1999 Progress Report: A Portable Device for Real-Time Measurement of the Size and Composition of Atmospheric AerosolsEPA Grant Number: R826769
Title: 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 Period Covered by this Report: October 1, 1998 through September 30, 1999
Project Amount: $580,963
RFA: Air Pollution Chemistry and Physics (1998) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Engineering and Environmental Chemistry
Objective:The goal of this research is to develop and field test a portable device for real-time size and composition measurements of atmospheric aerosols. Individual particles are sized with a commercial aerodynamic sizer and then ablated with a pulsed laser. Ions produced by the ablation process are analyzed by ion mobility. Each particle gives a unique mobility spectrum that can be related to chemical composition. Instrument development work will emphasize the adaptation of proven, field-worthy technologies toward the goal of correlated size and composition measurements. Fundamental work will establish and validate the link between mobility spectra and chemical composition.
During the report period, two devices were designed and built?one to perform laser ablation of bulk solids and one to perform laser ablation of individual aerosol particles. The former instrument was used to study fundamental aspects of laser ablation of nonvolatile materials and to develop a library of mobility spectra, while the latter instrument provided a way to detect these materials in airborne particles.
Laser ablation of bulk solids was performed in a temperature controlled, pneumatically sealed mobility spectrometer. A focused, pulsed laser beam (266 nm, 6 J/cm2) entered the spectrometer through a window and irradiated the surface at a 60 degree angle. To minimize the effect of scattered radiation, the reflected beam from the surface exited the spectrometer through a second window at a 60 degree angle to both the sample and entrance window. When ablated with ultraviolet radiation, metallic samples and graphite were found to give a single peak in the positive ion spectrum whose drift time changed with chemical composition. The negative spectra of these materials showed a single peak whose drift time was independent of chemical composition. The reduced mobility of this peak was consistent with formation of an O2- ion. This ion is an expected product of electron capture in air and suggests that the ablation process includes a photoelectron emission step. Samples composed of organic polymers gave broader and/or multiple peaks in the positive ion spectra that exhibited longer drift times than the peaks obtained from metallic samples.
Laser ablation of an individual aerosol particle was demonstrated with the second mobility spectrometer. In the initial experiments, an aerosol of phenanthrene particles (mean diameter = 700 nm, standard deviation = 150 nm) was generated with a nebulizer and drawn through the source region of the mobility spectrometer. A collinear laser beam (266 nm, 6 J/cm2) counter-propagated through the source region and was free fired at 10 Hz. The laser and particle beams were oriented perpendicular to the drift tube. When a particle was "hit" by the laser beam, a burst of ions was produced and subsequently characterized by mobility analysis. Laser pulses that resulted in the ablation of a particle were distinguished from ionization of the background gas when the ion signal exceeded a threshold value within a set time period after the laser had fired.
Single particle spectra obtained from the phenanthrene aerosol exhibited a single peak whose reduced mobility was consistent with that for the phenanthrene molecular ion. The signal to noise ratio of this peak was typically greater than 20 indicating that mobility spectrometry has the sensitivity needed to detect and characterize individual sub-micron particles.
It is interesting to compare these initial results to the more familiar method of real time single particle analysis, laser ablation mass spectrometry. In mass spectrometry, the particle is ablated in a vacuum. For phenanthrene particles, this results in the formation of low m/z fragment ions (particularly C+, C2+, and C3+). The molecular ion peak is typically more than an order of magnitude 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 sub-micron particles.