Aerodynamic Particle Sizing versus Light Scattering Intensity Measurement as Methods for Real-Time Particle Sizing Coupled with Time-of-Flight Mass Spectrometry Kimberly Salt, Christopher A. Noble, and Kimberly A. Prather* Department of Chemistry, University of California, Riverside, California 92521 Measurement of scattered light intensity and aerodynamic particle sizing are two methods that have recently been coupled with time-of-flight mass spectrometry for real-time determination of aerosol particle size and composition. An aerosol analysis technique recently developed in our laboratory, aerosol time-of-flight mass spectrometry, offers a unique experimental platform to evaluate both of these sizing techniques. This paper presents a comparison of results obtained with these two methods. Aerosols, as they exist in the atmosphere, are diverse and constantly fluctuating systems, providing a unique measurement challenge for analytical chemists. Particles in the low and submicrometer size range are of the most significant concern, as many pose potential health hazards due to their ability to penetrate the respiratory system, with aerodynamic particle size being the most influential parameter affecting the final location of particle deposition. 1 Due to health risks associated with the inhalation of fine aerosol particles, in 1987, the U.S. Environmental Protection Agency set national ambient air quality standard (NAAQS) concentration levels for PM 10 (particulate matter with an aerody- namic diameter less than or equal to 10 μm). 2-4 However, recent epidemiology studies, showing a possible link between increased health problems and high levels of ambient particulates, indicate that this size cutoff may not be adequate. 5-8 A major drawback to the current standard is that it is not chemically specific. Clearly, the chemical composition of a particle, as well as its size, will contribute to its ultimate biological and environmental reactivity. Thus, in order to establish a more appropriate size cutoff for particulates, it is imperative to develop analytical instrumentation which allows determination of the chemical speciation within particles in the size ranges under investigation. Such data will allow for more specific testing of the health effects of particles for a given size and chemical composition. With the goal of providing real-time information on particle size and chemical composition, our research group has developed a new aerosol analysis technique, aerosol time-of-flight mass spectrometry (ATOFMS). 9-11 Single-particle analysis is performed in the instrument using aerodynamic particle sizing 12,13 and time- of-flight mass spectrometry. The use of aerodynamic particle sizing is what makes ATOFMS different from all other single- particle time-of-flight mass spectrometric analysis methods. 14-19 Aerodynamic particle sizing involves measuring the time it takes for a particle to travel through two continuous-wave laser beams, a time which can be ultimately correlated to aerodynamic particle size upon system calibration using particles of known size. Other aerosol analysis methods using time-of-flight mass spectrometry rely on measurement of scattered light intensity resulting from an individual particle passing through a single laser for determi- nation of particle size. In most of these experimental configura- tions, the scattering signal also serves as an instantaneous trigger for a desorption/ ionization laser in the ion source region of the mass spectrometer. In addition to measurements of light scat- tering intensity, some methods employ a fixed time delay between scattered light detection and triggering of the desorption/ ionization pulse for particle size determination. 17 This method is useful for determining the size and analyzing particles in a relatively narrow size range. As a result, this is an excellent method of characterization when attempting to analyze particles of one size only; however, it is ineffective for analyzing all particles in a polydisperse aerosol sample. (1) Jensen, P. A.; O’Brien, D. Industrial Hygiene. In Aerosol Measurements: Principles, Techniques, and Applications; Willeke, K., Baron, P. A., Eds.; Van Nostrand Reinhold: New York, 1993; Chapter 24. (2) Environmental Protection Agency. Fed. Regist. 1987 , 52, 24634-24669. (3) Pope, C. A., III; Dockery, D. W.; Spengler, J. D.; Raizenne, M. E. Am. Rev. Respir. Dis. 1991 , 144, 668-674. (4) Chow, J. C.; Watson, J. G.; Ono, D. M.; Mathai, C. V. J . 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