Grantee Research Project Results
2001 Progress Report: A New Application of the Fundamental Physics of Atmospheric Pressure Ionization Mass Spectrometry to Ozone and Fine Particulate Formation Mechanisms
EPA Grant Number: R828179Title: A New Application of the Fundamental Physics of Atmospheric Pressure Ionization Mass Spectrometry to Ozone and Fine Particulate Formation Mechanisms
Investigators: O'Brien, Robert J. , Atkinson, Dean B. , Hard, Thomas M.
Current Investigators: O'Brien, Robert J. , Hard, Thomas M. , Atkinson, Dean B.
Institution: Portland State University
EPA Project Officer: Hahn, Intaek
Project Period: July 1, 2000 through June 30, 2002
Project Period Covered by this Report: July 1, 2001 through June 30, 2002
Project Amount: $223,574
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Water , Land and Waste Management , Air , Safer Chemicals
Objective:
The objectives of this research project involve the development and application of novel analysis procedures for the study of gas phase oxidation mechanisms relevant to the production of ozone, secondary volatile organic compound (VOC) oxidation products, and fine particulates in the atmosphere. Past work using a high resolution mass spectrometer has demonstrated that is was possible to generate and detect the reaction products of hydroxyl radical with VOCs added to the inlet of an atmospheric pressure ionization source. This work circumvents traditional problems with detecting transient and/or condensable products of these atmospherically relevant oxidation reactions, while conducting the reactions at atmospheric pressure. Without using any federal funding, we used a Varian Saturn 2000 Ion Trap Mass Spectrometer purchased by Portland State University. We have redesigned the source/reactor to allow more flexibility and better suit the ion trap mass spectrometer. Ion trap mass spectrometers are a highly sensitive and are a flexible type of bench-top mass spectrometer, which are only now coming into widespread use, mostly in gas chromatography/mass spectrometry (GC/MS) applications. The ability to conduct MS/MS experiments within the ion trap is a major advantage for these studies where several product ions with the same mass:charge ratio may be produced by the neutral reactions and ionization/fragmentation process. In the course of our studies, we have determined that many types of atmospherically relevant reactions, including ozonolysis of alkenes, are amenable to study by this system.
Progress Summary:
We have designed and tested two new interfaces between a commercial ion-trap mass spectrometer (ITMS) and atmospheric pressure reactors commonly used in the study of atmospheric chemical reactions. The instrument used is the Varian Saturn 2000 GC-MS system. We have removed the gas chromatograph (GC) and replaced it with a direct-sampling interface between the two atmospheric-pressure reactors and the ITMS. The interface produces flow rates similar to those previously supplied by the GC column. The interface is designed to be clean (not producing compounds detected by the mass spectrometer) and nonreactive with the compounds to be studied, through a combination of inherent inertness and short residence times within the interface.
Prior applications of MS to high pressure reaction systems have employed chemical ionization in the high-pressure region. We have chosen instead to perform the chemical (or electron) ionization in the low pressure region inside of the mass spectrometer to avoid complications in the high pressure reaction system and uncertainties associated with the transport of ions into the mass spectrometer. The resulting loss in ionization efficiency is partially offset by the increased sensitivity of the ITMS relative to other mass spectrometric methods. Another feature of the ITMS is the chemical ionization (CI) method, where individual ions are selected by mass:charge ratio after electron ionization formation and are then allowed to react with the neutral molecules entering through the interface. This significantly reduces the possibility that the ions observed in the mass spectrum may be produced by direct electron ionization/fragmentation of components of the reaction mixture or impurities. The use of CI (which produces far less fragmentation of compounds that electron ionization (EI)) significantly simplifies these studies, which always involve the simultaneous detection of a large number of neutral species.
The first interface is designed to sample a small (12 L) static reaction chamber. It consists of a tapered glass capillary, which conducts the reaction mixture from the static reaction chamber to the low pressure ionization region of the ITMS. The second interface is designed to sample the exhaust of a tubular atmospheric-pressure flow reactor. It consists of a 15 m stainless steel orifice placed only a few millimeters from the low pressure ionization region of the mass spectrometer. The predominant gas is helium in both reactors (required for operation of the ITMS) with sufficient oxygen added to convert nascent-free radicals to the peroxy forms that prevail under real atmospheric conditions. The other components of the reaction mixture, hydrocarbons, ozone, etc. are added at ppm levels with very short reaction times, which can be readily varied to probe the reaction mechanism.
We are applying both interface/reactor systems to the study of product distributions and kinetics in ozone-alkene reactions. The two reactors provide complementary information. In the static reactor, reaction times are much shorter than diffusion times, so we see only long-term products, distinguished by extraneous effects such as differential adsorption and desorption from surfaces. In the tubular reactor, with moveable injector, reaction times of 0.1 to 2 seconds allow us to observe the kinetic behavior of the reaction and the formation of earlier products.
With the static reactor, our preliminary explorations of ozone-alkene reactions showed the ability to detect the decay of reactants and growth of numerous products by chemical ionization. Similar results were obtained with electron ionization, but the mass spectra were considerably more complicated and difficult to interpret. More detailed experimental study of the reaction of ozone with 2,3-dimethylbutene (tetramethylethylene or TME), using both reactors, shows several distinct products. Ozone reaction with TME makes a useful benchmark study for this new method, because it has been extensively studied in the past. Some of the products were subjected to MS/MS analysis and the CID patterns were consistent with products identified in previous studies. In the flow reactor, the persistence of all these products when hydroxyl radicals are suppressed (by the addition of competing reagents) indicates that the detected products do not arise exclusively from the reaction of hydroxyl radicals with the alkene. The use of competitive hydroxyl radical scavengers also will allow us to establish the quantum yield of this key atmospheric channel in future studies of alkenes whose ozone reaction mechanisms are not as well established. The ability to observe the change in product yields of the ozone reaction simultaneously is a major strength of this approach.
Future Activities:
Further investigations of the ozone-alkene reaction system, using the ion trap mass spectrometer to measure reactant decay and product growth, in both the tubular flow reactor and the slow-flow chamber, are planned. After this, we will investigate the reactions of hydroxyl radical with a range of hydrocarbons.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 2 publications | 1 publications in selected types | All 1 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Wedian F, Atkinson DB. Ozone modulation of volatile hydrocarbons using membrane introduction mass spectrometry. Environmental Science & Technology 2002;36(19):4152-4155. |
R828179 (2001) R828179 (Final) |
not available |
Supplemental Keywords:
air, ozone, particulates, volatile organic compounds, VOC, oxidants, environmental physics, environmental chemistry., RFA, Scientific Discipline, Air, Toxics, particulate matter, air toxics, Environmental Chemistry, HAPS, VOCs, tropospheric ozone, Atmospheric Sciences, Engineering, Chemistry, & Physics, Environmental Engineering, hydroxyl radical, particle size, particulates, stratospheric ozone, aerosol particles, fine particles, mass spectrometry, hydrocarbon, biogenic modeling, air modeling, ozone, atmospheric pressure ionization, ambient air, ambient emissions, chemical composition, air pollution models, treatment, biogenic hydrocarbons, hydronium, biogenic hydrocarbon mixing, hydroxyl radicals, photochemical processes, fine particulate formation, airshed models, ambient aerosol particlesProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.