2009. "Contamination from electrically conductive silicone tubing during aerosol chemical analysis." Atmospheric Environment 43(17):2836-2839. Abstract Electrically conductive silicone tubing is used to minimize losses in sampling lines during the analysis of airborne particle size distributions and number concentrations. We report contamination from this tubing using gas chromatography-mass spectrometry (GC-MS) of filter-collected samples as well as by particle mass spectrometry. Comparison of electrically conductive silicone and stainless steel tubing showed elevated siloxanes only for the silicone tubing. The extent of contamination increased with length of tubing to which the sample was exposed, and decreased with increasing relative humidity.
2009. "Rapid Measurement of Emissions from Military Aircraft Turbine Engines by Downstream Extractive Sampling of Aircraft on the Ground: Results for C-130 and F-15 Aircraft." Atmospheric Environment 43(16):2612-2622. doi:10.1016/j.atmosenv.2009.02.012 Abstract Aircraft emissions affect air quality on scales from local to global. About 10% of the aviation fuel used in the U.S. is consumed by military aircraft, and emissions from this source are facing increasingly stringent environmental regulations, so improved methods for quickly and accurately determining emissions from existing and new engines are needed. This paper reports results of a study to advance the methods used for detailed characterization of military aircraft emissions, and provides emission factors for two aircraft; the F-15 fighter and the C-130 cargo plane. The new approach employs a strategy of outdoor ground-level sampling downstream behind operational military aircraft. This permits rapid change-out of the aircraft so that engines can be tested quickly on operational aircraft Measurements were made at throttle settings from idle to afterburner using a simple extractive probe in the dilute exhaust. Emission factors determined using this approach agree very well with those from the traditional method of extractive sampling at the exhaust exit. Emission factors are reported for CO2, CO, NO, NOx, and more than 60 hazardous and/or reactive organic gases. Intra-engine and engine to engine variability were assessed. For both engines, the effect of engine power on emissions was as expected, with higher power leading to reduced emission factors for CO and organic gases and higher emission factors for nitrogen oxides. At afterburner power, the F-15 engine yielded higher emission factors for CO and many organics and lower NOx emission factors compared with the military power throttle setting. The C-130 turboprop engine generally produced higher CO andorganic emissions and lower NOx emissions per unit of fuel consumed than the F-15 engines. Comparison of the emissions of nine hazardous air pollutants from these two engines with emissions from nine other aircraft engines also is discussed.
2009. "The Time Evolution of Aerosol Size Distribution Over the Mexico City Plateau." Atmospheric Chemistry and Physics 9(13):4261-4278. Abstract As part of the MILAGRO field campaign, the DOE G-1 aircraft was used to make measurements over and downwind of Mexico City with the objective of determining growth characteristics of aerosols from a megacity urban source. This study focuses on number concentration and size distributions. It is found that a 5-fold increase in aerosol volume is accompanied by about a 5-fold increase in accumulation mode number concentration. There is growth in aerosol volume because there are more accumulation mode particles, not because particles are larger. Condensation and volume growth laws were examined to see whether either is consistent with observations. Condensation calculations show that the growth of Aitken mode particles into the accumulation mode size range gives the required increase in number concentration. There are minimal changes in the accumulation mode size distribution with age, consistent with observations. Volume-growth in contrast yields a population of large particles, distinctly different from what is observed. Detailed model calculations are required to translate our observations into specific information on the volatility and properties of secondary organic aerosol.
