New mass spectrometry techniques for studying physical chemistry of atmospheric heterogeneous processes

Ion spectroscopy in methane activation

This review is devoted to ion spectroscopy studies of complexes relevant for the understanding of methane activation with metal ions and clusters. Methane activation starts with the formation of a complex with a metal ion. The degree of the interaction between an intact methane molecule and the ion can be monitored by the perturbations of C-H stretch vibrations in the methane molecule. Binding mediated by the electrostatic interaction results in a η³ type coordination of methane. In contrast, binding governed by orbital interactions results in a η² type coordination of methane. We further review the spectroscopic characterization of activation products of metal-methane reactions, such as the metal-carbene and carbyne products resulting from the interaction of selected 5d metals with methane. The focus of recent research in the field has shifted towards the investigation of interactions between methane and metal clusters. We show examples highlighting that metal clusters can be more reactive in methane activation reactions.

Chemistry of NOx and HNO3 Molecules with Gas-Phase Hydrated O.- and OH- Ions

The gas‐phase reactions of O^.−(H₂O)_n and OH⁻(H₂O)_n, n=20–38, with nitrogen‐containing atmospherically relevant molecules, namely NO_x and HNO₃, are studied by Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometry and theoretically with the use of DFT calculations. Hydrated O^.− anions oxidize NO^. and NO₂^. to NO₂⁻ and NO₃⁻ through a strongly exothermic reaction with enthalpy of −263±47 kJ mol⁻¹ and −286±42 kJ mol⁻¹, indicating a covalent bond formation. Comparison of the rate coefficients with collision models shows that the reactions are kinetically slow with 3.3 and 6.5 % collision efficiency. Reactions between hydrated OH⁻ anions and nitric oxides were not observed in the present experiment and are most likely thermodynamically hindered. In contrast, both hydrated anions are reactive toward HNO₃ through proton transfer from nitric acid, yielding hydrated NO₃⁻. Although HNO₃ is efficiently picked‐up by the water clusters, forming (HNO₃)_0–2(H₂O)_mNO₃⁻ clusters, the overall kinetics of nitrate formation are slow and correspond to an efficiency below 10 %. Combination of the measured reaction thermochemistry with literature values in thermochemical cycles yields ΔH_f(O⁻(aq.))=48±42 kJ mol⁻¹ and ΔH_f(NO₂⁻(aq.))=−125±63 kJ mol⁻¹.

Mass spectrometry as a tool to advance polymer science

In contrast to natural polymers, which have existed for billions of years, the first well-understood synthetic polymers date back to just over one century ago. Nevertheless, this relatively short period has seen vast progress in synthetic polymer chemistry, which can now afford diverse macromolecules with varying structural complexities. To keep pace with this synthetic progress, there have been commensurate developments in analytical chemistry, where mass spectrometry has emerged as the pre-eminent technique for polymer analysis. This Perspective describes present challenges associated with the mass-spectrometric analysis of synthetic polymers, in particular the desorption, ionization and structural interrogation of high-molar-mass macromolecules, as well as strategies to lower spectral complexity. We critically evaluate recent advances in technology in the context of these challenges and suggest how to push the field beyond its current limitations. In this context, the increasingly important role of high-resolution mass spectrometry is emphasized because of its unrivalled ability to describe unique species within polymer ensembles, rather than to report the average properties of the ensemble.

Accurate representations of the physicochemical properties of atmospheric aerosols: when are laboratory measurements of value?

Laboratory studies can provide important insights into the processes that occur at the scale of individual particles in ambient aerosol. We examine the accuracies of measurements of core physicochemical properties of aerosols that can be made in single particle studies and explore the impact of these properties on the microscopic processes that occur in ambient aerosol. Presenting new measurements, we examine here the refinements in our understanding of aerosol hygroscopicity, surface tension, viscosity and optical properties that can be gained from detailed laboratory measurements for complex mixtures through to surrogates for secondary organic atmospheric aerosols.

