Chemical Ionization Mass Spectrometry

bknightonchemistry.montana.edu

Dr. Knighton has been actively involved in the development and application of chemical ionization mass spectrometry for nearly 20 years. That interest continues today and is primarily focused on using drift tube reaction mass spectrometry for the quantification of trace level volatile organic compound (VOC) emissions from a wide variety of natural and anthropogenic sources. A significant fraction of this research effort involves field work where a proton transfer mass spectrometer (PTR-MS), a commercially produced drift tube reaction mass spectrometer, has been used to measure selected volatile organic trace gases in the ambient atmosphere and in the emissions from vehicles and aircraft. The PTR-MS instrument merges the concept of chemical ionization with that of the swarm technique of flow-drift tubes. Chemical ionization is based on H3O+ as the primary reagent ion, which does not react with the major components of clean air, but does react with most non-alkane VOC's via proton transfer reactions. The electric field used to transport the ions through the drift tube also provides sufficient additional energy to the ions to discourage association reactions with water molecules that are always present in any real sample. The selectivity provided by chemical ionization, control of unwanted hyfration reactions by using drift tube coupled with the sensitivity of mass spectrometry detection makes the PTR-MS a powerful analytical tool capable of rapid in-situ measurement of trace chemical species with proton affinities greater than that of water. Because the PTR-MS provides sensitive (sub-ppbv) real-time (~1 second) measurement of selected hydrocarbon components, it is ideally suited for monitoring systems that have rapidly changing chemical composition like that of engine exhaust emissions. The MSU PTR-MS system has been deployed in a wide variety of field programs including the measurement of aerosol trace gas precursor species in jet engine exhaust in the Aircraft Particle Emissions eXperiment (APEX)-2004, APEX2-2005 and APEX3-2005, on-board the Aerodyne Mobile Laboratory for the measurement of selected VOC's in Mexico City Urban Air Quality field measurement campaigns of 2002, 2003 and 2006. In addition, we have used the PTR-MS technique in the study of a diverse set of matrices including the monitoring of flavor compounds in human breath, volatile emission products of an endophytic fungus, Muscodor Albus, and those released from softwood lumber during the kiln drying process.

Keywords:
Analytical, Spectroscopy

To continue to refine and improve drift tube reaction mass spectrometry for the analysis of selected hydrocarbons in challenging measurement environments. The analysis of complex mixtures directly by chemical ionization spectrometry techniques like the PTR-MS is often complicated by ion fragmentation and ionization of interfering compounds. Two important air toxic compounds 1,3-butadiene and acrolein are listed as emission components in aircraft exhaust. Presently there are no existing technologies capable of reliably measuring these components in aircraft exhaust. Knowledge about the emissions of compounds from aircraft is important towards understanding the influence that commercial aircraft may have on human health. While the PTR-MS provides a response to both 1,3-butadiene and acrolein, interferences from other aircraft exhaust compounds notably water and the isomeric butenes makes the direct analysis of these componenets impossible. Funding from NSF and FAA is being used to work on a series of techniques to allow for the analysis of these compounds. These include: 1) Designing simple chemical scrubbers that selectively remove interfering compounds so that traditional PTR-MS techniques can be employed. 2) Discovering and learning how to produce new reagent ions that react selectively with either 1,3-butadiene and acrolein but not the other interfering compounds. 3) Develop MS/MS methods using a newly built triple quadrupole drift tube reaction mass spectrometer. The first study envisioned for this instrument is to use MS/MS to resolve mixtures of protonated acrolein C3H4OH+ and protonated butene C4H8H+.

Selected Publications

E. Velasco, B. Lamb, H. Westberg, E. Allwine, J. L. Arriaga-Colina, B. T. Jobson, M. Alexander, P. Prazeller, W. B. Knighton, T. M. Rogers, M. Grutter, S. C. Herndon, C. E. Kolb, M. Zavala, B. de Foy, R. Volkamer, L. T. Molina, M. J. Molina:
Distribution, magnitudes, reactivities, ration of diurnal patterns of volatile organic compounds in the Valley of Mexico during the MCMA 2002 and 2003 field campaigns
Atmos. Chem. Phys. Discuss., 6, 7563 (2006)

M. Zavala, S. C. Herndon, R. S. Slott, E. J. Dunlea, L. C. Marr, J. H. Shorter, M. Zahniser, W. B. Knighton, T. M. Rogers, C. E. Kolb, L. T. Molina and M. J. Molina:
Characterization of on-road emissions in the Mexico City Metropolitan Area using a mobile laboratory in chase and fleet average modes during the MCMA-2003 field campaign
Atmos. Chem. Phys. Discuss., 6, 4689 (2006).

Scott C. Herndon, Todd Rogers, Edward J. Dunlea, John T. Jaynes, RIchard miake-Lye and Berk Knighton:
Hydrocarbon emissions from in-use commercial aircraft during airport operations.
Environ. Sci. Technol. 40, 4406 (2006).

T. M. Rogers, E. P. Grimsrud, S. C. Herndon, J. T. Jayne, C. E. Kolb, E. Allwine, H. Westburg, B. K. Lamb, M. Zavala, L. T. Molina, M. J. Molina and W. B. Knighton:
On-road measurements of volatile organic compounds in the Mexico City metropolitan area using proton transfer reaction mass spectrometry
Int. J. Mass Spec., 252, 26 (2006)

Scott C. Herndon, John T. Jayne, Mark S. Zahniser, Douglas R. Worsnop, Berk Knighton, Eugene Allsine, Brian K. Lamb, Miguel Zavala, David D. nelson, J. Barry McManus, Joanne H. Shorter, Manjulas R. Canagaratna, Timothy B. Onash and Charles E. Kolb:
Characterization of urban pollutant emisssion fluxes and ambient concentration distributions using a mobile laboratory with rapid response instrumentation
Faraday Discuss., 130, 1 (2005)

M. Jiang, L. C. Marr, E. J. Dunlea, S. C. Herndon, J. T. Jayne, C. E. Kolb, W. B. Knighton, T. M. Rogers, M. Zavala, L. T. Molina, M. J. Molina:
Mobile laboratory measurements of black carbon, polycyclis aromatic hydrocarbons and other vehicle emissions in Mexico City
Atmos. Chem. Phys., 5, 3377 (2005)

David Ezra, Justin Jasper, Todd Rogers, Berk Knighton, Eric Grimsrud and Gary Strobel:
Proton transfer reaction-mass spectrometry as a technique to measure volatile emissions of Muscodor albus
Plant Science, 166, 1471 (2004)

Jennifer B. Mei, Gary A. Reineccius, W. Berk Knighton and Eric P. Grimsrud:
The Influence of Strawberry Yogurt Composition on Aroma Release
J. Agric. and Food Chem., 52, 6267 (2004)

Brad Fritz, Brian Lamb, Hal Westberg, Richard Folk, Berk Knighton and Eric Grimsrud:
Pilot- and full-scale measurements of VOC emissions from lumber drying on Inland Northwest species
Forest Prod. J., 54(6), 50 (2004)

Keywords:
Analytical, Spectroscopy