2009. "Evaluating Simulated Primary Anthropogenic and Biomass Burning Organic Aerosols during MILAGRO: Implications for Assessing Treatments of Secondary Organic Aerosols." Atmospheric Chemistry and Physics 9(16):6191-6215. Abstract Simulated primary organic aerosols (POA), as well as other particulates and trace gases, in the vicinity of Mexico City are evaluated using measurements collected during the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaigns. Since the emission inventories and dilution will affect predictions of total organic matter and consequently total particulate matter, our objective is to assess the uncertainties in predicted POA before testing and evaluating the performance of secondary organic aerosol (SOA) treatments. Carbon monoxide (CO) is well simulated on most days both over the city and downwind, indicating that transport and mixing processes were usually consistent with the meteorological conditions observed during MILAGRO. Predicted and observed elemental carbon (EC) in the city was similar, but larger errors occurred at remote locations since the CO/EC emission ratios in the national emission inventory were lower than in the metropolitan emission inventory. Components of organic aerosols derived from Positive Matrix Factorization and data from several Aerodyne Aerosol Mass Spectrometer instruments deployed both at ground sites and on research aircraft are used to evaluate the model. Predicted POA was consistently lower than the measured organic matter at the ground sites, which is consistent with the expectation that SOA should be a large fraction of the total organic matter mass. A much better agreement was found when predicted POA was compared with the sum of "primary anthropogenic" and "primary biomass burning" components on days with relatively low biomass burning, suggesting that the overall magnitude of primary organic particulates released was reasonable. The predicted POA was greater than the total observed organic matter when the aircraft flew directly downwind of large fires, suggesting that biomass burning emission estimates from some large fires may be too high. Predicted total observed organic carbon (TOOC) was also analyzed to assess how emission inventory estimates of volatile organic compounds may impact predictions of SOA.
2009. "Emission and Chemistry of Organic Carbon in the Gas and Aerosol Phase at a Sub-Urban Site Near Mexico City in March 2006 During the MILAGRO Study." Atmospheric Chemistry and Physics 9(10):3425-3442. Abstract Volatile organic compounds (VOCs) and carbonaceous aerosol were measured at a sub-urban site near Mexico City in March of 2006 during the MILAGRO study (Megacity Initiative: Local and Global Research Objectives). Diurnal variations of hydrocarbons, elemental carbon (EC) and hydrocarbon-like organic aerosol (HOA) were dominated by a high peak in the early morning when local emissions accumulated in a shallow boundary layer, and a minimum in the afternoon when the emissions were diluted in a significantly expanded boundary layer and, in case of the reactive gases, removed by OH. In comparison, diurnal variations of species with secondary sources such as the aldehydes, ketones, oxygenated organic aerosol (OOA) and water-soluble organic carbon (WSOC) stayed relatively high in the afternoon indicating strong photochemical formation. Emission ratios of many hydrocarbon species relative to CO were higher in Mexico City than in the U.S., but we found similar emission ratios for most oxygenated VOCs and organic aerosol. Secondary formation of acetone may be more efficient in Mexico City than in the U.S., due to higher emissions of alkane precursors from the use of liquefied petroleum gas. Secondary formation of organic aerosol was similar between Mexico City and the U.S. Combining the data for all measured gas and aerosol species, we describe the budget of total observed organic carbon (TOOC), and find that the enhancement ratio of TOOC relative to CO is conserved between the early morning and mid afternoon despite large compositional changes. Finally, the influence of biomass burning is investigated using the measurements of acetonitrile, which was found to correlate with levoglucosan in the particle phase. Diurnal variations of acetonitrile indicate a contribution from local burning sources. Scatter plots of acetonitrile versus CO suggest that the contribution of biomass burning to the enhancement of most gas and aerosol species was not dominant and perhaps not dissimilar from observations in the U.S.
