Grantee Research Project Results
Final Report: Speciation of Volatile and Reacting Compounds in Particulate Matter
EPA Grant Number: R823980Title: Speciation of Volatile and Reacting Compounds in Particulate Matter
Investigators: Johnston, Murray V. , Wexler, Anthony S.
Institution: University of Delaware
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1995 through September 30, 1998
Project Amount: $334,455
RFA: Exploratory Research - Chemistry and Physics of Air (1995) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Safer Chemicals
Objective:
In this work, methods to speciate volatile and reacting compounds in single particles by on-line laser desorption ionization mass spectrometry (Johnston and Wexler, 1995) were evaluated. Aerosols were sampled directly into a mass spectrometer where individual particles were ablated with a high energy pulsed laser bearn. Ions produced by the ablation event were mass analyzed and a complete mass spectrum was generated for each particle. The distribution of ions in the mass spectra were then used to determine the unique chemical composition of each particle. Potential advantages of this approach for ambient aerosol characterization are the ability to assess rapidly changing chemical compositions and the ability group particles on the basis of size and composition. Initial work emphasized sulfur and chromium since their environmental impacts are dependent upon chemical form -- sulfates contribute to acid deposition far more than sulfites, and Cr (VI3 is toxic while Cr (III) is a nutrient. As the work proceeded, these studies were expanded to include the effects of water and particle size on the ability to identify and quantitate chemical species in particles.Summary/Accomplishments (Outputs/Outcomes):
Sulfur. Single particles containing four common forms of particulate sulfur (methansesulfonate, hydroxymethanesulfonate, sulfate and sulfite) were studied to determine whether or not laser desorption ionization could perform sulfur speciation. Both dry and wet particles were investigated (Neubauer et al, l997; Neubauer et al, l998). The negative ion mode was found to be preferable to the positive ion mode for speciation. Methanesulfonate gave an intense CH3SO3- ion. Hydroxymethanesulfonate gave an intense OHCH2SO3- ion in particles containing a strong acid (for example, nitric acid and methanesulfonic acid) or a hydrogen donor (for example, ammonium sulfate). Otherwise, only SO2- and SO3- were observed. Sulfate gave a predominant SO4- or HSO4- ion, the latter being preferred in aqueous particles or in particles containing ammonium ions. Sulfite gave similar spectra to sulfate but the two could be distinguished on the basis of threshold laser irradiance. The threshold irradiance for detecting particulate sulfite was two to three times greater than particulate sulfate when using 248 nm radiation. Under ideal conditions, the relative amounts of sulfate, methanesulfate and hydroxymethanesulfate could be inferred from the relative intensities of marker ions detected in the mass spectra. However, since the relative intensities are also dependent upon particle size and the amount of particulate water, quantitation will be difficult in ambient aerosols.Chromium. Preliminary results (Neubauer et al, Int. J. Mass Spectrom. Ion Proc. (1995) 151, 77-87) showed that chromium (III), chromate and dichromate in particles can be distinguished by laser desorption ionization. Speciation was accomplished by quantitative measurement of the peak areas of various ions in the farnily CrxOy-. However, since the relative peak areas of these ions are also dependent upon particle size and water content, quantitation will be difficult in ambient aerosols. A note of caution particles containing significant amounts of chromate and dichromate absorb visible radiation. We have found that these particles do not efficiently scatter blue/green radiation. Therefore, the continuous laser used to detect these particles by light scattering in a single particle mass spectrometer should be in the red region (i.e. helium-neon or diode laser). A new version of the P.I.s single particle mass spectrometer removes the light scattering step and overcome this problem (Carson et al, 1997, Ge et al, 1998b).
