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Final Report: A Portable Device for Real-Time Measurement of the Size and Composition of Atmospheric Aerosols
EPA Grant Number: R826769Title: A Portable Device for Real-Time Measurement of the Size and Composition of Atmospheric Aerosols
Investigators: Johnston, Murray V. , Eiceman, Gary A.
Institution: University of Delaware , New Mexico State University - Main Campus
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1998 through September 30, 2001
Project Amount: $580,963
RFA: Air Pollution Chemistry and Physics (1998)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:The objective of this research project was to develop a portable device for real-time size and composition measurements of individual particles at atmospheric pressure. Particles were sized with a commercial aerodynamic sizer and then ablated with a pulsed laser. Ions produced by the ablation process were analyzed with an ion mobility spectrometer. Photons produced by the ablation process were analyzed with an optical spectrometer. For both chemical characterization methods, each particle gave a spectrum that could be related to its chemical composition. For ion analysis, fundamental studies with a tandem ion mobility spectrometer-mass spectrometer system allowed the relationship between mobility spectra and chemical composition to be studied.
Summary/Accomplishments (Outputs/Outcomes):The single particle spectrometer developed in this work has a modular design for easy conversion among multiple characterization methods. The heart of this device is a 3-inch cube that is configured to accept particle detection and sizing components from a commercial aerodynamic particle sizer. Particles enter from an inlet attached to one face of the cube. Particles emerging from the inlet are accelerated to a size-dependent velocity. A diode laser and a scatter collection optic and detector are mounted on two faces opposite each other and perpendicular to the inlet. The laser radiation is split into two beams to size the particles based on laser velocimetry. After a particle is detected and sized, it is ablated by a pulsed laser beam. The ablation laser radiation enters from the face opposite the inlet and counter-propagates collinearly with the particle beam. Ions or photons produced by laser ablation are analyzed along the remaining faces of the cube. Ions are analyzed on the basis of mobility in a constant electric field. Photons are analyzed with an optical spectrometer connected to the cube by a collection lens and fiber optic assembly. The entire device is one-third the size and mass of a conventional single particle mass spectrometer.
Much effort during the project period was directed toward understanding the relationship between ion mobility spectra, optical spectra, and chemical composition.
Organic polycyclic aromatic hydrocarbons (PAHs) and inorganic metals, polymers, and other solid materials were analyzed with the tandem ion mobility spectrometer-mass spectrometer system to study the relationship between mobility spectra and chemical composition. In these studies, bulk samples were ablated with a 266 nm laser beam at atmospheric pressure in the source region of the mobility spectrometer-mass spectrometer. Gas-phase ions with distinct mobilities were produced from six test PAHs by laser ablation. The ions produced were identified as molecular ions (molecule minus an electron) with the mass spectrometer. Mobility peaks were broader than those seen in gas-phase reactions, and this was attributed to Coulombic repulsion in the small volume region directly above the ablated surface. An ion shutter in the drift tube could be synchronized with the laser pulse to improve resolution. Negative ions also were detected from PAH samples, though these ions were mass identified as ions formed from air through the capture of electrons released from PAHs. No molecular ions from these compounds were observed in the negative ion spectra. Inorganic materials gave more complex positive ion spectra, consisting of broad unresolved peaks. Mass analysis of these ions indicated that they were composed of a distribution of molecular clusters. Negative ion spectra were very similar to those of the PAHs, consisting of a single intense peak whose mobility matched that of (H2O)nO2-.
Mobility spectra were obtained for individual particles in the 0.7 to 30 µm diameter range using radiation at 266 nm and 193 nm. The wavelength of the ablation laser was found to have little effect on the mobility spectra, although the particle hit rate was higher with 193 nm radiation. Single particle spectra obtained from PAH particles exhibited a single peak whose reduced mobility was consistent with that for the PAH molecular ion. The signal to noise ratios of these peaks typically were greater than 20 for circa 700 nm diameter particles, indicating that mobility spectrometry has the sensitivity needed to detect and characterize individual submicron particles.
It is important to compare the mobility spectra of PAHs to the more familiar method of real-time single particle analysis by laser ablation mass spectrometry. In mass spectrometry, the particle is ablated in a vacuum. For PAH particles, this results in the formation of low m/z fragment ions (particularly C+, C2+, and C3+). The molecular ion peaks typically are more than an order of magnitude and are less intense than these fragments. When ablation is performed at atmospheric pressure, little if any fragmentation is evident. Although ion detection at atmospheric pressure is less sensitive than in a vacuum, the sensitivity of mobility spectrometry is sufficient to detect submicron particles.
Single particle spectra also were obtained for a variety of inorganic salts (ammonium, alkali, transition metals, sulfate, and nitrate). Some spectra exhibited a single peak whose reduced mobility was similar to that obtained from the bulk solid. Other spectra exhibited a single, very sharp peak with a width that was limited by the rise time of the electronics and the reduced mobility that varied from particle to particle. These sharp peaks were thought to arise from highly photocharged particles.
Although, ion mobility spectrometry worked well for PAH characterization, it generally was less effective for inorganic materials owing to cluster formation, the dominance of O2- in the negative ion spectra, and photocharging. An alternative detection method, laser induced breakdown spectroscopy (LIBS), proved to work well for inorganic materials. LIBS was performed by ablating particles with a high energy laser pulse at 193, 248, and 355 nm. Similar spectra were obtained for each wavelength, provided that the laser pulse energy was suitably high. This approach was tested with circa 750 nm diameter particles containing salts of sodium, potassium, magnesium, calcium, copper, and iron. In each case, the metal ions present in the particle could be identified based upon the emission wavelengths detected.
Journal Articles on this Report: 1 Displayed | Download in RIS Format
| Other project views: | All 6 publications | 1 publications in selected types | All 1 journal articles |
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Young D, Douglas KM, Eiceman GA, Lake DA, Johnston MV. Laser desorption-ionization of polycyclic aromatic hydrocarbons from glass surfaces with ion mobility spectrometry analysis. Analytica Chimica Acta 2002;453(2):231-243. |
R826769 (Final) |
not available |
ambient air, particulates, metals, polycyclic aromatic hydrocarbon, PAH, organics, measurement methods, analytical, environmental chemistry. , Air, Scientific Discipline, RFA, Engineering, Chemistry, & Physics, indoor air, air toxics, particulate matter, Environmental Chemistry, Environmental Monitoring, aerosols, real time monitoring, indoor air quality, pulsed laser, chemical composition, air sampling, monitoring, aerodynamic sizer, particulates, atmospheric particles, spectroscopy, field monitoring, spectroscopic studies, ion exchange, atmospheric aerosol particles, ambient aerosol, particle size
Progress and Final Reports:
1999 Progress Report
2000 Progress Report
2001 Progress Report
Original Abstract