Grantee Research Project Results
2000 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.
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, 2000 through June 30, 2001
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 objective of this research project is to understand the complex oxidation products and atmospheric reaction mechanisms of atmospheric volatile organic compounds (VOCs) and air toxics. Aromatic and terpenic hydrocarbons are among the target atmospheric compounds studied. Oxidation products and atmospheric reaction mechanisms have proved difficult to analyze using conventional methods of analysis such as gas chromatography-mass spectrometry (GCMS).
Under past U.S. Environmental Protection Agency (EPA) support, we have developed an atmospheric pressure ionization source for a double-focusing, high resolution mass spectrometer (MS) and used it to study reactions that mimic atmospheric reactions, but occur in the ion source where hydroxyl radical is generated from the reaction 2H2O H3O+ + OH+ e-. The prior study was conducted with the collaboration of the Oregon Graduate Institute where the MS was located. This MS is no longer extant; the project proposed the development of an atmospheric pressure ionization (API) source for an ion trap MS (Saturn 2000) purchased by Portland State University (PSU) at no cost to the grant.
The high resolution MS used in the prior study cost more than $500,000 and occupied the greater part of a room, while the ion trap MS is a benchtop GCMS instrument that cost approximately $60,000. Both instruments have their particular strengths and weaknesses, with the ion trap representing newer technology, but the double focusing instrument encompassing a wide range of abilities. After receiving this grant, we contacted Andrew Grange who currently works for an EPA contractor in Las Vegas and who was a graduate student involved in the development of the API source for the high-resolution instrument. He put us in contact with used instrumentation vendors who indicated that high-resolution instruments (VG 7070 series) were coming onto the used instrument market at very reasonable prices. We developed contact with two such vendors with the intent of purchasing a used VG instrument onto which we could directly fit our existing API source. We realized that this purchase would take 3-6 months to accomplish, but appeared to be within our budgeted amount for new source development. Thus, we decided to postpone the initiation of grant research until such an instrument was acquired. Our plan was to apply, if necessary, for a no-cost extension to this grant. We would use the source development capital equipment budget to acquire the instrument and would not need to construct the source, because our previous source would be directly usable. The equipment vendor initially targeted a suitable instrument at Ohio State University (OSU), but after 2 months, this instrument proved to be nonfunctional. Another instrument was located at the University of Washington (UW) in Seattle. UW personnel assured us that this instrument was in good working condition. We attempted to purchase this instrument directly for $10,000, but property management insisted in placing it out on bid. This process took about 6 weeks, and no bids were received. At this point UW was willing to sell us the instrument for our available funds of $10,000. Prudently, we agreed to purchase the instrument if it proved to be in working condition as claimed. They were ultimately unable to get the instrument running in Seattle, at which point we declined its purchase and decided to proceed with the study using our existing ion trap MS.
Therefore, work on the study of the target atmospheric oxidation mechanisms commenced this summer. As originally proposed, the first phase of the study validated our novel experimental approach through the study of well-understood oxidation mechanisms of simple VOC compounds. The ability to reproduce known behavior will indicate the suitability of the technique for the study of unknown mechanisms. This work currently is underway.
It is advisable to briefly describe the operational parameters of the ion trap MS, which differ significantly from previous manifestations of mass spectrometry. In either magnetic sector, magnetic/electric sector or quadrupole MS instruments the ionization and detection occur essentially "on the fly." Ions are created, accelerated towards the detector, mass-selected, and detected in a very short time. In contrast, in the ion trap MS, ions are created and then "stored" in the "trap," where they undergo a circular motion. In this fashion, ions may be accumulated, giving greater sensitivity than some other forms of MS. Once sufficient ion numbers have been accumulated, the ion trap field is modulated to eject ions of a successive mass/charge ratio to the detector. Also, in contrast to traditional forms of MS, where high vacuum is required to prevent the collision of analyte ions with background gas, the ion trap operates most efficiently with a low background helium pressure. Varian Analytical markets our Saturn 2000 ion trap as an integrated GCMS instrument with dedicated software and hardware. We knew from the onset that this would provide one challenge in that the system is not dedicated for research. After commencing the study, we soon discovered a significant property of the ion trap GCMS system. Ion fragmentation patterns differ significantly from those obtained in other forms of MS for which extensive libraries have been developed. Thus, in submitting a known compound for ionization and then identification via library searching, we determined the following:
· Aromatic hydrocarbons (one target VOC family) are adequatly identified, although nuances such as differences among o-, m-, and p-xylene are not the same as with conventional MS.
· Alkane hydrocarbons have quite different fragmentation patterns such that the library search routine cannot correctly identify them.
· Aldehydes (target oxidation products) are intermediate.
· Other classes of VOCs currently are undetermined.
We contacted Varian, who seemed either unaware or unappreciative, but told us that we could construct our own libraries, which we have done.
