The Molecular Analysis of Sulfate Species in Environmental Aerosols Using Chemical Ionization Mass Spectrometry

Speciation of sulfurous acid, sulfuric acid and ammonium sulfate collected from the aerosol phase on a Fluoropore filter has been readily accomplished using techniques of chemical ionization mass spectrometry combined with thermal separation. Thermal separation of ammonium hydrogen sulfate from ammonium sulfate was not possible. Spectral separation of these species by selective ionization is proposed. Analysis of sulfate aerosols collected from ambient air and catalyzed vehicle emissions is described. It was found that sulfuric acid aerosol was rapidly converted to ammonium sulfate or ammonium hydrogen sulfate in the presence of ambient concentrations of ammonia. Ambient samples collected in the Detroit metropolitan area have been found to contain only trace quantities of sulfuric aicd. Sulfate samples collected from a dilution tube into which catalyzed vehicle exhaust was injected were found to contain significant quantities of ammonium sulfate in addition to sulfuric acid. The rate of sulfate specie volatilization from the filter media was found to be markedly affected by the ammonium sulfate particle size on the filter, thus necessitating the generation of appropriate standards. Impregnation of the collection filter with an appropriate material that will convert sulfuric acid upon collection to a species which is unaffected by ambient ammonia concentrations can permit subsequent quantitation of ambient H₂SO₄ by mass spectrometry.