Chemical Characterization of Aerosol Droplets via Droplet Assisted Ionization Mass Spectrometry
JOSHUA HARRISON, Thomas Hilditch, Kelvin Risby, Jim Walker, Bryan R. Bzdek, University of Bristol
Abstract Number: 220
Working Group: Instrumentation and Methods
Abstract
Droplet assisted ionization (DAI) is a voltage-free approach to create molecular ions from aerosol droplets for mass spectrometry analysis. This approach enables online, sensitive detection of analytes in aerosol droplets. The ionization mechanism for this method remains not fully understood. The role of water content was explored by sampling aqueous submicron aerosol into the customized inlet of a SYNAPT XS time-of-flight spectrometer. Aerosol number and mass concentrations were measured using a scanning mobility particle sizer. The droplet water content was controlled by changing the relative humidity (RH) around the aerosol either with a Nafion dryer to dry liquid droplets, a water boat to humidify dry particles, or a condensational growth chamber, activating the aerosol into supermicron liquid droplets. is important in the ionisation process, yielding a higher ion yield when more water, a protic solvent, is present. Ion yields were quantified from the ion count and sampled aerosol mass. Different analytes exhibited different RH-dependent ion yields that were consistent with each analyte’s hygroscopic response. Different analytes exhibited different RH-dependent ion yields that were consistent with each analyte’s hygroscopic response. For the ion yield of cortisol, a non-hygroscopic compound, there was no dependence on RH, whereas for angiotensin II, a mildly hygroscopic peptide compound, a clear increase (~2 orders of magnitude) at high RH compared to dry conditions was observed. Ammonium sulfate, a very hygroscopic inorganic compound, showed a strong dependence (~3 orders of magnitude) across a wide RH range. Ion yields were correlated to particle phase (solid, liquid) and water content. Mixtures of analytes in aerosol droplets convolute this understanding. Using the scanning mobility particle sizer, size-resolved ion yields were determined, indicating size-dependency of ionization efficiency. Furthermore, DAI was used to validate observed accelerated chemical reaction kinetics in the aerosol phase.