High-Resolution Mobility and Mass Spectrometry of Negative Ions Produced in a 241Am Aerosol Charger

This work concentrates on the simultaneous mobility and mass measurement of negative ions generated by the ionizing radiation in a Am-241 aerosol charger in N-2 (5.0), a 1:1-mixture of N-2 and synthetic air, pure synthetic air (5.0), and filtered laboratory air at approximate to 30% relative humidity. Therefore, a high-resolution mobility analyzer (UDMA) and an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF) were operated in series. Experiments with N-2 as carrier gas showed a dominating signal at an electrical mobility of 2.09cm(2)/Vs with 90% of the ions being nitrate based. The ion composition was altered after a baking-out to a spectrum with three strong mobility-peaks at Z(1) = 2.34cm(2)/Vs, Z(2) = 1.42cm(2)/Vs, Z(3) = 1.08cm(2)/Vs and a higher diversity of ions in the corresponding mass spectra. The carrier gas was gradually changed from N-2 (5.0) to a 1:1-mixture of N-2 with synthetic air and pure synthetic air (5.0), having only a minor effect on the overall pattern of the ion spectrum. Using room air leads to a domination of the nitrate based ions. The mobility-dependent transmission efficiency of the UDMA was modeled using an empirical, laminar diffusion deposition model. The data were further compared to an empirical mass-mobility relationship to evaluate the fragmentation of the ion clusters in the inlet of the mass spectrometer. This study suggests that the nitrate ion, NO3 (-), is found to be the dominant ion species produced in an aerosol charger, and that it may be mostly responsible for the charging of aerosol particles in negative polarity. Copyright 2014 American Association for Aerosol Research