Development and characterization of an aerosol time-of-flight mass spectrometer with increased detection efficiency

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 similar to0.5% for 95 nm and similar to47% for 290-nm PSL particles, while the particle detection efficiencies were measured to be similar to0.3% 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 similar to0.35 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), similar to30000 particles with aerodynamic diameters of <300 run 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.