AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
The Mass and Mobility Distributions of Ions Generated by a 10mCi Po-210 Alpha Particle Source as Measured by Differential Mobility Analysis-Mass Spectrometry
Mark Meredith, Carlos Larriba-Andaluz, Hui Ouyang, Ranganathan Gopalakrishnan, Derek Oberreit, CHRISTOPHER HOGAN JR., University of Minnesota
Abstract Number: 168 Working Group: Instrumentation and Methods
Abstract The unipolar diffusion charging rate and steady state bipolar charge distribution on aerosol particles are dependent on the mass and electrical mobility distributions of the charging ions. However, efforts to precisely determine these distributions have been scarce. We have used a parallel plate differential mobility analyzer coupled to a time of flight mass spectrometer (DMA-MS) to measure the two-dimensional mass-mobility distributions of both positive and negative ions produced by a 10 mCi Po-210 alpha particle source in dry air. In these experiments, ions were drawn into the DMA electrostatically, and the tubing involved was predominantly brass or stainless steel, with a small section of teflon tubing and delrin pastic housing the Po-210 source (keeping it isolated electrostatically from the DMA). The mass spectrometer had sufficiently high resolving power to enable clear identification of the atomic composition of detection ions. Overwhelmingly, both positive and negative ions are found to be ionized by protonation/deprotonation or electron addition/removal, and are composed of organic species from either tubing or which are commonly used in machining fluids (i.e. residues remaining on components after the manufacturing process), or from non-zero vapor pressure components of O-rings (polydimethylsiloxanes). The average masses and mobilities of negative ions are found to be less than those of their positive counterparts, and further narrower mass and mobility distributions are found for negative ions. Density functional theory (DFT) calculations were performed to determine candidate structures for the detected ions, with which gas molecule scattering calculations were applied to compare the predicted and measured mobilities for these ions. Generally, we find that scattering calculations with diffuse and inelastic reemission rules and considering the ion induced dipole potential, predict well the mobilities of ions.