Aerosol Cluster-Ion Interactions: The boundary between Chemical Ionization and Diffusion Charging

VINAY PREMNATH (1), Christopher J. Hogan, Jr. (1)

(1) University of Minnesota, Twin Cities

     Abstract Number: 144
     Last modified: April 24, 2010

Abstract
Knowledge of aerosol particle diffusion charging rates is of significant importance in the inversion of size distribution data taken with electrical mobility based instruments. For most nanoparticles, the diffusion charging rate is well-predicted from the limiting sphere theory of Fuchs. Central to this theory is lack of a back charging reaction, i.e. once an ion collides with a particle, the charge is transferred to the particle, and the charged particle will not lose charge upon collision with uncharged vapor molecules. However, it is also known that such back reactions readily occur in chemical ionization, where ions collide with gas molecules and charge transfer occurs. For sub 3 nm aerosol particles, i.e. aerosol clusters, a similar back reaction may occur, which would have drastic effects on cluster diffusion charging kinetics. We have used a combination of mass spectrometry based measurements and collision kinetic models to examine the size dependency of the charging back reaction between clusters and gas molecules. Experiments involved the generation of protonated gas phase amino acid (such as L-Leucine) cluster ions by electrospray ionization, and the introduction of base vapor (such as Trimethylamine) to facilitate the back reaction, wherein the vapor steals charge from amino acid cluster ions. The drop in signal intensity observed in the presence of base vapor confirmed the occurrence of the back reaction. Our results indicate that the discharge reaction rate is higher for smaller amino acid cluster ions compared to larger cluster ions, confirming that the rate of back reaction is size dependent. A model has also been developed to calculate charged cluster-gas molecule collision rates in the presence of image force and Van der Waals forces. In the limit of small clusters colliding with a gas molecule, the model effectively describes chemical ionization, while at increasing particle size the model converges to the free-molecular limit of Fuchs diffusion charging theory.