Plasma desorption mass spectrometry of organics at low temperatures

Abstract

The desorption/ionization of volatile hydrocarbons by Plasma Desorption (PD) produces a series of molecular ions. Among these are deprotonated molecular ions, which are not usually observed in PD. The H-loss phenomenon was examined as a function of structural features, i.e., the length of the alkane chains, and the size, saturation and nature of the substitution groups on aliphatic or aromatic rings. Specifically, the following compounds were studied: n-decane, n-octane, n-hexane, n-pentane, cyclooctane, cyclohexane, cyclohexene, 1,3-cyclohexadiene, benzene, toluene, ortho- xylene, meta-xylene, and para-xylene. PDMS is best carried out on solids; accordingly, the samples were frozen. The experiments were run using a time-of-flight mass spectrometer with a 252Cf source providing the fission fragments for PD. The event-byevent mode of desorption and detection was exploited with coincidence counting to study relationships between deprotonated molecular ions and H+. The results show that the H-loss is related to the structural features of the volatile hydrocarbons. Both the yield and number of deprotonated molecular ions changed with these structural features. The dominant molecular ion varied from (M-H)+ in saturated compounds to (M+H)+ in aromatic compounds. H-loss was characteristic of all the compounds studied and appeared to result from a sequential process governed mainly by the stability of the deprotonated products. The dominant molecular ions were related to those molecular ion species encountered in methane chemical ionization. The study of deprotonation by coincidence counting revealed a chemical relationship between deprotonated molecular ions and H+. There is a higher coincidental emission of H+ with highly deprotonated molecular ions than with less deprotonated molecular ions. This finding suggests that at least a portion of the deprotonation process occurs from multiply ionized species.