Development of an Aerodynamic Lens-Proton Transfer Reaction Mass Spectrometry (PTR-MS) Apparatus for Online Detection of Organic Compounds in Aerosol Particles

Atmospheric organic aerosols consist of both gas-phase organic compounds and particle-phase organic compounds. The components of aerosol particles (solid or liquid droplets) can undergo exchange or chemical reactions with gas-phase components, influencing atmospheric physical and chemical processes and potentially impacting atmospheric environment, climate, and human health. Methods for detecting organic compounds in aerosol particle phase are classified into offline and online detection. Offline detection methods involve collecting aerosol samples for analysis using chromatography-mass spectrometry, which is time-consuming and limited sensitivity. Online detection methods involve enriching aerosols through aerodynamic lens and then detecting them using laser ablation or electron impact ionization techniques. However, these methods usually generate a large number of ion fragments, making mass spectrum analysis difficult. Therefore, it is necessary to develop an online detection method for aerosol particle-phase organic compounds with soft ionization and high sampling efficiency. In the present work, a set of aerodynamic lens sampling systems was designed and developed for online detection and analysis of organic compounds in aerosol particle phase. This sampling system consists of a gas-phase stripping module for removing gas-phase organic compounds, an aerodynamic lens module for particle focusing and enrichment, and a thermal desorption module for extracting particle-phase organic compounds. The efficiency of gas-phase organic compound removal and the particle transmission rate of the gas-phase stripping module were firstly investigated. The results showed that gas-phase stripping module achieves a removal efficiency of 98.89% for gas-phase organic compounds, and the particle transmission rate ranges from 75.16% to 91.15% for particles with the diameters between 295 and 375 nm. Subsequently, the effectiveness of the aerodynamic lens was studied through theoretical simulations. The simulation results indicated that the transmission rate of particles in the diameters of 0.36-6.0 μm is above 90% in the designed aerodynamic lens. Finally, The designed aerodynamic lens sampling system with a homemade proton transfer reaction mass spectrometer (PTR-MS) was integrated to develop an aerodynamic lens-PTR-MS device. This device was tested by detecting simulated aerosol samples containing reaction products ofα-pinene and ozone. The mass spectrometry results demonstrated that the intensity changes of the examined ten ions are closely related to the presence ofα-pinene, confirming that the device can achieve online detection of organic compounds in aerosols ranging from submicron to several micrometers.