dehydrogenation chemistry, a field-induced proton transfer from a molecular ion to a neighbouring molecule is suggested to produce an [M - H] + ion and an [M-H] . radical after elimination of molecular hydrogen, which in secondary reactions form an alkene ion or a dimer ion. Multiple dehydrogenation occurs by repeating this reaction sequence with other parts of molecules having long alkyl chains. The primary reaction is inhibited by the admixture of molecules with lower ionization energies than those of the alkanes.