This paper describes the development and characterization studies of a more efficient aerosol time-of-flight mass spectrometer (ATOFMS), showing results for the on-line detection and determination of the size and chemical composition of single fine (100-300 nm) and ultrafine (<100 nm) particles. An aerodynamic lens inlet was implemented, replacing the converging nozzle inlet used on conventional ATOFMS instruments. In addition, the light scattering region was modified to enhance the scattering signals for smaller particles. Polystyrene latex spheres (PSL) with aerodynamic diameters ranging from 95 to 290 nm were used to characterize the particle sizing efficiency (product of particle transmission efficiency and particle scattering efficiency), particle detection efficiency (product of particle sizing efficiency and particle hit rate), and particle beam profile and perform instrument calibration. At number concentrations of <20 particles/cm(3), the particle sizing efficiencies were determined to be approximately 0.5% for 95 nm and approximately 47% for 290-nm PSL particles, while the particle detection efficiencies were measured to be approximately 0.3% for 95 nm and 44% for 290-nm PSL particles. This represents a significant increase (i.e., at least 3 orders of magnitude) in detection efficiencies for smaller particles over the conventional ATOFMS. In addition, the beam profiles for PSL particles of various sizes were measured in the ion source of the mass spectrometer and follow a Gaussian distribution with a full width at half-maximum of approximately 0.35 mm. The resulting higher detection efficiencies allow ATOFMS to obtain higher temporal resolution measurements of the composition of fine and ultrafine individual particles as demonstrated in initial ambient measurements in La Jolla, CA. At typical ambient particle number concentrations of 10(2)-10(3) particles/cm(3), approximately 30 000 particles with aerodynamic diameters of <300 nm were detected with average 24-h hit rates of 30% for particles between 50 and 300 nm. This advancement, allowing for high temporal resolution measurements of the composition of smaller particles with higher efficiency, adds to a growing number of instruments that can chemically characterize individual fine and ultrafine particles, with the goal of providing new insights into a number of areas including environmental and material sciences, health effects studies, industrial hygiene, and national security.

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Given growing concerns over the observed relationship between ultrafine particles and adverse human health effects, there is a major need in the community performing human/animal exposure studies for methods that can be used for the generation of high concentrations of ultrafine particles (<100 nm) with controllable compositions. The Palas spark discharge generator (Palas GFG 1000) is commonly used to generate "soot-like" particles for such studies. However, before such methods can be used routinely in the lab, it is important to assess the chemical variability and reproducibility of the ultrafine particles produced using such techniques. The goal of this study involves performing the on-line assessment of the chemical variability of individual ultrafine and fine (50-300 nm) particles produced by a Palas generator. The aerodynamic size and chemical composition of 12 C and 13 C elemental carbon (EC), composite iron-carbon (Fe-12 C), and welding particles were analyzed using aerosol time-of-flight mass spectrometry, and in general highly reproducible single-particle mass spectra were obtained. When using pure graphite ( 12 C) electrodes, EC particles were produced with sizes peaking in the ultrafine mode and 96% of the mass spectra containing distinct C + n (n = 1-3) envelopes at m/z 12, 24, and 36. In contrast, the size mode of the particles generated from isotopically labeled 13 C graphite electrodes peaked in the accumulation mode, with 73% of the particles producing EC carbon ion cluster patterns at m/z 13 ( 13 C + ), 26 ( Observed differences between the 12 C and 13 C particle spectra are most likely due to their different surface properties, with 13 C particles more effectively adsorbing semivolatile organic species originating in the particle-free dilution air. Homogeneous metal particles were also generated from Fe-12 C and welding rods with almost all (92% and 97%, respectively) of the spectra showing reproducible Fe/Mn/Cr and Fe/ 12 C ion ratios.