Probing surfaces of atmospherically relevant organic particles by easy ambient sonic-spray ionization mass spectrometry (EASI-MS)

EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

Both ambient and laboratory-generated particles can have a surface composition different from the bulk, but there are currently few analytical techniques available to probe these differences. Easy ambient sonic-spray ionization mass spectrometry (EASI-MS) was applied to solid, laboratory-generated particles with core–shell morphologies formed from a variety of dicarboxylic acids. The soft ionization facilitated parent peak detection for the two compounds, from which the depth probed could be determined from the relative signal intensities. Two different configurations of a custom-made nebulizer are reported that yield different probe depths. In the “orthogonal mode,” with the nebulizer ∼10 centimeters away from the particle stream and at a 90° angle to the MS inlet, evaporation of the nebulizer droplets forms ions before interaction with the particles. The probe depth for orthogonal mode EASI-MS is shown to be 2–4 nm in these particle systems. In the “droplet mode”, the nebulizer and particle streams are in close proximity to each other and the MS inlet so that the particles interact with charged liquid droplets. This configuration resulted in full dissolution of the particles and gives particle composition similar to that from collection on filters and extraction of the particles (bulk). These studies establish that EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

A comprehensive protocol for chemical analysis of flame combustion emissions by secondary ion mass spectrometry

RATIONALE

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is used to provide detailed information on the surface chemical composition of soot. An analytical protocol is proposed and tested on a laboratory flame, and the results are compared with our previous measurements provided by two-step laser mass spectrometry (L2MS).

METHODS

This work details: (1) the development of a dedicated apparatus to sample combustion products from atmospheric flames and deposit them on substrates suitable for TOF-SIMS analysis; (2) the choice of the deposition substrate and the material of the sampling line, and their effect on the mass spectra; (3) a method to separate the contributions of soot and condensable gas based on impact deposition; and finally (4) post-acquisition data processing.

RESULTS

Compounds produced during flame combustion are detected on the surface of different deposition substrates and attributed a molecular formula based on mass defect analysis. Silicon and titanium wafers perform similarly, while the surface roughness of glass microfiber filters results in a reduced mass resolution. The mass spectra obtained from the analysis of different locations of the deposits obtained by impaction show characteristic patterns that are attributed to soot/condensable gas.

CONCLUSIONS

A working method for the analysis of soot samples and the extraction of useful data from mass spectra is proposed. This protocol should help to avoid common experimental issues like sample contamination, while optimizing the setup performance by maximizing the achievable mass resolution.

A cautionary note on the effects of laboratory air contaminants on ambient ionization mass spectrometry measurements

RATIONALE

Ambient ionization mass spectrometry methods are convenient, sensitive and require little sample preparation. However, they are susceptible to species present in air surrounding the mass spectrometer. This study identifies some challenges associated with the potential impacts of indoor air contaminants on ionization and analysis involving open-air methods.

METHODS

Unexpected effects of volatile organic compounds (VOCs) from floor maintenance activities on ambient ionization mass spectrometry were studied using three different ambient ionization techniques. Extractive electrospray ionization (EESI), direct analysis in real time (DART) and ionization by piezoelectric direct discharge (PDD) plasma were demonstrated in this study to be affected by indoor air contaminants. Identification of contaminant vapors was verified by comparison with standards using EESI-MS/MS product ion scans.

RESULTS

Emissions of diethylene glycol monoethyl ether and ethylene glycol monobutyl ether are identified from floor stripping and waxing solutions using three ambient ionization mass spectrometry techniques. These unexpected indoor air contaminants are capable of more than 75% ion suppression of target analytes due to their high volatility, proton affinity and solubility compared with the target analytes. The contaminant vapors are also shown to form adducts with one of the target analytes.

