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1999 Progress Report: Real-Time Analysis of PAH Bound to Size-Resolved Atmospheric Particles by Tandem Time of Flight Mass Spectrometers
EPA Grant Number: R825391Title: Real-Time Analysis of PAH Bound to Size-Resolved Atmospheric Particles by Tandem Time of Flight Mass Spectrometers
Investigators: Smith, Kenneth A. , Worsnop, Douglas R.
Institution: Massachusetts Institute of Technology , Aerodyne Research Inc.
Current Institution: Massachusetts Institute of Technology
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1996 through September 30, 1999 (Extended to November 30, 2000)
Project Period Covered by this Report: October 1, 1998 through September 30, 1999
Project Amount: $375,000
RFA: Exploratory Research - Air Engineering (1996)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:Polycyclic aromatic hydrocarbons (PAHs) adsorbed on atmospheric particles are of concern because PAHs are known to be mutagenic. Measurements of the amount of PAH associated with different aerosol size fractions are critical for a complete understanding of the environmental fate of and human exposure to fine particles containing PAH. The goal of this research is to develop and demonstrate an aerosol mass spectrometer capable of quantifying PAHs associated with individual size-segregated atmospheric particles in real time. Progress Summary:
The instrument operates by drawing ambient aerosol into a vacuum system through an aerodynamic lens that focuses particles into a narrow beam and efficiently transmits the particles through the vacuum system where they collide on a resistively heated surface. The PAH components in/on the particle flash vaporize upon contact with the heater and the gas phase molecular constituents are then photo-ionized by UV laser resonance-enhanced multi-photon ionization (REMPI). The ionized molecular PAH species are classified using time-of-flight (TOF) molecular mass spectrometric analysis. The technique provides complete mass spectrometric information for individual particles. Particle aerodynamic size is determined by measuring particle time of flight using a mechanical particle beam chopper. This is performed by firing the laser at specific time intervals synchronized to the phase of the particle beam chopper with subsequent detection of ion signals in the TOF spectrometer. This particle sizing scheme takes advantage of the size-dependent distribution of particle velocities generated by the expansion of the aerosol into the vacuum.
During the reporting period, we have further developed and quantified the aerodynamic particle sizing technique. Size-dependent particle velocity, particle beam width, and particle transmission and collection efficiency have been measured as a function of particle sizes. Particles in the size range of 60 -1000 nm travel at velocities ranging from 260 to 90 m s-1, respectively. Particle beam widths are less than 1 mm diameter and transmission/collection efficiencies reach 100 percent. Only with a knowledge of quantitative particle collection efficiency, is it possible to associate measured particle count rates with the actual number density of ambient particles sampled. The experimental measurements are supported by fluid dynamics modeling results. Our modeling results also indicate that the collection efficiency can be essentially unity over the entire particle size range of ~50 to 5000 nm if the inlet geometry of the aerodynamic lens system is optimized. This will significantly enhance the applicability of the instrument in analyzing ambient particles.
Also during this reporting period, preliminary measurements using laboratory-generated PAH particles have been conducted that illustrate the applicability of the TOF mass spectrometric REMPI detection scheme. Time-of-flight spectral of pyrene and perylene particles flash-vaporized and ionized by the excimer laser are shown in Figure 1. In the figure, the noise at t=0 indicates the time of the laser firing. Following the laser firing, two ion peaks are observed at 17 microseconds and at 19 microseconds, which correspond to the parent peaks for pyrene (202 AMU) and perylene (252 AMU), respectively. Particle velocity and, hence, particle aerodynamic size is measured by firing the laser with a specified time delay after the open gate of the particle beam chopper. The measurements on PAH particles demonstrate that the REMPI process coupled with molecular TOF mass spectrometer provides a selective and, hence, sensitive means of particle size-resolved PAH ionization/classification. To further improve single particle detection sensitivity, the vaporization heater is being moved closer to the axis of molecular TOF tube. This requires that the heater (which is a metal surface) be floated to the potential of the TOF ion optics, which is approximately 4000 V.
Figure 1. TOF mass spectrum of laboratory-generated pyrene and perylene particles following thermal flash vaporization and photo-ionization by an excimer laser at 248 nm.
Future Activities:The next stage of the work will focus on two issues: (1) optimizing the coupling between the particle vaporizer, the excimer laser, and the ion optical components of the TOF molecular mass spectrometer in order to improve single particle detection sensitivity; and (2) performing calibration measurements using PAH absorbed on soot generated from a flat flame. Finally, the instrument will be used to sample ambient aerosol and soot particles derived from diesel engines to demonstrate that the instrument is able to provide a complete mass spectrum for single size-resolved particles containing PAH molecules with high sensitivity and selectivity.
Journal Articles on this Report: 1 Displayed | Download in RIS Format
| Other project views: | All 9 publications | 2 publications in selected types | All 2 journal articles |
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Jayne JT, Leard DC, Zhang XF, Davidovits P, Smith KA, Kolb CE, Worsnop DR. Development of an aerosol mass spectrometer for size and composition analysis of submicron particles. Aerosol Science and Technology 2000;33(1-2):49-70. |
R825391 (1999) R825391 (2000) R825391 (Final) R825253 (Final) |
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exposure, risk, risk assessment, health effects, ecological effects, carcinogen, environmental chemistry, monitoring, transportation. , Air, Scientific Discipline, RFA, Engineering, Chemistry, & Physics, air toxics, particulate matter, Environmental Chemistry, mobile sources, emissions measurement, fate, ambient emissions, ultraviolet excimer laser, vapor plume, fine particles, aerosol particles, combustion, monitoring, fine particulates, air quality models, particulates, atmospheric particles, flight mass spectrometer, mutagenic pollutants, PAH
Progress and Final Reports:
1997 Progress Report
Original Abstract
2000 Progress Report
Final Report