Journal of Aerosol Science, Vol.90, 36-50, 2015
The mass-mobility distributions of ions produced by a Po-210 source in air
We utilized a differential mobility analyzer-mass spectrometer (DMA-MS) with a high resolving power (similar to 45) parallel plate DMA to measure the masses and mobilities of positive and negative ions generated in dry, particle free air with a 10 mCi Po-210 a-irradiation source. We are able to chemically identify 6 positive ions and 25 negative ions in spectra. The positive ions detected were generated by protonation, and included protonated acetone (59 Da), protonated bis-(2-ethylhexyl)phthalate (391 Da), and protonated cyclic polydimethylsiloxanes. The negative ions detected were all deprotonated, and included Teflon-derived perfluoralkanoate ions, long chain alcohol derived alkoxide ions, and the nitrate ion, which, despite its low mass and high mobility was the most abundant ion in negative spectra. Most detected ions can be traced back to system components or to compounds used in the manufacture of system components. Though clearly the detected ions are system specific, the conditions employed were similar to those used in many laboratory scale aerosol measurement systems. The mobility-mass relationship for measured ions was compared to prior mass-mobility measurements of ions produced via alpha a-irradiation, as well as to the predictions of gas molecule scattering calculations, which took as inputs semi-emipirical AM1 based structural models. Through this comparison we show that (1) a universal mobility mass relationship cannot be applied to all ions and (2) for most ions, gas molecule scattering with diffuse scattering rules can be used to accurately predict ion mobilities. Ion spectra collected with the same system but several months apart were also compared, demonstrating that although similar species are detected, the generated ion spectrum can vary over time. Finally, Brownian dynamics simulations with measured polydisperse ion properties were utilized to determine the expected steady-state charge distribution for particles exposed to the measured ions. Via simulations, we Find that the presence or absence of the negative nitrate ion has a strong effect on the steady-state charge distribution and that in most systems it is high concentrations of this ion (relative to others) which can bias charge distributions to negative polarity. Conversely, absence of the nitrate ion shifts charge distributions towards the positive mode. (C) 2015 Elsevier Ltd. All rights reserved.