Making NMR analysis of intact atmospheric samples more feasible: Comprehensive multiphase 1H/13C NMR analysis of air particles collected on electrostatic plates

Air particles are critically important in various atmospheric processes and directly impact human and environmental health. NMR can provide unique insights into air particulate properties, composition and structure. However, atmospheric samples are especially challenging to analyze due to their complexity, heterogeneity, and limited sample sizes. Comprehensive multiphase (CMP) nuclear magnetic resonance (NMR) spectroscopy holds great potential for understanding the composition and interactions of air particles since it combines solid, gel, and liquid-state NMR capabilities into a single probe. This allows particles to be studied as-is (i.e. "intact"), without drying, extraction, or other pre-treatments that could fractionate the samples or alter the particle-water interface. Two sampling techniques are examined. The first, impaction, is a conventional and widely used approach, which collected insufficient material for NMR analysis. The second approach uses an electric field to ionize atmospheric materials before collecting them on electrostatic plates of opposite charge. The material is scraped from the collection plates making recovery easy. On average, the electrostatic collector (∼$160 USD per unit) recovered ∼13 times the mass compared to the impactor, providing enough material for NMR analysis (∼20-40 mg). Samples were collected on a seasonal basis from British Columbia and Ontario (Canada), alongside year-long monthly sampling at the Ontario location. CMP NMR is applied to understand the composition and chemical distributions within sub-phases when swollen with water. This work introduces a combined sampling and NMR analysis approach that should be of interest to all researchers focused on better understanding the source, reactivity and structure of air-particulates.