Improving Single-Particle Chemical Composition Analysis through Homogenization of an Excimer Ablation Laser Beam

WENDY FLORES-BRITO, Coty Jen, Ryan Sullivan, Carnegie Mellon University

     Abstract Number: 186
     Working Group: Instrumentation and Methods

Abstract
Aerosols are often present as an external mixture of internally mixed particles with complex chemical compositions. The physicochemical properties of individual aerosol particles have been shown to greatly influence ice nucleation, cloud microphysics, and the hygroscopic growth and deposition of inhaled particles. Obtaining chemical composition and mixing state quantitative information through single particle mass spectrometry is limited due to matrix effects and laser–particle interaction issues during the process of laser desorption/ionization (LDI) of particles. Homogenizing the LDI’s laser beam is a necessary measure to improve the laser–particle interactions and a key step towards obtaining more reproducible and quantitative single particle physicochemical information. A custom–made, variable beam delivery system (BDS) was used to homogenize the 193 nm Excimer LDI laser for the commercially available LAAPTOF (laser ablation aerosol time-of-flight) mass spectrometer. Un–homogenized, homogenized, and collimated beam profiles were captured and analyzed with a beam profiling camera and software. Visual and quantitative inspection of the results demonstrated that homogenization was achieved for the homogenized and collimated beam profiles. As an additional quantitative measure of homogenization, the Wilcoxon Rank Sum test of equal medians was utilized to compare each row versus column of the averaged fluence from the beam profiles. The resulting p-value maps used to evaluate the flat-top quality produced by the BDS exhibited a better and more uniform distribution for the collimated case, which would be the beam profile that interacts with the particle beam inside the LAAPTOF. Particle hit fraction and shot-to-shot variance studies were performed to evaluate the system’s improvement after homogenization. The similarity of individual particle mass spectra acquired from homogeneous test particles was assessed with and with out the BDS homogenization of the LDI laser beam. These promising results will lead to more reproducible and quantitative online aerosol particle analysis.