Role of Relative Humidity in Controlling Ion Yields in Droplet Assisted Ionization

JOSHUA HARRISON, Kelvin Risby, Thomas Hilditch, Jim Walker, Bryan R. Bzdek, University of Bristol

     Abstract Number: 176
     Working Group: Instrumentation and Methods

Abstract
Droplet assisted ionisation (DAI) is a voltage-free approach to create molecular ions from microscopic liquid droplets by passing them through a temperature-controlled capillary interface into a mass spectrometer. This approach is highly versatile, enabling sensitive detection of analytes in aqueous nanodroplets while avoiding artifacts from offline measurements. However, the ionization mechanism is not fully understood. The presence of a protic solvent like water is essential to the ionization process, but the effect of solvent has not been systematically explored. Here, the role of solvent on ion yield is explored by sampling plumes of submicron aerosol generated by atomizing aqueous solutions into the customized inlet of a SYNAPT XS time-of-flight mass spectrometer. The droplet water content was controlled by changing the relative humidity (RH) around the aerosol either with a Nafion dryer (to dry liquid droplets) or a water boat (to humidify dry particles). Aerosol number and mass concentrations were measured using a scanning mobility particle sizer. 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. For example, the ion yield for cortisol, a non-hygroscopic compound, showed no dependence on RH, whereas that for angiotensin II, a mildly hygroscopic peptide compound, displayed a clear increase (~2 orders of magnitude) at high RH compared to dry conditions. Finally, the ion yield for ammonium sulphate, 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 (adsorbed water or solution droplet). The results demonstrate the importance of a protic solvent to generate ions during the rapid breakup of liquid droplets. The application of this approach to explore accelerated chemical reactions will also be discussed.