Fragmentation reactions of molecular ions and dications of indoleamines

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Abstract

The combination of techniques such as two-dimensional mass spectrometry and mass analysed ion kinetic energy spectrometry (with and without collision gas) in the study of gas-phase ion chemistry of indole, gramine, tryptamine and serotonine enabled us to establish the fragmentation pathways of their singly- and doubly-charged molecular ions generated by electron impact. The measurement of the kinetic energy released in the charge separation reactions of the molecular dications produced information on the intercharge distance on the transition state for the fragmentation, assuming that it arises entirely from the coulombic energy released. For the singly-charged molecular ions of indoleamines, loss of the neutral molecule CH2=NR (R = H or CH3), by hydrogen transfer from the amino (or methyl) group to the indole nucleus, is the only fragmentation occurring in the third field-free region of the mass spectrometer. Upon collision of the molecular ion with He gas, a competing process of bond cleavage by loss of the radical •CH2NH2 (or •N(CH3)2 for gramine) is observed for tryptamine and serotonine. The doubly-charged ions dissociate by elimination of neutral molecules or radicals to generate doubly-charged fragment ions or by charge separation reactions. The intercharge distance, calculated from the kinetic energy released in the fragmentation, was, whenever possible, correlated with charge location in the ion.

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