The sorption of vapor molecules from nanoparticles (heterogeneous uptake) in the gas phase plays an important role in new particle formation in the atmosphere as well as in soot formation. Separation techniques can additionally be devised to isolate gas phase analytes which are similar in mass and structure to one another, yet exhibit differential amounts of heterogeneous uptake. However, in spite of both its importance in natural processes and its potential use in separations, heterogeneous vapor uptake for sub-5 nm entities is poorly understood, primarily because of a dearth of available techniques to examine vapor molecule sorption and desorption. This talk focuses on the recent development of techniques to examine heterogeneous uptake, all of which make use of ion mobility spectrometry. Specifically, we use a combination of differential mobility analysis with mass spectrometry (DMA-MS), as well as differential mobility analysis-drift tube ion mobility spectrometry to infer the chemical composition and collision cross sections of both organic and inorganic nanoclusters. During measurements, we seed the ion mobility spectrometers (the DMA and the drift tube) with controlled amounts of water vapor, and correlate shifts in measured mobility with the extent of water vapor uptake. We demonstrate that it is possible to characterize the extent of heterogeneous vapor uptake by nanoclusters down to the single vapor molecule level. Further, measurements may be compared directly to theoretical predictions of the extent of heterogeneous vapor uptake, based on both classical theories or modern computational approaches.
Chris Hogan is a McKnight Land-Grant Assistant Professor in the department of Mechanical Engineering at the University of Minnesota. He received his BS degree in Biological & Environmental Engineering from Cornell University in 2004, and his PhD degree in Energy, Environmental, & Chemical Engineering from Washington University in 2008. After studying as a Postdoctoral Associate at Yale University in 2008-2009, he joined the faculty at the University of Minnesota in July 2009. He is the recipient of the 2011 Sheldon K. Friedlander Award for “Outstanding PhD dissertation in a field of aerosol science and technology”, and the 2013 Marian Smoluchowski Award for “Outstanding contributions in aerosol science”. Currently, his laboratory group, the Nanoparticle Physics Laboratory, focuses on the analysis of mass, momentum, and energy transport processes in nanoparticle-laden aerosols and colloids.