2008. "Photooxidation of Alpha-Pinene at High Relative Humidity in the Presence of Increasing Concentrations of NOx." Atmospheric Environment 42(20):5044-5060. doi:10.1016/j.atmosenv.2008.02.026 Abstract The photooxidation of ~1 ppm alpha-pinene in the presence of increasing concentrations of NO2 was studied in a Teflon chamber at relative humidities from 70 - 88% and temperatures from 296 - 304 K. The loss of alpha-pinene and formation of gas phase products were followed using proton transfer reaction mass spectrometry (PTR-MS). Gas phase reaction products measured by PTR-MS and their yields include formaldehyde (5 + 1%), formic acid (2.5 + 1.4%), methanol (0.6 + 0.3%), acetaldehyde (3.9 + 1.7%), acetic acid (10 + 2%), acetone (11.5 + 3.1%), pinonaldehyde (22 + 6%), and pinene oxide (0.9 + 0.1%). There was evidence of organic nitrates in the gas phase and small peaks were tentatively assigned to norpinonaldehyde, 4-oxopinonaldehyde, propanedial, 2,3-dioxobutanal and 3,5,6-trioxoheptanal or 3-hydroxymethyl-2,2-dimethylcyclobutylethanone. The formation and growth of new particles were followed using a scanning mobility particle sizer (SMPS), and their chemical composition was probed using single particle mass spectrometry (SPLAT II). SPLAT II analysis also provided measurements of the vacuum aerodynamic diameters of the newly formed secondary organic aerosol (SOA) particles and, in combination with the electrical mobility diameter, a particle density of 1.21 + 0.02 g cm-3 was calculated, 20% larger than often assumed in calculating SOA yields. SPLAT II showed that the suspended SOA consisted of a complex mixture of organic nitrates and organics, possibly including pinonic acid, pinic acid and trans-sobrerol. Three-wavelength light scattering measurements made using an integrating nephelometer were consistent with particles having a refractive index characteristic of organic compounds, but the data could not be well matched at all three wavelengths with a single refractive index. The effect of addition of cyclohexane or NO on particle formation showed that ozonolysis was the major mechanism of SOA formation in this system. However, unlike simple ozonolysis, organic nitrates are formed in both the gas and particle phases. Identifying and measuring specific organic nitrates in both the gas and particle phases in air may help to elucidate why SOA formation has been reported in field studies to be associated with polluted urban areas, yet the carbon in these particles is largely contemporary, i.e., non-fossil fuel carbon.
2008. "Nitrate Ion Photochemistry at Interfaces: A New Mechanism for Oxidation of alpha-Pinene." Physical Chemistry Chemical Physics. PCCP 10(21):3063-3071. doi:10.1039/b719495a Abstract The photooxidation of 0.6 - 0.9 ppm α-pinene in the presence of a deliquesced thin film of NaNO3, and for comparison increasing concentrations of NO2, was studied in a 100 L Teflon® chamber at relative humidities from 70 − 88% and temperatures from 296 − 304 K. The loss of α-pinene and the formation of gaseous products were followed with time using proton transfer mass spectrometry. The yields of gas phase products were smaller in the NaNO3 experiments than in NO2 experiments. In addition, pinonic acid, pinic acid, trans-sobrerol and other unidentified products were detected in the extracts of the wall washings only for the NaNO3 photolysis. These data indicate enhanced loss of α-pinene at the NaNO3 thin film during photolysis. Supporting the experimental results are molecular dynamics simulations which predict that α-pinene has an affinity for the surface of the deliquesced nitrate thin film, enhancing the opportunity for oxidation of the impinging organic gas during the nitrate photolysis. This new mechanism of oxidation of organics may be partially responsible for the correlation between nitrate and the organic component of particles observed in many field studies, and may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. In addition, photolysis of nitrate on surfaces in the boundary layer may lead to oxidation of co-adsorbed organics.
2008. "Effects of aerosol organics on cloud condensation nucleus (CCN) concentration and first indirect aerosol effect." Atmospheric Chemistry and Physics 8(21):6325-6339. Abstract Abstract. Aerosol microphysics, chemical composition, and CCN properties were measured on the Department of Energy Gulfstream-1 aircraft during the Marine Stratus/ Stratocumulus Experiment (MASE) conducted over the coastal waters between Point Reyes National Seashore and Monterey Bay, California, in July 2005. Aerosols measured during MASE included free tropospheric aerosols, marine boundary layer aerosols, and aerosols with high organic concentration within a thin layer above the cloud. Closure analysis was carried out for all three types of aerosols by comparing the measured CCN concentrations at 0.2% supersaturation to those predicted based on size distribution and chemical composition using K¨ohler theory. The effect of aerosol organic species on predicted CCN concentration was examined using a single hygroscopicity parameterization.
