Atmospheric sampling Townsend discharge ionization mass spectrometry for analysis of gas-phase mixtures

Atmospheric sampling Townsend discharge ionization (ASTDI) mass spectra of 65 different volatile organic compounds within 12 different functional groups have been evaluated. The sample air was introduced directly into the reduced pressure Townsend ion source by means of a deactivated capillary. The positive ion ASTDI mass spectrum of dry background air was dominated by the two precursor ions, O2+ (100%) and NO+ (30%). Hydride abstraction, proton transfer, charge exchange and NO+ addition produced mass spectra with detailed fragmentation useful for structural elucidation. Quasi-molecular ions (e.g., [M + H]+) and adducts ions (e.g., [M + NO]+) indicated the molecular weight for nearly all the investigated compounds. H3O+ emerged in the background spectrum of humid sample air, which produced, e.g., [M + H]+ from hydrogen transfer for most oxygenated compounds. By dilution of the sample air with methane prior to the Townsend source, O2+ was depleted and C2H5+ became the most intense reagent ion. This favored formation of [M + H]+, whereas NO adduct ions diminished in the resulting ASTDI(Me) spectra. Identification of the molar weight and functional group was possible, since each functional group exhibited a unique combination of quasi-molecular and NO+ adduct ions.