Ionization efficiency of evolved gas molecules from aerosol particles in a thermal desorption aerosol mass spectrometer: Laboratory experiments

Thermal desorption aerosol mass spectrometers (TDAMSs) with electron ionization are widely used for quantitative analysis of aerosol chemical composition, and the ionization efficiency of evolved gas molecules from aerosol particles is an important parameter for such analysis. We performed laboratory experiments using a custom-made TDAMS to investigate the key factors affecting ionization efficiency. Ammonium chloride (NH4Cl) and ammonium iodide (NH4I) were used as test compounds because their thermal decomposition products are expected to be simple (dominated by ammonia (NH3) and hydrogen halide (HX)). The ion signals originating from NH3 and HX were measured by altering the position of the ionizer relative to the vaporization point. The ratio of ion signal from NH3 to that from HX increased with increasing divergence angle of evolved gas plumes, which suggests that the angular distribution of gas molecules could be an important factor affecting the ionization efficiency. Copyright (c) 2018 American Association for Aerosol Research