2008. "The Time Evolution of Aerosol Composition Over the Mexico City Plateau." Atmospheric Chemistry and Physics 8(6):1559-1575. Abstract The time evolution of aerosol concentration and chemical composition in a megacity urban plume was determined based on 8 flights of the DOE G-1 aircraft in and downwind of Mexico City during the March 2006 MILAGRO field campaign. A series of selection criteria are imposed to eliminate data points with non-urban emission influences. Biomass burning has urban and non-urban sources that are distinguished on the basis of CH3CN and CO. In order to account for dilution in the urban plume, aerosol concentrations are normalized to CO which is taken as an inert tracer of urban emission, proportional to the emissions of aerosol precursors. Time evolution is determined with respect to photochemical age defined as −Log10 (NOx/NOy). The geographic distribution of photochemical age and CO is examined, confirming the picture that Mexico City is a source region and that pollutants become more dilute and aged as they are advected towards T1 and T2, surface sites that are located at the fringe of the City and 35 km to the NE, respectively. Organic aerosol (OA) per ppm CO is found to increase 7 fold over the range of photochemical ages studied, corresponding to a change in NOx/NOy from nearly 100% to 10%.
2008. "Development of a Micropyrolyzer for Enhanced Isotope Ratio Measurement." Industrial and Engineering Chemistry Research 47(22):8625-8630. doi:10.1021/ie8009236 Abstract This paper presents design, fabrication and testing of a micro scale reactor for the pyrolysis of organic compounds. The reactor system described here is suitable for use in enhanced isotope ratio measurement in a continuous flow mode. A characteristic of such a system is it can be utilized to pyrolyze organic compounds with sample size 20-50 times smaller than conventional. Results have shown that organic compounds, such as 1-butanol, ethanol, and ethanol amine, can be fully decomposed to desired products CO and H2, at temperature of 1200oC, which is 200oC lower than conventionally reported. Undesired products methane and CO2 are eliminated in the pyrolysis process. The proof-of-concept experimental results clearly demonstrate that the micro pyrolyzer can be readily integrated with isotope ratio mass spectrometer (IRMS) to differentiate between different sources of the same materials.
2007. "Distribution, Magnitudes, Reactivities, Ratios and Diurnal Patterns of Volatile Organic Compounds in the Valley of Mexico During the MCMA 2002 & 2003 Field Campaigns." Atmospheric Chemistry and Physics 7(2):329-353. Abstract A wide array of volatile organic compound (VOC) measurements was conducted in the Valley of Mexico during the MCMA-2002 and 2003 field campaigns. Study sites included locations in the urban core, in a heavily industrial area and at boundary sites in rural landscapes. In addition, a novel mobile-laboratory-based conditional sampling method was used to collect samples dominated by fresh on-road vehicle exhaust to identify those VOCs whose ambient concentrations were primarily due to vehicle emissions. Four distinct analytical techniques were used: whole air canister samples with Gas Chromatography/Flame Ionization Detection (GC-FID), on-line chemical ionization using a Proton Transfer Reaction Mass Spectrometer (PTR-MS), continuous real-time detection of olefins using a Fast Olefin Sensor (FOS), and long path measurements using UV Differential Optical Absorption Spectrometers (DOAS). The simultaneous use of these techniques provided a wide range of individual VOC measurements with different spatial and temporal scales. The VOC data were analyzed to understand concentration and spatial distributions, diurnal patterns, origin and reactivity in the atmosphere of Mexico City. The VOC burden (in ppbC) was dominated by alkanes (60%), followed by aromatics (15%) and olefins (5%). The remaining 20% was a mix of alkynes, halogenated hydrocarbons, oxygenated species (esters, ethers, etc.) and other unidentified VOCs. However, in terms of ozone production, olefins were the most relevant hydrocarbons. Elevated levels of toxic hydrocarbons, such as 1, 3-butadiene, benzene, toluene and xylenes, were also observed. Results from these various analytical techniques showed that vehicle exhaust is the main source of VOCs in Mexico City and that diurnal patterns depend on vehicular traffic in addition to meteorological processes. Finally, examination of the VOC data in terms of lumped modeling VOC classes and its comparison to the VOC lumped emissions reported in other photochemical air quality modeling studies suggests that some alkanes are underestimated in the emissions inventory, while some olefins and aromatics are overestimated.
