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 ionizatio n mass spectrometry technique that uses proton transfer reactions with H3O ions for on-line measurements of organic trace gases in air. The instrumen t is calibrated for benzene and toluene, and the humidity dependence is inv estigated. The observed humidity dependence is explained using a simple mod el that calculates the distribution of H3O+. (H2O)(n) (n = 0, 1, 2, 3) clus ter ions in the reactor. These findings were verified in a field intercompa rison by comparing PTR-MS measurements of benzene and toluene with GC (gas chromatograph) analyses of gas samples. Isoprene, acetone, acetonitrile, me thanol, dimethyl sulfide, and acetaldehyde were also investigated, and no h umidity dependence was found, except for isoprene, when larger clusters wer e used as primary ions. (C) 2001 Elsevier Science B.V.