Measurements of benzene and toluene in ambient air using proton-transfer-reaction mass spectrometry: calibration, humidity dependence, and field intercomparison

PTR-MS (proton transfer reaction-mass spectrometry) is a chemical ionization mass spectrometry technique that uses proton transfer reactions with H3O+ ions for on-line measurements of organic trace gases in air. The instrument is calibrated for benzene and toluene, and the humidity dependence is investigated. The observed humidity dependence is explained using a simple model that calculates the distribution of H3O+. (H2O)(n) (n = 0, 1, 2, 3) cluster ions in the reactor. These findings were verified in a field intercomparison by comparing PTR-MS measurements of benzene and toluene with GC (gas chromatograph) analyses of gas samples. Isoprene, acetone, acetonitrile, methanol, dimethyl sulfide, and acetaldehyde were also investigated, and no humidity dependence was found, except for isoprene, when larger clusters were used as primary ions. (C) 2001 Elsevier Science B.V.