2007. "Effect of Hydrophobic Primary Organic Aerosols on Secondary Organic Aerosol Formation from Ozonolysis of α-Pinene." Geophysical Research Letters 34(20):Paper # L20803. doi:10.1029/2007GL030720 Abstract Semi-empirical secondary organic aerosol (SOA) models typically assume a well-mixed organic aerosol phase even in the presence of hydrophobic primary organic aerosols (POA). This assumption significantly enhances the modeled SOA yields as additional organic mass is made available to absorb greater amounts of oxidized secondary organic gases than otherwise. We investigate the applicability of this critical assumption by measuring SOA yields from ozonolysis of α-pinene (a major biogenic SOA precursor) in a smog chamber in the absence and in the presence of dioctyl phthalate (DOP) and lubricating oil seed aerosol. These particles serve as surrogates for urban hydrophobic POA. The results show that these POA did not enhance the SOA yields. If these results are found to apply to other biogenic SOA precursors, then the semi-empirical models used in many global models would predict significantly less biogenic SOA mass and display reduced sensitivity to anthropogenic POA emissions than previously thought.
2006. "Atmospheric Oxidation in the Mexico City Metropolitan Area (MCMA) during April 2003." Atmospheric Chemistry and Physics 6(9):2753-2765. Abstract The Mexico City Metropolitan Area (MCMA) study in April 2003 provided a unique opportunity to examine atmospheric oxidation in a megacity that has more pollution than US and European cities. Most atmospheric constituents that are important for atmospheric oxidation, including the free radicals hydroxyl (OH) and hydroperoxyl (HO2), were measured. OH typically reached 7x106 cm-3, comparable to amounts observed in US cities, but HO2 reached 40 pptv in the early afternoon, substantially more than observed in most US cities. A steady-state chemistry model was able to simulate the measured OH and HO2 to within well within measurement and model errors bars except for nighttime, when measured OH was 5 times modeled and measured HO2 was 2.5 times modeled, and during morning rush hour, when measured HO2 was ~5 times modeled. We observed similar comparisons in US cities. The agreement between the measured and calculated OH reactivity to within uncertainties indicates that the volatile organic compounds important for atmospheric oxidation are known. The high calculated instantaneous ozone production rate from HO2 measurements is consistent with the high ozone levels typically observed in MCMA.
2005. "Flux Measurements of Volatile Organic Compounds from an Urban Landscape." Geophysical Research Letters 32(20):L20802, doi: 10.1029/2005GL023356. Abstract Direct measurements of volatile organic compound (VOC) emissions that include all anthropogenic and biogenic emission sources in urban areas are a missing requirement to evaluate emission inventories and constrain current photochemical modelling practices. Here we demonstrate the use of micrometeorological techniques coupled with fast-response sensors to measure urban VOC fluxes from a neighborhood of Mexico City, where the spatial variability of surface cover and roughness is high. Fluxes of olefins, methanol, acetone, toluene and C2-benzenes were measured and compared with the local gridded emission inventory. VOC fluxes exhibited a clear diurnal pattern with a strong relationship to vehicular traffic. Recent photochemical modeling results suggest that VOC emissions are significantly underestimated in Mexico City1, but the measured VOC fluxes described here indicate that the official emission inventory2 is essentially correct. Thus, other explanations are needed to explain the photochemical modelling results.
2005. "On-line Analysis of Organic Compounds in Diesel Exhaust Using a Proton Transfer Reaction Mass Spectrometer (PTR-MS)." International Journal of Mass Spectrometry 245(1-3):78-89. Abstract Chemical ionization mass spectrometry using H3O+ proton transfer in an ion drift tube (PTR-MS) was used to measure volatile organic compound (VOC) concentrations on-line in diesel engine exhaust as a function on engine load. The purpose of the study was to evaluate the PTR-MS instrument as an analytical tool for diesel engine emissions abatement research. Measured sensitivities determined from gas standards were found to be between 30% and 100% greater than calculated sensitivities. A slight humidity dependent sensitivity was observed for non-polar species, implying that reactions with H+(H2O)2 were important for some organics. The mass spectra of diesel exhaust were complex but displayed a pattern of strong ion signals at 14n+1 (n=3..8) masses, with a relative ion abundance similar to that obtained from electron impact ionization of alkanes. Laboratory experiments verified that C8-C16 n-alkanes and C8-C13 1-alkenes react with H3O+ in dissociative proton transfer reaction resulting in alkyl cation ion products, primarily m/z 41, 43, 57, 71 and 85. Monitoring the sum of these ions signals may be useful for estimating alkane emissions from unburnt diesel fuel. Alkane fragmentation likely simplified the diesel exhaust mass spectrum and reduced potential mass interferences with isobaric aromatic compounds. It is shown that the relative abundances of VOCs changed as a function of engine load. Concentrations of aldehydes and ketones dominated those of aromatic species with formaldehyde and acetaldehyde estimated to be the most abundant VOCs in the PTR-MS mass spectrum at all engine loads. The relative abundances of benzene and toluene increased with engine load indicating their pyrogenic origin. The relative abundance of alkanes, aromatics, aldehydes, and alcohols was broadly consistent with literature publications of diesel exhaust analysis by gas chromatography. About 75% of the organic ion signal could be assigned. On line analysis of diesel exhaust using this technology may be valuable tool for diesel engine emission research.
