Development of Dipolar Proton Transfer Reaction Mass Spectrometer for Real-time Monitoring of Volatile Organic Compounds in Ambient Air

Volatile organic compounds (VOCs) in ambient air can participate in photochemical reactions, which lead to the generation of secondary pollutants such as ozone and aerosol. So real-time and accurate monitoring of atmospheric VOCs plays an important role in the study of the causes of air pollution. On the basis of proton transfer reaction mass spectrometry (PTR-MS) research, a novel dipolar proton transfer reaction mass spectrometer (DP-PTR-MS) for real-time and on-line monitoring of atmospheric VOCs was developed. Compared with conventional PTR-MS with one kind of reagent ion H3O+ , DP-PTR-MS had three kinds of reagent ions H3O+ , OH‒ , (CH3)2COH+ , which could be switched according to the actual detection need. So DP-PTR-MS can improve the qualitative ability and expand the detection range effectively. The reagent ion H3O+ can be used for detecting VOCs whose proton affinities are greater than that of H2O. The reagent ion OH‒ can be used to identify VOCs cooperating with the reagent ion H3O+ , and can also be used for detecting some inorganic substances such as CO2. The reagent ion (CH3)2COH+ can be used for accurately detecting NH3 under interference elimination circumstances. The limit of detection (LOD) and sensitivity of DP-PTR-MS were measured by using six kinds of standard gases. The results showed that the LOD for detecting toluene was 7 × 10‒12 (V/V) and the sensitivity for detecting ammonia reached 126 cps/10‒9 (V/V). The ambient air in Hefei city was on-line and real-time monitored for continuous 78 h with DP-PTR-MS. The results showed that the newly developed DP-PTR-MS could be used for long-term and real-time monitoring atmospheric VOCs at the concentration of 10‒12 (V/V) level. DP-PTR-MS is an important tool to the study of the causes of atmospheric pollution and the monitoring of trace VOCs emissions.