Particulate Water. Particulate water has a large effect on the laser desorption ionization mass spectra of single particles (Neubauer et al, 1997; Neubauer et al, 1998; Ge et al, 1998a). As the water content increases, marker ions for water (O-, OH- in negative ion spectra; M(H2O)n+ in the positive ion spectra) increase in intensity. Aqueous particles exhibit a two to four fold higher threshold laser fluence, a lower total ion current at a comparable laser fluence, and less peak broadening at high laser fluences than their dry particle counterparts. The transition between "dry" and "wet" particle spectra depends upon particle composition. The transition can occur as an abrupt change over a narrow humidity range or a gradual change over a broad humidity range. In most cases studied, water remained associated with the particle well below the deliquescence relative humidity (DRH), yielding spectra similar to those of aqueous droplets. Particulate phase water below the DRH appeared to be the result of surface adsorption under ambient conditions rather than condensational growth in the mass spectrometer inlet. (Condensational growth is discussed in Mallina et al, 1997.) Marker ions for methanesulfonate and sulfate were subject to signal quenching in multicomponent aqueous droplets containing chloride and nitrate. In contrast, all components could be detected in dry particles regardless of composition.
Particle Size. Particle size influences both the absolute and relative signal intensities of marker ions produced by laser desorption ionization. When a high laser fluence is used to ablate micron-size particles, the absolute signal intensities of marker ions for inorganic components in particles tend to increase linearly with droplet volume, indicating a volume desorption mechanism (Mansoori et al, l998). In contrast, the absolute intensities of marker ions for organic components show little particle size dependence in the micron-size range; however, the concentration dependencies are consistent with a surface adsorption model which suggests that only molecules at or near the droplet surface are efficiently ablated and ionized by the laser (Mansoori et al, 1997; Mansoori et al, 1998).
Since micron-size particles are not completely ablated by the laser pulse, composition gradients within the particle can affect the relative signal intensities of marker ions in the mass spectra. When a micron-size multicomponent inorganic particle dries, the surface layer composition is given by the eutonic composition derived from a phase diagram of water activity vs. composition and is independent of the bulk composition of the particle (Ge et al, 1996). A consequence of this process is that minor components tend to be concentrated at the particle surface while major components are enriched in the particle core. Since material near the particle surface is preferentially ablated and ionized, the laser desorption ionization mass spectra exhibit enhanced
signal intensities of marker ions for minor components. In contrast, particles under ca. 100 nm diameter are completely ablated by the laser pulse and the relative intensities of marker ions in the laser desorption ionization mass spectra give a quantitative measure of the total composition of the particle, not just the surface composition (Carson et al, 1997b; Ge et al, 1998b).
Real-Time Measurements. Laser desorption ionization was used to characterize the product of an aerosol reaction in real time (Carson et al, 1997a). Sodium chloride aerosols were mixed with vapor phase ammonia and nitric acid to produce an arnmonium nitrate surface layer. With a low laser irradiance, only material near the particle surface was ablated and ionized. The mass spectra were independent of the amount of ammonium nitrate deposited and simply indicated a nitrate-enriched surface layer. With a higher laser irradiance, both the surface layer and core were ablated, and relative intensities of marker ions in the mass spectra reflected the total composition. That is, the relative intensities of marker ions produced from the ammonium nitrate surface layer increased with increasing surface Laser thickness. Thus, the laser irradiance dependence allowed the surface and total compositions of the particles to be distinguished.