2004. "Control System for an Ion Trap Mass Spectrometer." Scientific Computing & Instrumentation (April 2004):18-20. Abstract This paper describes the development of a control system for an ion trap mass spectrometer that is both inexpensive and flexible, allowing application of ion trap technology to a number of research areas. This project is part of the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a U.S. Department of Energy national scientific user facility, located at Pacific Northwest National Laboratory (PNNL) in Richland, Washington. EMSL offers a comprehensive array of cutting-edge resources that are available to its users. Among these resources are a number of custom-built ion trap mass spectrometers (ITMS) that are are devoted to stuides in scientific areas including atmopsheric chemistry, diesel catalysis, biological applications as well as applications in defense and homeland security. The primary thrust of this effort is to develop a basic hardware and software platform for ion trap mass spectrometry (ITMS) that will allow the construction and deployment of systems with capabilities tailored to the specific needs of a variety of scientific applications. The emphasis will be on the use of chemical ionization techniques to address the need for highly sensitive and specific measurements as well as analytical needs dictated by research areas such as atmospheric chemistry, catalysis, materials science, biology, and inhalation toxicology. This ITMS platform can also be used to develop and deploy systems for critical applications such as chemical weapons detection and nuclear proliferation monitoring.
2004. "Membrane Inlet Proton Transfer Reaction Mass Spectrometry (MI-PTRMS) for direct measurements of VOCs in water." International Journal of Mass Spectrometry 239(2-3):171-177. Abstract The use of a membrane inlet proton transfer reaction mass spectrometry (MI-PTRMS) system was investigated for the quantitative analysis of VOCs directly from water. Compounds playing an important role in environmental, biological and health issues such as methanol, acetonitrile, acetone, dimethylsulfide (DMS), isoprene, benzene, and toluene have been analyzed both in fresh and salty water. The system shows very good sensitivity, reproducibility, and a linear response of up to five orders of magnitude. The detection limit for DMS is about 100 ppt and for methanol is about 10 ppb both in fresh and salty water. The response time of the various compounds across the membrane is on the order of a few minutes. This fast response and the fact that the PTRMS can perform absolute measurements without the necessity of calibration make the system suitable for on-line and on-site measurements of VOCs directly from water.
2004. "Investigation of Fundamental Physical Properties of a Polydimethylsiloxane (PDMS) Membrane using a Proton Transfer Reaction-Mass Spectrometry (PRTMS)." International Journal of Mass Spectrometry 239(2-3):179-186. Abstract A membrane introduction proton transfer reaction mass spectrometry (MI-PTRMS) has been employed for the characterisation of a polydimethylsiloxane (PDMS) membrane. For this purpose the diffusion and partition coefficients (which serves as a measure for solubility) have been determined experimentally for different classes of chemical compounds both non-polar and polar species, i.e. aromatics, alcohols, ketones. It turned out that not only polar compounds exhibit strong interaction with a hydrophobic membrane such as the PDMS, but also non polar compounds as trimethylbenzene or propylbenzene which bear a relevant number of methyl groups or an alkyl chain show strong interaction with a PDMS membrane. Stronger interaction analyte-membrane leads to a slower diffusion coefficient and larger partition coefficient. The effect of the temperature on the diffusion coefficient and partition coefficient is also investigated. At higher temperature diffusion becomes faster and solubility lower. Permeability is calculated from diffusion and partition coefficients and activation energy are derived from corresponding Arrhenius plots. The MI-PTRMS system shows detection limits in the order of tens of pptv and it’s linear over five orders of magnitude.