CONCLUSIONS

The common practice in MS analysis of subtracting a background air spectrum may not be appropriate if the presence of ionizable air contaminants alters the spectrum in unexpected ways. For example, VOCs released into air from floor stripping and waxing are capable of causing ion suppression of target analytes.

Efficient scavenging of Criegee intermediates on water by surface-active cis-pinonic acid

Criegee intermediates efficiently react with surface-active cis-pinonic acid rather than linear alkyl organic acids of similar size, or interfacial water molecules at air-aqueous interfaces.

Progress in the Analysis of Complex Atmospheric Particles

This article presents an overview of recent advances in field and laboratory studies of atmospheric particles formed in processes of environmental air-surface interactions. The overarching goal of these studies is to advance predictive understanding of atmospheric particle composition, particle chemistry during aging, and their environmental impacts. The diversity between chemical constituents and lateral heterogeneity within individual particles adds to the chemical complexity of particles and their surfaces. Once emitted, particles undergo transformation via atmospheric aging processes that further modify their complex composition. We highlight a range of modern analytical approaches that enable multimodal chemical characterization of particles with both molecular and lateral specificity. When combined, these approaches provide a comprehensive arsenal of tools for understanding the nature of particles at air-surface interactions and their reactivity and transformations with atmospheric aging. We discuss applications of these novel approaches in recent studies and highlight additional research areas to explore the environmental effects of air-surface interactions.

"Sizing" Heterogeneous Chemistry in the Conversion of Gaseous Dimethyl Sulfide to Atmospheric Particles

The oxidation of biogenic dimethyl sulfide (DMS) emissions is a global source of cloud condensation nuclei. The amounts of the nucleating H2SO4(g) species produced in such process, however, remain uncertain. Hydrophobic DMS is mostly oxidized in the gas phase into H2SO4(g) + DMSO(g) (dimethyl sulfoxide), whereas water-soluble DMSO is oxidized into H2SO4(g) in the gas phase and into SO4(2-) + MeSO3(-) (methanesulfonate) on water surfaces. R = MeSO3(-)/(non-sea-salt SO4(2-)) ratios would therefore gauge both the strength of DMS sources and the extent of DMSO heterogeneous oxidation if Rhet = MeSO3(-)/SO4(2-) for DMSO(aq) + ·OH(g) were known. Here, we report that Rhet = 2.7, a value obtained from online electrospray mass spectra of DMSO(aq) + ·OH(g) reaction products that quantifies the MeSO3(-) produced in DMSO heterogeneous oxidation on aqueous aerosols for the first time. On this basis, the inverse R dependence on particle radius in size-segregated aerosol collected over Syowa station and Southern oceans is shown to be consistent with the competition between DMSO gas-phase oxidation and its mass accommodation followed by oxidation on aqueous droplets. Geographical R variations are thus associated with variable contributions of the heterogeneous pathway to DMSO atmospheric oxidation, which increase with the specific surface area of local aerosols.

Multistage Reactive Transmission-Mode Desorption Electrospray Ionization Mass Spectrometry

Elucidating reaction mechanisms is important for advancing many areas of science such as catalyst development. It is often difficult to probe fast reactions at ambient conditions with high temporal resolution. In addition, systems involving reagents that cross-react require analytical methods that can minimize interaction time and specify their order of introduction into the reacting system. Here, we explore the utility of transmission mode desorption electrospray ionization (TM-DESI) for reaction monitoring by directing a microdroplet spray towards a series of meshes with micrometer-sized openings coated with reagents, an approach we call multistage reactive TM-DESI (TM (n) -DESI, where n refers to the number of meshes; n = 2 in this report). Various stages of the reaction are initiated at each mesh surface, generating intermediates and products in microdroplet reaction vessels traveling towards the mass spectrometer. Using this method, we investigated the reactivity of iron porphyrin catalytic hydroxylation of propranolol and other substrates. Our experimental results indicate that TM (n) -DESI provides the ability to spatially separate reagents and control their order of introduction into the reacting system, thereby minimizing unwanted reactions that lead to catalyst deactivation and degradation products. In addition, comparison with DESI-MS analyses (the Zare and Latour laboratories published results suggesting accessible reaction times <1 ms) of the reduction of dichlorophenolindophenol by L-ascorbic acid suggest that TM (1) -DESI can access reaction times less than 1 ms. Multiple meshes allow sequential stages of desorption/ionization per MS scan, increasing the number of analytes and reactions that can be characterized in a single experiment.