Because the Varian Saturn 2000 is intended to be a dedicated GCMS system, significant alterations had to be performed to adapt this instrument for the proposed research. Modifications accomplished to date include the following:
· We removed the fixtures associated with the connection of the chromatography column to the inlet Helium carrier gas and to the MS itself, and replaced them with a small, simple, prototype reaction system designed to give us familarity with the applicability of the ion trap to the proposed research. This has involved remachining of some dedicated Varian components in our machine shop. In these modifications, we have employed the previous gas chromatography (GC) oven as a temperature controlled reactor. Initial experimentation has shown (as anticipated) that a glass reactor (glass is polar) has unacceptable adsorbance (retention) of polar VOC oxidation products. Thus, after initial experimentation, we have switched to small teflon reactors. We are able to fabricate these reactors in the lab. These reactors are easily cleaned or replaced after they become contaminated with oxidation products of previous reactions. Furthermore, their adsorptive properties are superior to those of glass.
· The resident software on the Saturn 2000 is dedicated to the acquisition of a mass chromatogram. Thus, we have had to adapt our experimentation to mimic those of a mass chromatogram. Furthermore, the data storage format of the Saturn 2000 (being entirely dedicated to chromatography) is wholly inadequate for our purposes. This latter problem has been overcome by a combination of software we have written ourselves, and subsidiary programs which Varian has been kind enough to provide us with and which work in concert with the dedicated software.
· A major problem that has been successfully overcome is sample introduction to the Saturn 2000. It expects a flow of approximately 2 std cc/min of helium carrier gas eminating from the end of the chromatography capillary column. This is a major consideration for two reasons: (1) mass spectrometric processes are currently predicated on a Helium environment within the ion trap; and (2) the MS pumping system cannot maintain sufficient vacuum with higher flow rates.
Initially, we employed a variety of flow regulating valves at the remachined MS entrance without satisfactory performance. However, we found a newly available commercial flow regulating valve, which gives excellent performance in our initial, prototype experimentation.
· Although the MS has no pressure gauge, we can regulate the pressure within the MS to the correct level by monitoring the current of the turbomolecular pump while adjusting the valve.
· In the targeted GCMS approach of the Saturn 2000, compounds to be identified and quantified are eluted successively from the GC column to the ion trap where they are ionized, fragmented and identified by library searching (see previous). In our oxidation study, several different oxidation products will be present simultaneously within the ion trap.
· The source reconciliation program UNMIX, developed under EPA support by Ron Henry of the University of South Carolina (USC), has been obtained. This program is designed to take as input the time varying composition of atmospheric aerosol or VOC compounds. After state-of-the-art statistical analysis, UNMIX outputs the individual source emission profiles. Although UNMIX has only been applied to source characterization, we have recognized that the problem of assignment of individual ions to fragmentation of multiple parent ions is an identical problem, and therefore should be completely amenable to solution of UNMIX. To that end, we have conducted an extensive program of testing and quantification of the abilities and reliability of UNMIX. This work should be separately publishable.
With these developments, we have commenced experimentation with the study of known oxidation processes as described above. For our proof-of-principle study, we have choosen the important atmospheric ozone/alkene reaction. Initially, our experiments have been directed to the reaction of ozone with simple alkenes. This approach is taken with the goal to quantify oxidation products before attacking more complex oxidation pathways of more complex VOC compounds. As two examples, we have followed the reaction of 2-pentene and tetrametnylethylene within our miniature reactor upon the addition of ozone. Although these reactions are (in fact) quite complex, they can be oversimplified with the following description.
O2 |
CH3-CH2=CH2-CH2-CH3 + O3 CH3CH=O + CH3CH2CH=O |
O2 | |
(CH3) 2C=C(CH3) 2 + O3 | 2 (CH3)2=O |
In the former, two aldehydes are generated as products, while in the latter, a ketone is generated. Additional minor products also are expected. We have been successful in following the course of this experiment, identifying and quantifying the major products, as well as some of the minor products.
Currently, we are studying these reactions within the GC reactor mounted within the GC oven. In addition, we have documented the fragmentation patterns and quantitative response of expected aldehydic products. An extensive range of experiments has been conducted to determine the most appropriate conditions under which to conduct these studies.
Future Activities:
We will continue to study these reactions within the GC reactor mounted within the GC oven.
Journal Articles:
No journal articles submitted with this report: View all 2 publications for this projectSupplemental Keywords:
volatile organic compound, VOC, VOC oxidation, fine particulates, air, RFA, toxics, atmospheric sciences, engineering, chemistry, physics, environmental chemistry, hazardous air pollutants, HAPS, particulate matter, tropospheric ozone, aerosol particles, air modeling, air pollution models, airshed models, ambient aerosol particles, ambient air, ambient emissions, atmospheric pressure ionization, biogenic hydrocarbon mixing, biogenic hydrocarbons, biogenic modeling, chemical composition, fine particle formation, fine particles, fine particulate formation, hydrocarbon, hydronium, hydroxyl radical, hydroxyl radicals, mass spectrometry, ozone, particle size, particulates, photochemical processes, stratospheric ozone., RFA, Scientific Discipline, Air, Toxics, particulate matter, air toxics, Environmental Chemistry, HAPS, VOCs, Atmospheric Sciences, tropospheric ozone, Environmental Engineering, Engineering, Chemistry, & Physics, 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.