2003. "Proton Transfer Reaction Ion Trap Mass Spectrometer." Rapid Communications in Mass Spectrometry 17(14):1593-1599. Abstract Proton transfer reaction mass spectrometry is a relatively new field that has attracted a great deal of interest in the last few years. This technique uses H₃Oþ as a chemical ionization (CI) reagent to measure volatile organic compounds (VOCs) in the parts per billion by volume (ppbv) to parts per trillion by volume (pptv) range. Mass spectra acquired with a proton transfer reaction mass spectrometer (PTR-MS) are simple because proton transfer chemical ionization is ‘soft’ and results in little or no fragmentation. Unfortunately, peak identification can still be difficult due to isobaric interferences. A possible solution to this problem is to couple the PTR drift tube to an ion trap mass spectrometer (ITMS). The use of an ITMS is appealing because of its ability to perform MS/MS and possibly distinguish between isomers and other isobars. Additionally, the ITMS duty cycle is much higher than that of a linear quadrupole so faster data acquisition rates are possible that will allow for detection of multiple compounds. Here we present the first results from a proton transfer reaction ion trap mass spectrometer (PTR-ITMS). The aim of this study was to investigate ion injection and storage efficiency of a simple prototype instrument in order to estimate possible detection limits of a second-generation instrument. Using this prototype a detection limit of 100 ppbv was demonstrated. Modifications are suggested that will enable further reduction in detection limits to the low-ppbv to high-pptv range. Furthermore, the applicability of MS/MS in differentiating between isobaric species was determined. MS/MS spectra of the isobaric compounds methyl vinyl ketone (MVK) and methacrolein (MACR) are presented and show fragments of different mass making differentiation possible, even when a mixture of both species is present in the same sample. However, MS/MS spectra of acetone and propanal produce fragments with the same molecular masses but with different intensity ratios. This allows quantitative distinction only if one species is predominant. Fragmentation mechanisms are proposed to explain the results.
2003. "Proton Transfer Reaction Ion Trap Mass Spectrometer." Rapid Communications in Mass Spectrometry 17(14):1593-1599. Abstract Proton Transfer Reaction Mass Spectrometry (PTR-MS) is a relatively new field that has attracted a great deal of interest in the last several years. This technique uses H3O+ as a chemical ionization (CI) agent for measuring volatile organic compounds (VOCs) in the parts per billion by volume (ppbv) - parts per trillion by volume (pptv) range. PTR-MS mass spectra are simple because the ionization method of proton transfer is “soft”, resulting in little or no fragmentation. Unfortunately, the simplicity of the mass spectra can cause problems in peak identification due to isobaric interferences. A possible solution to this problem is to couple the PTR drift tube to an ion trap mass spectrometer (ITMS). ITMS is appealing because of the ability to perform MS/MS and possibly distinguish between isomers and other isobars. Additionally, the ITMS duty cycle is much higher than that of a linear quadrupole so faster data acquisition rates can be realized for detection of multiple compounds. We present here the first results from a Proton Transfer Reaction Ion Trap Mass Spectrometer (PTR-ITMS). The aim of this study was to investigate ion injection and storage efficiency of a simple prototype interface in order to estimate possible detection limits of a second generation instrument. Using this prototype a detection limit of 100 ppbv was demonstrated for the PTR-ITMS. Modifications are suggested that will enable further reduction in detection limits to the low ppbv to pptv range. Furthermore the applicability of MS/MS to differentiate between isobaric species was determined. MS/MS spectra of the isobaric compounds methyl vinyl ketone (MVK) and methacrolein (MACR) are presented and show fragments of different mass making a differentiation possible even when a mixture of both species is present in the same sample. MS/MS spectra of acetone and propanal produce fragments with the same molecular weight but different ratios, allowing quantitative distinction only if one species is predominant. Fragmentation mechanisms are proposed to explain the results.
