Application of Direct Analysis in Real Time-Mass Spectrometry (DART-MS) to the study of gas-surface heterogeneous reactions: focus on ozone and PAHs

Ambient mass spectrometry imaging of food natural products by angled direct analysis in real time high-resolution mass spectrometry

Direct surface analysis in ambient conditions provides information on the position and chemical composition of an object at the time of investigation. An angled sampling probe is developed in this work for direct analysis in real time (DART) ionization high-resolution mass spectrometry. The DART ion source and the interface were modified for improved surface resolution, increased ion transfer efficiency, as well as enabling two-dimensional surface scanning. The angled probe DART-MS system was used for investigating a variety of food samples including fruit peels, ginseng root, plant leaves and sections of radish. Abundant signals and distinct chemical profiles are obtained in seconds, and spatial distribution of different molecules across the sample surfaces can be observed. In addition, the developed system can quickly identify the chemical changes when the surfaces were treated. The method is capable of directly evaluating food sample surfaces with different shapes, hardness, and conditions, without any sample pretreatments.

Nanospray Laser-Induced Plasma Ionization Mass Spectrometry for Sensitive and High-Coverage Analysis of Broad Polarity Organic Pollutants

Accurate, sensitive, and high-coverage analysis of various organic pollutants in complex samples and single cells is significant in investigating their environmental behaviors and toxic effects. Here we proposed a nanospray laser-induced plasma ionization mass spectrometry (nLIPI-MS) method for sensitive and high-coverage analysis of broad polarity organic pollutants under ambient and open-air conditions. The nLIPI-MS method combines nanospray with laser-induced plasma ionization, enabling direct analysis without sophisticated sample pretreatment. For the analysis of nonpolar and very weakly polar organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and alkyl-PAHs, the nLIPI-MS method showed desirable analytical performance, with limits of detection of as low as the μg/L level. For moderately polar organic pollutants such as p-phenylenediamine quinones (PPD-Qs), nLIPI-MS displayed significant enhanced sensitivity by 1 to 2 orders of magnitude in comparison to nanoelectrospray ionization (nESI)-MS. For more polar organic pollutants such as per- and polyfluoroalkyl substances (PFASs), nLIPI-MS also showed good analytical performance, with a sensitivity improvement of 1.3- to 2.7-fold over that of nESI-MS. The sensitivity of nLIPI-MS for the analysis of PPD-Qs and PFASs reached as low as the ng/L level, enabling the analysis of ultratrace levels of these pollutants in complex samples and even single cells. Our experimental results demonstrated that nLIPI-MS was an extensive ambient MS method, showing desirable analytical performance for the analysis of organic pollutants with broad polarity ranges.

When Does Multiphase Chemistry Influence Indoor Chemical Fate?

Human chemical exposure often occurs indoors, where large variability in contaminant concentrations and indoor chemical dynamics make assessments of these exposures challenging. A major source of uncertainty lies in the rates of chemical transformations which, due to high surface-to-volume ratios and rapid air change rates relative to rates of gas-phase reactions indoors, are largely gas-surface multiphase processes. It remains unclear how important such chemistry is in controlling indoor chemical lifetimes and, therefore, human exposure to both parent compounds and transformation products. We present a multimedia steady-state fugacity-based model to assess the importance of multiphase chemistry relative to cleaning and mass transfer losses, examine how the physicochemical properties of compounds and features of the indoor environment affect these processes, and investigate uncertainties pertaining to indoor multiphase chemistry and chemical lifetimes. We find that multiphase reactions can play an important role in chemical fate indoors for reactive compounds with low volatility, i.e., octanol-air equilibrium partitioning ratios (Koa) above 108, with the impact of this chemistry dependent on chemical identity, oxidant type and concentration, and other parameters. This work highlights the need for further research into indoor chemical dynamics and multiphase chemistry to constrain human exposure to chemicals in the built environment.

