Design and development of an ultrafine particle reflection-time-of-flight mass spectrometer

Abstract

The primary motivation for the research is to study the effect of small particles on global climate. The study can also help to understand the dynamics involved with gas to particle conversion, which is being debated to be a rich source for atmospheric aerosol. However, the most important use would be to study health effects, since small particles easily diffuse into the lungs, with seemingly little physiological filtration mechanism. The research work involves the design, development and characterization of a single-ultrafine-particle mass spectrometer. The instrument aerodynamically size selects fine and ultrafine aerosol particles (size range 20 nm-1 []m), with a constant Stokes number, and focuses them into a vacuum chamber. This is achieved by changing the upstream pressure of the inlet, which changes aerodynamic drag experienced by the particle. After its entry into the chamber, the particle is ablated by a high power excimer laser, which produces ions from the original molecular constituents. Reflectron time-of-flight mass spectrometry is utilized to analyze the ions, and thus the chemical composition of the particle that was hit. The present work is aimed to overcome the shortcomings of previous instruments, while allowing for increased portability. The instrument is designed, fabricated and experimentally characterized. The first phase involves analysis of the particle beam generated by the inlet. An atomizer generates aerosols from a solution of 5% oleic acid and ethanol. The polydisperse aerosol is passed through a differential mobility analyzer to make a monodisperse mixture, which is transmitted through the inlet. The monodisperse particle beam is intercepted by glass slides, and the spot sizes are indicative of the beam shape and width at the slide positions. A theoretical analysis of the fluid flow field and particle trajectory is developed to correlate with the experimental results. The second phase involves calibration of the mass spectra, and measuring some particulate composition from the laboratory room air.