Off-line LDI-TOF-MS monitoring of simultaneous inorganic and organic reactions on particles levitated in a laboratory environment

Simultaneous inorganic and organic reactions occur on tropospheric particles. The outcomes of these parallel reaction pathways, and particularly the minor reaction products that collectively sum to an appreciable fraction of the particle's mass, are the focus of this work. To characterize condensed phase organic, inorganic, and metal-organic reaction products, we are developing methodology that involves, firstly, the creation of picoliter droplets that carry net charge that are trapped and levitated by an electrodynamic balance in a laboratory environment for the duration of the experiment. Following the rapid evaporation of volatile solvents from the droplets (i.e. seconds), either a residual droplet or a particle remains levitated depending on the atmospheric conditions inside the levitation chamber and the composition of the starting solution from which the initial droplets were created. These droplet residues, whose initial composition is known, are then exposed to gaseous compounds and oxidants that cause transformation of the composition of the levitated droplets/particles by multiphase and heterogeneous reactions that proceed dependent on the species condensing and reacting on the particles. The droplets/particles are then deposited onto a substrate that allows subsequent, and detailed characterization by instrumental methods. In the studies reported here, laser desorption ionization time of flight mass spectrometry was used to characterize reaction products that had formed on the levitated droplets/particles. This apparatus and methodology were used to study the effect of the inorganic reaction of nitric acid vapor with NaCl(aq) and NaCl(s) on the formation of imines by dehydration reaction between the functional groups of a primary amine and an aldehyde. The product yield of imines was found to be dependent on the sequence of introduction of the gaseous reactants to the levitation chamber.