NEGATIVE ION CHEMICAL IONIZATION MASS SPECTROMETRY OF POLYCYCLIC AROMATIC HYDROCARBONS

The negative ion chemical ionization (NICI) mass spectrometry of thirty-nine polycyclic aromatic hydrocarbons (PAH) was investigated. The methane NICI mass spectra of PAH are dominated by the molecular anion for species which show high NICI sensitivity, and by the molecular anion plus adduct ions for insensitive species. PAH adducts are formed under NICI conditions between the molecular PAH species in the gas phase and radical species present in the methane plasma. The alkyl-PAH adduct species is subsequently ionized by resonance capture of a low energy thermal electron. NICI spectra acquired using methane and deuteromethane as the reagent gases were used to elucidate two reaction schemes leading to the formation of the observed adduct ions. The absolute sensitivities for individual PAH varied over more than three orders of magnitude under methane NICI conditions, and the sensitivity and selectivity of detection for isomeric PAH were found to depend on the pressure and temperature in the ion source of the mass spectrometer. The selectivity of detection observed for parent PAH species was also observed for their alkylated homologues. The selectivity of NICI for isomeric PAH was used for the qualitative and quantitative determination of trace levels of PAH and alkylated-PAH in samples of a petroleum crude oil, diesel particulate matter, and urban air particulate matter.