Heterogeneous Reactivity of Nitric Acid with Nascent Sea Spray Aerosol: Large Differences Observed between and within Individual Particles

Current climate and atmospheric chemistry models assume that all sea spray particles react as if they are pure NaCl. However, recent studies of sea spray aerosol particles have shown that distinct particle types exist (including sea salt, organic carbon, and biological particles) as well as mixtures of these and, within each particle type, there is a range of single-particle chemical compositions. Because of these differences, individual particles should display a range of reactivities with trace atmospheric gases. Herein, to address this, we study the composition of individual sea spray aerosol particles after heterogeneous reaction with nitric acid. As expected, a replacement reaction of chloride with nitrate is observed; however, there is a large range of reactivities spanning from no reaction to complete reaction between and within individual sea spray aerosol particles. These data clearly support the need for laboratory studies of individual, environmentally relevant particles to improve our fundamental understanding as to the properties that determine reactivity.

CO2 diffusion in champagne wines: a molecular dynamics study

Although diffusion is considered as the main physical process responsible for the nucleation and growth of carbon dioxide bubbles in sparkling beverages, the role of each type of molecule in the diffusion process remains unclear. In the present study, we have used the TIP5P and SPC/E water models to perform force field molecular dynamics simulations of CO2 molecules in water and in a water/ethanol mixture respecting Champagne wine proportions. CO2 diffusion coefficients were computed by applying the generalized Fick's law for the determination of multicomponent diffusion coefficients, a law that simplifies to the standard Fick's law in the case of champagnes. The CO2 diffusion coefficients obtained in pure water and water/ethanol mixtures composed of TIP5P water molecules were always found to exceed the coefficients obtained in mixtures composed of SPC/E water molecules, a trend that was attributed to a larger propensity of SPC/E water molecules to form hydrogen bonds. Despite the fact that the SPC/E model is more accurate than the TIP5P model to compute water self-diffusion and CO2 diffusion in pure water, the diffusion coefficients of CO2 molecules in the water/ethanol mixture are in much better agreement with the experimental values of 1.4 - 1.5 × 10(-9) m(2)/s obtained for Champagne wines when the TIP5P model is employed. This difference was deemed to rely on the larger propensity of SPC/E water molecules to maintain the hydrogen-bonded network between water molecules and form new hydrogen bonds with ethanol, although statistical issues cannot be completely excluded. The remarkable agreement between the theoretical CO2 diffusion coefficients obtained within the TIP5P water/ethanol mixture and the experimental data specific to Champagne wines makes us infer that the diffusion coefficient in these emblematic hydroalcoholic sparkling beverages is expected to remain roughly constant whathever their proportions in sugars, glycerol, or peptides.

Imaging nanocarbon materials: soot particles in flames are not structurally homogeneous

For the first time, nascent soot particles are probed by using helium-ion microscopy (HIM). HIM is a technique that is similar to scanning electron microscopy (SEM) but it can achieve higher contrast and improved surface sensitivity, especially for carbonaceous materials. The HIM microscope yields images with a high contrast, which allows for the unambiguous recognition of smaller nascent soot particles than those observed in previous transmission electron microscopy studies. The results indicate that HIM is ideal for rapid and reliable probing of the morphology of nascent soot, with surface details visible down to approximately 5 nm, and particles as small as 2 nm are detectable. The results also show that nascent soot is structurally and chemically inhomogeneous, and even the smallest particles can have shapes that deviate from a perfect sphere.