AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Mechanism of Ion Formation by Droplet Assisted Ionization
MICHAEL J. APSOKARDU, Justin Krasnomowitz, Devan E. Kerecman, Yao Zhang, Shuai Jiang, Murray Johnston, University of Delaware
Abstract Number: 161 Working Group: Instrumentation and Methods
Abstract Droplet assisted ionization (DAI) has been developed for online molecular composition measurements directly from aerosol droplets. Analysis with DAI is done by passing droplets through a custom-built, stainless-steel capillary tube (69mm length; 1mm o.d.; 0.5mm i.d.) that has been interfaced with a Waters Synapt G2-S quadrupole ion mobility time-of-flight mass spectrometer. The DAI capillary has the option to be temperature-controlled between 25 and 850°C. Previous work has focused on determining the optimum conditions to maximize analyte ion signal intensities. One such factor is temperature of the capillary inlet, where a greatly enhanced signal is often seen at elevated temperatures. This temperature dependence provides a unique opportunity to examine the fundamentals of how gas phase ions are formed. Using the temperature dependence of absolute ion signal intensities, in combination with aerodynamic modeling of the capillary inlet, these data are reconstructed as an Arrhenius plot to give the activation energy (Ea) for ion formation. For the test compound polypropylene glycol, Ea is estimated to be 41.3 ± 3.1 kJ/mol. This value is not so different from the enthalpy of vaporization of water at 25°C, which is ~45 kJ/mol. Other test compounds give Ea values in the 30 to 40 kJ/mol range, indicating that the formation mechanism is strongly related to evaporation of water, but also depends on characteristics of the species being analyzed. Presented here are experimental measurements of Ea made with DAI in combination with theoretical calculations made by molecular dynamics simulations to understand the respective roles of solvent and analyte for producing ions from droplets. The results suggest that ions are ejected from the droplet surface into the gas phase with a surrounding cluster of water molecules. The water molecules are lost inside the mass spectrometer prior to mass analysis. Also, to be discussed are the implications of this mechanism to the application of DAI for online molecular characterization of aerosols.