Kinetics of hypochlorous acid reactions with organic and chloride-containing tropospheric aerosol

Chlorine plays an important role in tropospheric oxidation processes, in both marine and continental environments. Although modeling studies have explored the importance of halogen chemistry, uncertainty remains in associated chemical mechanisms and fundamental kinetics parameters. Prior kinetics measurements of multiphase halogen recycling reactions have been largely performed with dilute, bulk solutions, leaving unexplored more realistic chemical systems which have high solute concentrations and are internally mixed with both halide and organic components. Here, we address the multiphase kinetics of gaseous HOCl using an aerosol flow tube and aerosol mass spectrometer to study its reactions with particulate chloride, using atmospherically relevant particle acidity, solute concentrations, and ionic strength. We also investigate the chemistry that results when biomass burning (BB) aerosol components and chloride are internally mixed. Using pH-buffered deliquesced particles, we show that the rate constant for reaction of dissolved HOCl with H+ and Cl- at high relative humidity (RH) (80-85%) is within a factor of two of the literature value for bulk phase conditions. However, at lower RH values (60-70%) where the particles are considerably more concentrated, the rate constant for chloride loss from the particles is an order of magnitude higher. For pure organic compounds commonly found in biomass burning (BB) aerosol, such as coniferaldehyde, salicylic acid and furfural, an increase in the aerosol chlorine content occurs with HOCl exposure, indicating the formation of organochlorine species. Together, these independent findings explain results for internally mixed aerosol particles with both chloride and BB components present where we observed behavior consistent with both chloride loss and organochlorine formation occurring simultaneously upon HOCl exposure. Our results indicate that chlorine recycling via HOCl uptake by chloride-containing particles will occur in the atmosphere efficiently over a wide range of RH conditions, even when reactive organic compounds are present in the same particles as chloride. Simultaneously, formation of organochlorine compounds, which are commonly toxic, is likely occurring when reactive organic components are present.

Direct analysis in real-time mass spectrometry: Observations of helium, nitrogen and argon as ionisation gas for the detection of small molecules using a single quadrupole instrument

RATIONALE

Direct analysis in real time is typically performed using helium as the ionisation gas for the detection of analytes by mass spectrometry (MS). Nitrogen and argon are found with abundance in the air and provide a cheaper and greener alternative to the use of helium as ionisation gas. This study explores the use of helium, nitrogen and argon as ionisation gas for the detection of organic compounds.

METHODS

Four illicit drugs, two amino acids and five explosives were chosen as target analytes to understand selectivity, sensitivity and linearity when helium, nitrogen or argon was used as the ionisation gas with the direct analysis in real time (DART) source. Analysis was carried out on a Waters Acquity QDa single quadrupole mass spectrometer.

RESULTS

Calibration curves over the range of 5-100 ng were produced for each analyte using the different ionisation gases to assess the instrument response. Nitrogen gave a higher response to concentration than helium or argon; however, the lowest limits of detection were observed when helium was used.

CONCLUSIONS

All the target analytes were detected using DART-MS with helium, nitrogen or argon as the ionisation gas. Whereas helium provided the highest sensitivity, nitrogen produced reasonable limits of detection and had good linearity across the concentration range explored, suggesting it provides a greener and cheaper alternative to helium.

CHEMICAL REACTION MONITORING BY AMBIENT MASS SPECTROMETRY

Chemical reactions conducted in different media (liquid phase, gas phase, or surface) drive developments of versatile techniques for the detection of intermediates and prediction of reasonable reaction pathways. Without sample pretreatment, ambient mass spectrometry (AMS) has been applied to obtain structural information of reactive molecules that differ in polarity and molecular weight. Commercial ion sources (e.g., electrospray ionization, atmospheric pressure chemical ionization, and direct analysis in real-time) have been reported to monitor substrates and products by offline reaction examination. While the interception or characterization of reactive intermediates with short lifetime are still limited by the offline modes. Notably, online ionization technologies, with high tolerance to salt, buffer, and pH, can achieve direct sampling and ionization of on-going reactions conducted in different media (e.g., liquid phase, gas phase, or surface). Therefore, short-lived intermediates could be captured at unprecedented timescales, and the reaction dynamics could be studied for mechanism examinations without sample pretreatments. In this review, via various AMS methods, chemical reaction monitoring and mechanism elucidation for different classifications of reactions have been reviewed. The developments and advances of common ionization methods for offline reaction monitoring will also be highlighted.

