Paper spray mass spectrometry for the analysis of picoliter droplets

Coupled Interfacial and Bulk Kinetics Govern the Timescales of Multiphase Ozonolysis Reactions

Chemical transformations in aerosols impact the lifetime of particle phase species, the fate of atmospheric pollutants, and both climate- and health-relevant aerosol properties. Timescales for multiphase reactions of ozone in atmospheric aqueous phases are governed by coupled kinetic processes between the gas phase, the particle interface, and its bulk, which respond dynamically to reactive consumption of O₃. However, models of atmospheric aerosol reactivity often do not account for the coupled nature of multiphase processes. To examine these dynamics, we use new and prior experimental observations of aqueous droplet reaction kinetics, including three systems with a range of surface affinities and ozonolysis rate coefficients (trans-aconitic acid (C₆H₆O₆), maleic acid (C₄H₄O₄), and sodium nitrite (NaNO₂)). Using literature rate coefficients and thermodynamic properties, we constrain a simple two-compartment stochastic kinetic model which resolves the interface from the particle bulk and represents O₃ partitioning, diffusion, and reaction as a coupled kinetic system. Our kinetic model accurately predicts decay kinetics across all three systems, demonstrating that both the thermodynamic properties of O₃ and the coupled kinetic and diffusion processes are key to making accurate predictions. An enhanced concentration of adsorbed O₃, compared to gas and bulk phases is rapidly maintained and remains constant even as O₃ is consumed by reaction. Multiphase systems dynamically seek to achieve equilibrium in response to reactive O₃ loss, but this is hampered at solute concentrations relevant to aqueous aerosol by the rate of O₃ arrival in the bulk by diffusion. As a result, bulk-phase O₃ becomes depleted from its Henry's law solubility. This bulk-phase O₃ depletion limits reaction timescales for relatively slow-reacting organic solutes with low interfacial affinity (i.e., trans-aconitic and maleic acids, with k_rxn ≈ 10³-10⁴ M^-1 s^-1), which is in contrast to fast-reacting solutes with higher surface affinity (i.e., nitrite, with k_rxn ≈ 10⁵ M^-1 s^-1) where surface reactions strongly impact the observed decay kinetics.

An Open Port Sampling Interface for the Chemical Characterization of Levitated Microparticles

Several studies have reported ionization methods to classify the chemical composition of levitated particles held in an electrodynamic balance using mass spectrometry (MS). These methods include electrospray-based paper spray (PS) ionization, plasma discharge ionization, and direct analysis in real-time (DART) ionization, with each showing advantages and disadvantages. Our recent work demonstrated that PS ionization could yield accurate data for the chemical evolution of mixed component particles undergoing evaporation. However, measurements were performed using an internal standard to account for and correct the inherent variability in the PS ionization source. Here, we explore a new electrospray-based method coupled to particle levitation-the Open Port Sampling Interface (OPSI), which provides many advantages over the PS method, with few disadvantages. In this application note we report experiments in which micron-sized particles, containing analytes such as citric acid, maleic acid, and tetraethylene glycol, were levitated and optically probed to determine their size and mass. Subsequent transfer of individual levitated particles into the OPSI allowed for the ionization and mass spectrometry analysis of these particles. We discuss the stability and reproducibility of MS measurements, demonstrate effective quantitation in both positive and negative mode, and determine the sensitivity of the OPSI to a range of analyte mass present in levitated particles. Importantly, we show stability of the OPSI over >6 h without the need for normalizing signal variations with an internal standard in the sample, demonstrating robust application of the OPSI to measurements over extended periods of time.

Electrodynamic balance–mass spectrometry reveals impact of oxidant concentration on product composition in the ozonolysis of oleic acid

Electrodynamic balance–mass spectrometry measurements reveal how the ozone concentration influences the product composition of oleic acid aerosol droplets.

Measuring the Chemical Evolution of Levitated Particles: A Study on the Evaporation of Multicomponent Organic Aerosol

Single-particle levitation methods provide an effective platform for probing the physical properties of atmospheric aerosol via micrometer-sized particles. Until recently, chemical composition measurements on levitated particles were limited to spectroscopy, yielding only basic chemical information. Here, we describe, benchmark, and discuss the applications of an approach for probing the physical properties and chemical composition of single levitated particles using high-resolution mass spectrometry (MS). Using a linear quadrupole electrodynamic balance (LQ-EDB) coupled to paper spray mass spectrometry, we report accurate measurements of the evolving size within 5 nm (using broadband light scattering) and relative composition (using MS) of evaporating multicomponent levitated particles in real time. Measurements of the evaporation dynamics of semivolatile organic particles containing a range of n-ethylene glycols (n = 3, 4, and 6) in various binary and ternary mixtures were made under dry conditions and compared with predictions from a gas-phase diffusion evaporation model. Under assumptions of ideal mixing, excellent agreement for both size and composition evolution between measurements and models were obtained for these mixtures. At increased relative humidity, the presence of water in particles causes the assumption of ideality to break down, and the evaporative mass flux becomes a function of the mole fraction and activity coefficient. Through compositionally resolved evaporation measurements and thermodynamic models, we characterize the activity of organic components in multicomponent particles. Our results demonstrate that the LQ-EDB-MS platform can identify time-dependent size and compositional changes with high precision and reproducibility, yielding an effective methodology for future studies on chemical aging and gas-particle partitioning in suspended particles.

Combining Mass Spectrometry of Picoliter Samples with a Multicompartment Electrodynamic Trap for Probing the Chemistry of Droplet Arrays

Single droplet levitation provides contactless access to the microphysical and chemical properties of micrometer-sized samples. Most applications of droplet levitation to chemical and biological systems use nondestructive optical techniques to probe droplet properties. To provide improved chemical specificity, we coupled a multicompartment quadrupole electrodynamic trap (QET) with single droplet mass spectrometry. Our QET continuously traps a monodisperse droplet population (tens to hundreds of droplets) and allows for the simultaneous sizing of a single droplet using its Mie scattering pattern. Single droplets are subsequently ejected into the ionization region of an ambient pressure inlet mass spectrometer. We optimized two complementary soft ionization techniques for picoliter aqueous droplets: (1) paper spray (PS) ionization and (2) thermal desorption glow discharge (TDGD) ionization. Both techniques detect oxygenated organic acids in single droplets, with signal-to-noise ratios >100 and detection limits on the order of 10 pg. Sensitivity and reproducibility across single droplets are driven by the droplet deposition location and spray stability in PS-MS and the ionization region humidity and analyte evaporation rate in TDGD-MS. Importantly, the analyte evaporation rate can control the TDGD-MS quantitative capability because high evaporation rates result in significant ion suppression. This effect is mitigated by optimizing the vaporization temperature, droplet size range, and analyte volatility. We demonstrate quantitative and reproducible measurements with a droplet internal standard (<10% RSD) and compare the sensitivity of PS-MS and TDGD-MS. Finally, we demonstrate the application of QET-MS to the study of heterogeneous chemical kinetics with the reaction of gas phase O₃ and aqueous maleic acid droplets.