Conclusions:
The results of this work provide a clearer picture of the relationship between particle composition and laser desorption ionization mass spectra. For micron-size particles, the surface layer of the particle is preferentially analyzed. The consequence is that relative signal intensities of marker ions in the mass spectra may not indicate the total composition of the particle if a concentration gradient exists between the surface layer and interior of the particle. For dry particles that have previously undergone one or more deliquescence-efflorescence cycles, the surface layer composition will reflect the eutonic composition that is given by a phase diagram of water activity vs. composition. This process tends to enrich the surface layer with minor components and enhance the signal intensities of marker ions for these species in the mass spectra. Aqueous particles give very different spectra from their dry particle counterparts. In addition, quenching of methanesulfonate and sulfate marker ions in the mass spectra of aqueous multicomponent particles is observed. Thus, it is recommended that ambient measurements in humid environments be performed in a manner such that water is removed from the particles prior to analysis by laser desorption ionization.Journal Articles on this Report : 12 Displayed | Download in RIS Format
Other project views: | All 29 publications | 12 publications in selected types | All 12 journal articles |
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Type | Citation | ||
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Carson PG, Johnston MV, Wexler AS. Laser desorption/ionization of ultrafine aerosol particles. Rapid Communications in Mass Spectrometry 1997;11(9):993-996. |
R823980 (Final) |
not available |
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Carson PG, Johnston MV, Wexler AS. Real-time monitoring of the surface and total composition of aerosol particles. Aerosol Science and Technology 1997;26(4):291-300. |
R823980 (Final) |
not available |
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Ge Z, Wexler AS, Johnston MV. Multicomponent aerosol crystallization. Journal of Colloid and Interface Science 1996;183(1):68-77. |
R823980 (Final) |
not available |
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Ge Z, Wexler AS, Johnston MV. Deliquescence Behavior of multicomponent aerosols. Journal of Physical Chemistry A 1998;102(1):173-180. |
R823980 (Final) |
not available |
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Ge Z, Wexler AS, Johnston MV, Wexler AS. Laser desorption/ionization of single ultrafine multicomponent aerosols. Environmental Science & Technology 1998;32(20):3218-3223. |
R823980 (Final) |
not available |
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Johnston MV, Wexler AS. MS of individual aerosol particles. Analytical Chemistry 1995;67(23):721A-726A. |
R823980 (Final) |
not available |
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Mallina RV, Wexler AS, Johnston MV. Particle growth in high-speed particle beam inlets. Journal of Aerosol Science, March 1997;28(2):223-238. |
R823980 (Final) |
not available |
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Mallina RV, Wexler AS, Johnston MV. High-speed particle beam generation: simple focusing mechanisms. Journal of Aerosol Science 1999;30(6):719-738. |
R823980 (Final) R826234 (1999) R826234 (Final) |
Exit |
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Mansoori BA, Johnston MV, Wexler AS. Laser desorption ionization of size resolved liquid microdroplets. Analytica Chimica Acta, February 1998;359(1-2):185-191. |
R823980 (Final) |
not available |
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Mansoori BA, Johnston MV, Wexler AS. Matrix-assisted laser desorption/ionization of size- and composition selected aerosol particles. Analytical Chemistry 1996;68(20):3595-3601. |
R823980 (Final) |
not available |
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Neubauer KR, Johnston MV, Wexler AS. On-line analysis of aqueous aerosols by laser desorption ionization. International Journal of Mass Spectrometry and Ion Processes, April 1997;163(1-2):29-37. |
R823980 (Final) |
not available |
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Neubauer KR, Johnston MV, Wexler AS. Humidity effects on the mass spectra of single aerosol particles. Atmospheric Environment, August 1998;32(14-15):2521-2529. |
R823980 (Final) |
not available |
Supplemental Keywords:
Ambient air, particulates, sulfates, acid rain, dissolved solids, analytical, measurement methods, RFA, Scientific Discipline, Air, Toxics, Geographic Area, particulate matter, air toxics, Physics, State, HAPS, Chemistry, Environmental Monitoring, Atmospheric Sciences, Engineering, Chemistry, & Physics, EPA Region, ambient aerosol, particulates, exposure and effects, Delaware (DE), acid volatile sulfide, aerosol particles, mass spectrometry, oxidation, Chromium, Region 3, air sampling, chemical composition, chemical detection techniques, sulfur, Sulfur dioxide, analytical chemistry, environmental contaminants, atmospheric aerosol particles, Chromium Compounds, urban air , atmospheric deposition, rapid single particle mass spectrometryRelevant Websites:
http://www.udel.edu/chem/johnston/http://www.me.udel.edu/wexler/
Progress 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.