Evidence for Simultaneous Generation of Ion Pairs in Laser Mass Spectrometry

Abstract

Our research group has been concerned with the laser mass spectra of organic compounds; particular emphasis has been on fragmentation processes and the mechanisms which produce organic ions. Frequently, positive and negative ions occur in pairs raising the question of whether ion formation can result from charge exchange reactions producing an ion pair in a single step (1,2). Pair-production could be brought about by intermolecular transfer of atoms, groups, or electrons, or by intramolecular charge separation. The general reactions for these possibilities can be summarized below: $$ {\text{2GM}}{\xrightarrow{{{\text{hv}}}}^{\text{ + }}}{\text{M}}{{\text{G}}_{\text{2}}}{\text{ + }}{{\text{M}}^{\text{ - }}} $$ (1) where G = H or an alkyl group $$ {\text{AB}}\xrightarrow{{{\text{hv}}}}{{\text{A}}^{\text{ + }}}{\text{ + }}{{\text{B}}^{\text{ - }}} $$ (2) The above reactions imply pair-production in a single step, which distinguishes them from indirect processes which can produce the same ions. General examples of the latter are: $$ {\text{GM}}\xrightarrow{{{\text{hv}}}}{{\text{G}}^{\text{ + }}}{\text{ }}\xrightarrow{{{\text{GM}}}}{\text{MG}}_{\text{2}}^{\text{ + }} $$ (3) $$ {\text{AB}}\xrightarrow{{{\text{hv}}}}{\text{A}}{{\text{B}}^{\text{ + }}}^{\text{*}} \to {{\text{A}}^{\text{ + }}}{\text{ + B}} $$ (4) Production of pairs of ions in the laser mass spectra of a variety of systems is a well-documented fact. Formation of (M + H)+ and (M − H)− ions is common for molecules containing acidic and basic groups; for example, carboxylic acids, amino acids, and phenols. Also, in both the field desorption and laser mass spectra of zwitterionic compounds, ions such as (M + CH3)+ and (M − CH3)− are observed for the same compound. Similarly, pair production from a single molecule is possible; for example, esters (R1COOR2) show R1CO+ and OR2 − peaks in their positive and negative ion laser mass spectra, respectively. Almost all aromatic hydrocarbons show peaks corresponding to M+ in their positive ion spectra, and many show M− ions in their negative ion spectra. This raises the possibility of intermolecular electron transfer reactions to produce positive and negative ions.