Chemi-ionization produced by the catalytic combustion of a hydrocarbon

A mixture of 1000 p.p.m. propane and air was found to produce gas-phase ions when passed over a platinum filament, heated to ∼ 700°C or more. The ions were detected by direct sampling into a mass spectrometer. No negative ions were observed-only positively charged ones. It appears that these ions originate from chemi-ionization reactions occurring heterogeneously on the surface of the platinum. However, alkali metal impurities in the platinum contribute to the observed mass spectrum of positive ions. The observed spectra contain the relatively stable ions Na+, K+, and H3O+ and their hydrates. In fact, the original chemi-ions (possibly CHO+) disappear rapidly and form the more stable ions observed. These studies confirm the usefulness of heated platinum wires for sensing the presence of hydrocarbon gases in air. © 1991