Abstract
Surface-assisted oxidation reactions in the chemical ionization (CI) source have been studied using oxidation yield determinations and oxygen-18 incorporation measurements with 18O 2 or H 2 18O added to the negative ion chemical ionization buffer gas (N 2 or Ar). Oxidation yield measurements show that O 2, but not H 2O, affects the oxidation yield. Using 18O 2, 18O incorporation was found to be consistently lower in the ionized products of surface-assisted reactions compared with the products of gas-phase ion/molecule reactions. In addition, the degree of 18O incorporation was influenced by ion source temperature, with an increase in ion source temperature reducing the degree of incorporation. Although H 2O has a negligible effect upon the efficiency of surface-assisted oxidation reactions, water-derived oxygen is efficiently incorporated by the products of surface-assisted oxidation reactions. In experiments with H 2 18O + O 2 mixtures a correlation between the 18O content of the oxidized products and the 18O content of filament-derived ReO 4 − was observed. This correlation, and the detection of rhenium oxides on ion source surfaces, suggest that filament-derived rhenium oxides may play a role in the activation of ion source surfaces towards the selective surface-catalyzed oxidation of aromatic hydrocarbons.
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