Grantee Research Project Results
2004 Progress Report: Particle Sampler for On-Line Chemical and Physical Characterization of Particulate Organics
EPA Grant Number: R831077Title: Particle Sampler for On-Line Chemical and Physical Characterization of Particulate Organics
Investigators: Smith, Kenneth A. , Worsnop, Douglas R. , Boudries, Hacene
Current Investigators: Smith, Kenneth A. , Worsnop, Douglas R.
Institution: Massachusetts Institute of Technology , Aerodyne Research Inc.
EPA Project Officer: Chung, Serena
Project Period: October 1, 2003 through August 1, 2006 (Extended to September 30, 2007)
Project Period Covered by this Report: October 1, 2003 through August 1, 2004
Project Amount: $410,000
RFA: Measurement, Modeling, and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5) (2003) RFA Text | Recipients Lists
Research Category: Air , Air Quality and Air Toxics , Particulate Matter
Objective:
The overall objective of this research project is to develop an innovative particle sampler, which can be coupled to already available commercial or research-grade analytical instruments for on-line, size-resolved analysis of individual organic species. This coupling will enable: (1) high-efficiency separation of particles from the gas phase; (2) concentration of collected aerosol on a cryo-cooled surface under high vacuum (avoiding problems associated with filter sampling and solvent extractions); and (3) direct injection of desorbed species from particle samples into analytical instruments.
This program is a collaborative effort between the research groups led by Professor Kenneth A. Smith at the Massachusetts Institute of Technology (MIT) and Dr. Douglas R. Worsnop at Aerodyne Research, Inc. (ARI).
Progress Summary:
The aim of Year 1 of the project was to: (1) design and build the particle sampler; (2) couple the miniature particle collector to the vacuum chamber; and (3) test the heating and cooling rate of the miniature particle collector, leak integrity, and preliminary aerosol sampling.
Each of the above technical tasks was accomplished. The first prototype particle sampler was designed and constructed at MIT in collaboration with ARI, and a preliminary evaluation was performed.
Design of the Prototype Particle Sampler
The schematic of the combined miniature particle collector and the vacuum chamber is shown in Figure 1. It consists of three main sections:
Aerosol Beam Generation Chamber . Aerosols are sampled with a unique aerodynamic particle lens developed at the University of Minnesota (Liu, et al., 1995a; Liu, et al., 1995b), which focuses the particles into a well-defined beam approximately 1 mm in diameter. The performance of the aerodynamic lens was studied extensively during the development phases of the Aerosol Mass Spectrometer by the ARI-MIT team (Zhang, et al., 2002; Zhang, et al., 2004). The collection efficiency of this lens as a function of size is near unity between 60 nm and 750 nm and decreases to about 70 percent for aerosol sizes outside this range.
Particle Sizing Chamber. The focused particle beam exiting the lens enters the particle sizing chamber maintained at a low pressure (10-6 Torr). The size-dependent particle velocities created by the gas expansion provide a means of obtaining the particle aerodynamic diameter by measuring particle time of flight. We plan to develop a dual chopper system that will aerodynamically select a specific size bin so that we will have the potential to size resolve the collected aerosol.
Particle Collection Chamber. Particles are collected by directing the focused particle beam into the miniature particle collector. This collector is composed of a specifically designed miniature aerosol collection surface with an integrated resistance heater and cooling system using liquid nitrogen, allowing the aerosol collector to be operated at temperatures varying between -170 ° C and 600 ° C.
Coupling the Miniature Particle Collector to the Vacuum Chamber
The miniature particle collector was designed, constructed, and successfully integrated to the vacuum chamber. This system allows aerosol collection at low temperatures, between ambient and -170ºC, followed by desorption of organic species present in/on collected particle and finally injection/transfer to a separate detection system with a high-purity carrier gas. The miniature particle collector also was connected to two external 4-port valves, which allow for the necessary sampling cycles of particle samplin g and analysis. A typical sampling cycle of the particle sampler consists of four stages: backflush, sampling, vaporization/desorption, and transfer/injection. Preliminary tests show that a heating rate of 100°C minutes-1 was achieved using a 15W miniature cartridge heater. A liquid nitrogen cooling system also was incorporated into the miniature aerosol collector allowing for sampling at sub-ambient temperatures. The heating and cooling system was electronically controlled through the particle sampler control panel box. A photograph of the first particle sampler prototype is presented in Figure 2.
Preliminary Laboratory Testing
The particle sampler prototype was connected to a commercial gas chromatograph-mass spectrometer ([GC/MS], HP 5890 Series II and HP 5971). The connection was simple to make and involved bypassing the carrier gas flow to the GC/MS and directing it via 1/16” heated stainless steel line to the particle sampler prototype. The outlet of the particle sampler was then connected to the injector port of the GC/MS. This coupling was straightforward and required no modification of the GC/MS gas and inlet ports. The particle sampler successfully collected laboratory generated aerosols from oleic acid for subsequent analysis by mass spectrometry. By coupling the particle sampler with a GC/MS system, the ability to separate and generate mass spectra of the individual organic components was demonstrated.
The initial test consisted of sampling laboratory-generated aerosol using pure oleic acid. Particles were generated using an atomizer (TSI, Model 3076, Minneapolis, MN) and then dried using a silica gel desiccant.
