Aerosol Ion Mobility based Techniques for the Improved Analysis of Chemical Mixtures

Abstract

Particle and ion separations in the gas phase are typically based upon ion mobility (K), also often called the electrical mobility, or simply, the mobility. The mobility is the proportionality coefficient between the steady velocity a particle (charged) moves with and the magnitude of an applied external electric field driving motion. At low electric field strengths, particles are in thermal equilibrium with ambient gas molecules, and the mobility is a constant value independent of field strength. Under these conditions, the mobility is largely a function of particle size, and can be linked to particle diameter. Meanwhile, at high electric field, the translational kinetic energy of charged particles and ions exceeds the thermal energy of gas molecules, and this leads to deviations from thermal equilibrium. Under these conditions the mobility is a function of the field strength, specifically the ratio of the field strength to the gas number density (E/N). The goal of the studies described here was to exploit ion mobility measurement principles in numerous new ways, at both low and high field strengths, to develop particle analysis techniques amenable not only to aerosol particles, but also to particles from liquid suspensions introduced into the gas phase via sprays. The first portion of my dissertation research focuses on an air-jet nebulizer-IMS system consisting of a nanoparticle nebulizer (NPN), a differential mobility analyzer (DMA), and a condensation particle counter (CPC) for the size analysis of chemical mechanical planarization (CMP) slurries. For silica slurries, an air-jet nebulizer-IMS system showed better repeatability and capability for multimodal size distributions. For non-silica slurries, the air-jet nebulizer-IMS system, DLS, and EM differed from each other with peak size shifts of 10 nm or less. The second portion of my dissertation focuses on an IMS-IMS system consisting of two nano DMAs to examine vapor binding to protein molecules in the gas phase. These experiments were performed to determine if vapor binding, leading to mobility shifts, was vapor and protein specific, which would lead to expanded separation capabilities with IMS. In the experiments, the first DMA determined the mobility of protein ions at atmospheric pressure conditions and the second DMA examined shifts in their mobility after the introduction of condensable vapor molecules. It is found that low charge state protein ions adsorb water, nonane, and 1-butanol vapor molecules and the affinity of protein ions to nonane is shown to be higher than to butanol or water when - Köhler theory is applied to experimental results. The third portion of my dissertation research focuses on an IMS-DMS system consisting of a DMA and a field asymmetric ion mobility spectrometer (FAIMS). This system allows a tandem mobility analysis by separating ions both at low field limit and at high field limit. Importantly DMA-FAIMS analysis also enables determination of the actual mobility versus E/N function in a single system. This study also results in the realization of a DMA-FAIMS system and demonstrates the capability of separating ions with the same mobility; benefitting analysis methods in atmospheric new particle formation events and detection of pesticide volatility. The last portion of my dissertation focuses on a Langevin dynamics simulation of particulate film deposition with polydisperse and agglomerated particles. While distinct from the other studies in that it is numerical, the simulations depend upon modeling particle equations of motion, which are also the fundamental equations governing IMS separation. Simulation-deposited films are characterized based their porosities and pore size distributions which are incorporated into calculation of thermal conductivities. The results suggest that the pore size distribution is highly dependent on porosity regardless of other parameters and particle deposited films can achieve comparable thermal conductivities to conventional aerogels.