Dept. of Environmental Science and Engineering
Oregon Graduate Center
Most analytical techniques used to study atmospheric oxidation have lacked the sensitivity necessary to determine oxidation products directly in the gas phase. Loss or alteration of products can occur during concentration or separation steps before analysis. Atmospheric pressure ionization (API) mass spectrometry is potentially a very powerful technique for such studies. API provides high sensitivity for hydrocarbon oxidation products containing heteroatoms (0 and N). An API source for a VG 7070E-HF mass spectrometer was fabricated and successfully operated at an accelerating potential of 6 kV to provide medium resolution (4,800, 10% valley, m/z 92, 1% of the maximum ion signal). This resolution is more than sufficient to separate mass peaks due to oxidized hydrocarbons having the same nominal mass, but differing by -CH2CH2- vs. >C=O groups. The mass spectra obtained can contain hydrated, fragmented, and molecular ions; multiple ions from each analyte in a gas mixture complicate spectra and decrease sensitivity. Manipulation of three variables, the relative humidity of the sample gas, the pressure in the corona discharge region, and the potential across the collisionally induced dissociation region, can be used to simplify the mass spectra, to maximize sensitivity for analytes, and to differentiate between hydrated, fragmented, and molecular ions. Spurious oxidation products formed by free radical reactions with OH radicals generated in the API source interfered with oxidation studies. Strategies for suppressing peaks in the mass spectra due to the interfering chemistry are demonstrated: 1) operating at the lowest sustainable corona discharge current, 2) adding CO to scavenge OH radicals, and 3) modestly increasing the flow of sample gas through the API source. A dynamic range of two decades and a detection limit of 8 ppb are demonstrated for benzaldehyde, and factors causing nonlinearity in the response curve are discussed. Several toluene oxidation products, identified after concentration or separation steps in previous studies, were found in our reaction vessel gas. Using medium resolution the mass peaks due to these products were separated from each other and from background peaks. Exact masses were then determined and used to assign molecular formulae to the ions corresponding to the product peaks.
Grange, Andrew H., "Medium resolution atmospheric pressure ionization mass spectrometry" (1988). Scholar Archive. 255.