Polydisperse oleic acid particles were sampled for 3 minutes at a flow rate of 100 mL minutes -1 through a 100 mm critical orifice. The collection temperature was set to -25 °C. After a suitable collection time had elapsed, the collection chamber was sealed from the high-vacuum separation chambers and the particle collector was gradually heated to 250 °C for 5 minutes. This caused desorption of the collected particles as a vapor, which was transferred by a flow of carrier gas to the GC/MS for chemical analysis. The chromatographic separation was performed using a 25 m long, 0 .2 mm i.d. capillary column (Fused Silica HP Ultra2, Crosslinked, 5% Ph Me Silicone). Ultra high purity helium was used as the carrier gas at a flow rate of 1 mL minute-1. The temperature program was: an initial GC oven temperature of 40 ° C for 2 minutes, followed by an increase to 310 ° C at 12 ° C minutes-1, and then held for an additional 10 minutes. The HP mass spectrometer, operated in the 70eV electron impact ionization mode (scanning m/z 30 – 350), was used to perform the chemical analysis. A typical total ion chromatogram of pure oleic acid is shown in Figure 3.
Figure 3. Total Ion Chromatogram Obtained From the Particle Sampler of Laboratory-Generated Oleic Acid
This preliminary study shows successful particle collection and mass spectral analysis of laboratory-produced organic aerosol species. This strongly indicates that a combined particle sampler-GC/MS system will be able to successfully collect and analyze the organic fraction of ambient atmospheric aerosols. Similar experiments will be performed during Year 2 of the project to include testing of several different laboratory-generated aerosols (such as hydrocarbons, polycyclic aromatic hydrocarbons, lubricating oil, diesel fuel) and ambient air.
Conclusion
A prototype particle sampler for on-line sampling was successfully designed and constructed. This prototype also was easily interfaced with a commercial GC/MS. Preliminary experiments to evaluate the potential and performance of the particle sampler as a means for the chemical analysis of organic aerosols were conducted. The major results are summarized below:
- Efficient heating and cooling rates were measured during the sampling and desorption processes, respectively.
- Easy connection and coupling to commercial GC/MS systems. The heated transfer lines were connected to a GC injector with little or no modifications to the commercial instruments.
- Laboratory-generated aerosols of oleic acid sampled by the particle sampler were demonstrated.
Future Activities:
We are in the process of performing further experiments and redesigning the prototype to better optimize and characterize the particle sampler for ambient aerosol measurements. In particular, the following tasks need to be addressed during Year 2 of the project:
- Reduce the dead volume inside the collection chamber to near zero by modifying the carrier gas pathway.
- Improve the isolation valve to eliminate the leak between the aerosol collection chamber and the vacuum system.
- Incorporate a cryo-trap module before chromatographic injection to have instantaneous injection.
- Investigate and determine the optimum conditions for sampling time, desorption time, and desorption carrier flow.
- Investigate and evaluate the influence and effect of any evaporation losses that may occur during sampling by varying the aerosol collection temperature.
References:
Liu BYH, Ziemman PJ, Kittelson DB, McMurry PH. Generating particle beams of controlled dimensions and divergence: I. Theory of particle motion in aerodynamic lenses and nozzle expansions. Aerosol Science and Technology 1995a;22:293-313.
Liu P, Ziemman PJ, Kittelson DB, McMurry PH. Generating particle beams of controlled dimensions and divergence: II. Experimental evaluation of particle motion in aerodynamic lenses and nozzle expansions. Aerosol Science and Technology 1995b;22:314-324.
Zhang X, Smith KA, Worsnop DR, Jimenez J, Jayne JT, Kolb C. A numerical characterization of particle beam collimation by an aerodynamic lens-nozzle system: Part I. An individual lens or nozzle. Aerosol Science and Technology 2002;36:617-631.
Zhang X, Smith KA, Worsnop DR, Jimenez J, Jayne JT, Kolb C, Morris J, Davidovits P. Numerical characterization of particle beam collimation: Part II. Integrated aerodynamic lens-nozzle system. Aerosol Science and Technology 2004;38:619-638.
Journal Articles:
No journal articles submitted with this report: View all 6 publications for this projectSupplemental Keywords:
particle sampler, organic speciation, on-line analysis, size-resolved, PM 2.5, GC/MS, air, ecosystem protection/environmental exposure and risk, research, atmospheric sciences, environmental chemistry, environmental monitoring, monitoring, monitoring/modeling, particulate matter, PM, aerosol analyzers, air quality model, air quality models, air sampling, analytical chemistry, atmospheric chemistry, atmospheric dispersion models, atmospheric measurements, chemical characteristics, gas chromatography, human exposure, human health effects, modeling, modeling studies, monitoring stations, particle size,, RFA, Scientific Discipline, Air, Ecosystem Protection/Environmental Exposure & Risk, RESEARCH, particulate matter, Environmental Chemistry, Monitoring/Modeling, Monitoring, Environmental Monitoring, Ecological Risk Assessment, particle size, atmospheric dispersion models, atmospheric measurements, analysis of organic particulate matter, chemical characteristics, human health effects, air quality models, monitoring stations, gas chromatography, air quality model, air sampling, modeling, analytical chemistry, particulate matter mass, particle sampler, modeling studies, aerosol analyzersProgress 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.