10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Mechanistic Insights into the Ionization of Airborne Nanoparticles via Droplet Assisted Ionization (DAI)
DEVAN E. KERECMAN, Michael J. Apsokardu, Murray Johnston, University of Delaware
Abstract Number: 554 Working Group: Instrumentation
Abstract Online methods for molecular analysis of secondary organic aerosol have the potential for fast time resolution measurements while consuming a minimal amount of sample. Understanding the ionization mechanisms behind these techniques allows for their optimization and successful application. Our group has developed an online technique, Droplet Assisted Ionization (DAI), where liquid droplets are sampled through a heated capillary into the mass spectrometer. The solvent medium assists formation of ions from analytes in the droplet, but the ionization mechanism and corresponding experimental conditions required for optimum detection are poorly understood. Current theories regarding the mechanism of DAI suggest an ionization process similar to electrospray ionization, but do not clearly explain how the initial charge is induced on the droplet. The work described here offers insight into the ion formation process by using two peptide-based aerosols, bradykinin and angiotensin II, as well as a polymer-based aerosol, poly(propylene)-glycol. The ion signal is found to increase linearly with aerosol mass concentration. There is no particle size dependence other than the fact that a large particle has more mass (and gives a correspondingly higher signal) than a small particle. A detectable signal is obtained for an aerosol mass flow into the inlet of about 80 pg/s. For the peptide aerosols, water is crucial for ionization. Drying an aqueous aerosol to remove water decreases the analyte signal intensity by 2-3 orders of magnitude. A similar reduction in signal intensity is observed when drying droplets composed of 50/50 water/acetonitrile solvent. Inlet temperature has a smaller but still important effect: signals are about an order of magnitude higher for inlet temperatures above 500 C than for lower temperatures. We find that additives, which increase the ion yield in a conventional inlet ionization experiment (e.g. 3-nitrobenzonitrile in Matrix Assisted Ionization, MAI), have little or no effect in DAI. This result suggests that such additives in a conventional experiment serve the purpose to create an aerosol, which is not needed in our work since aerosols are already being analyzed. The results of this study elucidate the conditions for efficient analysis of secondary organic aerosol by DAI.