2003. "Detecting Isobaric VOCs." Analytical Chemistry 75(17):371A. Abstract Volatile organic compounds (VOCs) can be monitored by proton transfer reaction (PTR) MS, in which a PTR drift tube is coupled to a quadrupole mass spectrometer. In this technique, compounds undergo proton transfer reactions when they collide with H₃O⁺ ions. This soft chemical ionization method, which results in little fragmentation, allows researchers to detect trace levels of VOCs from relatively uncomplicated spectra. Sometimes, the m/z values of VOCs are similar or identical, rendering it impossible to make a positive identification. Thus, Michael Alexander and co-workers at the Pacific Northwest National Laboratory, San Francisco State University, and Innsbruck University (Austria) developed a novel instrument that couples a PTR drift tube to an ion trap mass spectrometer, which can perform MS/MS, to produce characteristic fragments of isobaric species.
2003. "Gas-Phase Hydrolysis of SOCL₂ at 297 and 309 K: Implications for Its Atmospheric Fate ." Journal of Physical Chemistry A 107(32):6183-6190. Abstract The gas-phase hydrolysis of thionyl chloride (SOCl₂) has been investigated at 297 K and 309 K in a mixing chamber using FTIR spectroscopy. Reagent concentrations of ~80 ppmv SOCl2 at 1, 18, 45 and 63% relative humidity (RH) were studied at 309 K, while humidities of ~3, 6, 10, 12, 18, 34, 48, 68 and 85% were used in the 297 K studies, all at a total pressure of ~1 atm in synthetic air. In each experiment, an aliquot of SOCl₂(g) was rapidly introduced into a chamber at fixed RH, quickly reaching a maximum and then decreasing exponentially as monitored by time-resolved infrared spectroscopy. The only observed reaction products were HCl(g) and SO₂(g) which were formed in a molar ratio of ~2:1. The exponential SOCl₂ decay curves directly showed the reaction to be first order in SOCl₂, and together with the RH-dependent data reveal the reaction to be first order in H₂O as well. The 297 K and 309 K hydrolysis rate constants were measured as 5.0 ? 1.3 x 10-21 and 4.3 ? 1.5 x 10-21 cm3/molecule-sec, respectively. Confirming previous results, SOCl₂ absorbed only at lambda < 290 nm, which suggests hydrolysis as the main loss mechanism in the troposphere at ambient temperatures and humidities.
2003. "Membrane Introduction Proton-Transfer-Reaction Mass Spectrometry." International Journal of Mass Spectrometry 223-224:763-770. Abstract The combination of membrane introduction mass spectrometry (MIMS) and proton transfer reaction mass spectrometry (PTR-MS) is explored. The PTR-MS is used to measure properties of a well-characterized membrane material, poly-dimethylsiloxane (PDMS). It is found that the ability of the PTR-MS to measure absolute concentrations in real-time makes it an ideal tool for the characterization of membrane properties and the interaction of the membrane with multiple organic species. Values for the diffusion coefficients of several molecules are measured and found to be in agreement with literature values. Time modulation of the analyte across the membrane is explored as a method of resolving isobaric interferences for different chemical species. This is demonstrated for acetone and propanal. Finally, the benefit of combining MIMS with PTR-MS is demonstrated by the direct analysis of organic species in the headspace of a hot water solution where the high humidity would not allow analysis using the PTR-MS alone.
2002. "Surface Decontamination of Simulated Chemical Warfare Agents Using a Nonequilibrium Plasma with Off-Gas Monitoring." IEEE Transactions on Plasma Science 30(4):1454-1459. Abstract InnovaTek is developing a surface decontamination technology that utilizes active species generated in a nonequilibrium corona plasma. The plasma technology was tested against DMMP, a simulant for the chemical agent Sarin. GC-MS analysis showed that a greater than four log10 destruction of the DMMP on an aluminum surface was achieved in a 10 minute treatment. An ion-trap mass spectrometer was utilized to collect time-resolved data on the treatment off-gases. These data indicate that only non-toxic fragments of the broken down DMMP molecule were present in the gas phase. The technology is being further refined to develop a product that will not only decontaminate surfaces but will also sense when decontamination is complete