Photochemically Driven Peptide Formation in Supersaturated Aerosol Droplets.

The condensation of amino acids into peptides plays a crucial role in protein synthesis and is thus essential for understanding the origins of life. However, the spontaneous formation of peptides from amino acids in bulk aqueous media is energetically unfavorable, posing a challenge for elucidating plausible abiotic mechanisms. In this study, we investigate the formation of amide bonds between amino acids within highly supersaturated aerosol droplets containing dicyandiamide (DCD), a cyanide derivative potentially present on primordial Earth. Metastable states, i.e. supersaturation, within individual micron-sized droplets are studied using both an optical trap and a linear quadrupole electrodynamic balance. When irradiated with intense visible light, amide bond formation is observed to occur and can be monitored using vibrational bands in Raman spectra. The reaction rate is found to be strongly influenced by droplet size and kinetic modelling suggests that it is driven by the photochemical product of a DCD self-reaction. Our results highlight the potential of atmospheric aerosol particles as reaction environments for peptide synthesis and have potential implications for the prebiotic chemistry of early Earth.

Chemically Resolved Evaporation Dynamics of Dicarboxylic Acid Mixtures in Solid Particles.

The evaporation rate and corresponding vapor pressure of dicarboxylic acids have been the subject of numerous scientific studies over the years, with reported values spanning several orders of magnitude. Recent work has identified the importance of considering the phase state of the material during evaporation, likely accounting for some of the variability in measured vapor pressures. In the homologous series of dicarboxylic acids, the phase state under dry conditions may be crystalline or amorphous, with particles of odd-carbon-numbered acids exhibiting tendencies to remain amorphous and spherical. Although measurements of vapor pressures for pure components make up most of the available literature data, for many applications, these compounds are not present in isolation. Additionally, many systems containing a semi-volatile material exist in a solid state, especially under dry and low relative humidity conditions. In this work, we explore the evaporation of compounds present in mixed solid-state particles. Specifically, we use single particle levitation coupled with mass spectrometry to measure the evolving composition of solid particles containing mixtures of glutaric acid and succinic acid, glutaric acid and adipic acid, and malonic acid and succinic acid. Under dry conditions, these systems exhibit non-spherical geometries consistent with crystallization of one or both components into an organic crystal. Our measurements allow the evaporation of each component in the mixture to be characterized independently and effective vapor pressures of the pure components to be inferred. The resulting vapor pressures are compared against pure component vapor pressures. We demonstrate that these mixtures exhibit thermodynamic ideality but can be influenced by limited diffusion in the solid phase. These are the first results in the literature that explore the thermodynamic and kinetic factors that control the evaporative evolution of mixed solid-state particles.

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.

Connecting the Phase State and Volatility of Dicarboxylic Acids at Elevated Temperatures.

The partitioning of semivolatile organic molecules between condensed phases and the vapor phase has broad application across a range of scientific disciplines, with significant impacts in atmospheric chemistry for regulating the evolving composition of aerosol particles. Vapor partitioning depends on the molecular interactions and phase state of the condensed material and shows a well-established dependence on temperature. The phase state of solid organic material is not always well-defined, and many examples can be found for the formation of amorphous subcooled liquid states rather than crystalline solids. This can lead to significant changes to vapor equilibrium processes by modifying the thermodynamics and kinetics of evaporation. Here, we explore the influence of phase state on the evaporation dynamics of a series of straight-chain dicarboxylic acids across a range of above-ambient temperatures. These molecules show an odd/even alteration in some of their properties based on the number of carbon atoms that may be connected to their phase state under dry conditions. Using a newly developed linear-quadrupole electrodynamic balance, we levitate single particles containing the sample and expose them to dry conditions across a range of temperatures (ambient to ∼350 K). Using the rate of evaporation measured from the change in the size or relative mass, we derive the vapor pressure and enthalpy of vaporization. Light scattering data allows for unambiguous identification of the phase of the particles (crystal vs amorphous) allowing the vapor equilibrium properties to be attributed to a particular state. This work highlights a new experimental method for characterizing vapor pressures of low volatility substances and extends the temperature range of available data for the vapor pressure of terminal dicarboxylic acids. These measurements show that crystalline and subcooled liquid states persist at elevated temperatures and provide a direct comparison between subcooled and crystal phases under the same experimental conditions.

Paper spray mass spectrometry for the analysis of picoliter droplets.

Recent experimental efforts have shown that single particle levitation methods may be effectively coupled with mass spectrometry (MS) using paper spray (PS) ionization for compositional analysis of picoliter droplets. In this work, we characterize the response of PS-MS to analytes delivered in the form of picoliter droplets and explore its potential for identification and quantification of these samples. Using a microdroplet dispenser to generate droplets, we demonstrate sensitivity to a range of oxygenated organic molecules typical of compounds found in atmospheric secondary organic aerosol. We assess experimental factors that influence the reproducibility and sensitivity of the method and explore the linearity of the system response to increasing analyte mass in droplets containing single or multicomponent analytes. We show that the ratio of analyte signal from multicomponent samples may be used to characterize the relative composition of the system. These measurements demonstrate that the droplet PS-MS method is an effective tool for qualitative and quantitative analysis of single picoliter droplets containing picogram levels of analyte. The potential applications of this technique for characterizing the composition of levitated particles will be discussed.