Detection of Ultratrace Oxidized Products in Spontaneous Microdroplet Oxidation Chemistry Experiments: Exploring Potential Sources of Measurement Variability

Abstract

Recent observations of unexpected, spontaneous oxidation reactions occurring in the presence of water microdroplets that otherwise do not occur in bulk water challenge our understanding of interfacial water chemistry. If such processes occur generally in water-based droplets suspended in air, there are remarkable implications for atmospheric chemistry, sustainability, and airborne disease transmission, among others. However, the literature contains both positive and null outcomes when attempting to detect spontaneously produced oxidized species in droplets generated with and without an applied voltage. To incorporate spontaneous oxidation microdroplet chemistry into, e.g., atmospheric chemistry models, it will be crucial to identify sources of variability across experiments and determine whether these processes are occurring in neutral, contact-free droplets or as an experimental artifact of, e.g., contact electrification. To identify potential sources of measurement variability, we interrogated microdroplet oxidation chemistry in the context of two surprising observations from the literature: the spray-based degradation of perfluorooctanoic acid (PFOA) and the oxidation of gaseous volatile organic compounds (VOCs, e.g., ethane). We readily observed the spray-based degradation of PFOA, observing three times more degradation than previously reported, and demonstrated that the presence of oxygen can promote enhanced degradation. However, we could not detect the oxidation of VOCs without using an ultratrace detection scheme based on cryogenic focusing of analytes. Our ultratrace detection scheme revealed oxidation products at concentrations 5–6 orders of magnitude lower than expected. We identify several sources of potential variability, including the presence of ionic and nonionic solutes in the microdroplets, which promoted oxidation at concentrations as low as 10–15 M in the bulk water. We also demonstrate that variable droplet charging can suppress or enhance observed oxidation, depending on polarity. Our observations suggest that microdroplet oxidation reactions may be a general phenomenon in sprays, although with dramatic variability between experiments, and we make no claims as to the specific oxidation mechanism responsible for these observations. On the basis of our measurements, we recommend strategies to minimize variability and enhance reproducibility when performing spontaneous microdroplet chemistry experiments at ambient conditions with no applied voltages.