Ambient electric arc ionization for versatile sample analysis using mass spectrometry

A novel, convenient ambient electric arc ionization (AEAI) device was developed as a mass spectrometry ion source for versatile sample analysis. AEAI could be considered as a soft ionization technique in which the protonated ion ([M + H]⁺) is the main ion species with little or no in-source fragmentation for most analytes. Coupled with a high-resolution Orbitrap mass spectrometer, AEAI could be applied to the analysis of a variety of organic compounds having a wide range of polarities, ranging from non-polar species such as polybenzenoid aromatic hydrocarbons (PAHs) to highly polar species such as amino acids. With its versatile capabilities in the mass spectrometric analysis of small molecules, AEAI has the potential to be an alternative to traditional ionization methods such as electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and electron impact (EI) ionization. The limitations of AEAI are also discussed.

Indoor Surface Chemistry: Developing a Molecular Picture of Reactions on Indoor Interfaces

Chemical reactions on indoor surfaces play an important role in air quality in indoor environments, where humans spend 90% of their time. We focus on the challenges of understanding the complex chemistry that takes place on indoor surfaces and identify crucial steps necessary to gain a molecular-level understanding of environmental indoor surface chemistry: (1) elucidate key surface reaction mechanisms and kinetics important to indoor air chemistry, (2) define a range of relevant and representative surfaces to probe, and (3) define the drivers of surface reactivity, particularly with respect to the surface composition, light, and temperature. Within the drivers of surface composition are the roles of adsorbed/absorbed water associated with indoor surfaces and the prevalence, inhomogeneity, and properties of secondary organic films that can impact surface reactivity. By combining laboratory studies, field measurements, and modeling we can gain insights into the molecular processes necessary to further our understanding of the indoor environment.

Chemical Composition, Spatial Homogeneity, and Growth of Indoor Surface Films

Organic films on indoor surfaces are ubiquitous, but details about their composition and growth over timescales less than a month are not fully understood. To address these gaps in understanding, organic film samples in an apartment unit were collected over the course of 17 days using passive samplers and analyzed in a non-targeted manner using direct analysis in real-time mass spectrometry (DART-MS). Overall, the chemical composition observed across various locations within the apartment are very similar. Mass spectra also show clear evidence for the growth of semi-volatile compounds from natural sources and consumer products, such as carboxylic acids and plasticizers. Certain compounds show evidence for equilibration, mostly consistent with surface partitioning models based on octanol-air partition coefficients (K_oa). Compounds which have higher molecular weight or larger K_oa values tend to equilibrate later, leading to an overall shift in the composition of the film as a function of collection time. Growth rates of film thickness are at least 0.05 nm/day based on a limited number of individually calibrated ions.

The atmospheric chemistry of indoor environments

Through air inhalation, dust ingestion and dermal exposure, the indoor environment plays an important role in controlling human chemical exposure. Indoor emissions and chemistry can also have direct impacts on the quality of outdoor air. And so, it is important to have a strong fundamental knowledge of the chemical processes that occur in indoor environments. This review article summarizes our understanding of the indoor chemistry field. Using a molecular perspective, it addresses primarily the new advances that have occurred in the past decade or so and upon developments in our understanding of multiphase partitioning and reactions. A primary goal of the article is to contrast indoor chemistry to that which occurs outdoors, which we know to be a strongly gas-phase, oxidant-driven system in which substantial oxidative aging of gases and aerosol particles occurs. By contrast, indoor environments are dark, gas-phase oxidant concentrations are relatively low, and due to air exchange, only short times are available for reactive processing of gaseous and particle constituents. However, important gas-surface partitioning and reactive multiphase chemistry occur in the large surface reservoirs that prevail in all indoor environments. These interactions not only play a crucial role in controlling the composition of indoor surfaces but also the surrounding gases and aerosol particles, thus affecting human chemical exposure. There are rich research opportunities available if the advanced measurement and modeling tools of the outdoor atmospheric chemistry community continue to be brought indoors.

Kinetics and Condensed-Phase Products in Multiphase Ozonolysis of an Unsaturated Triglyceride

Ozone is an important oxidant in the environment. To study the nature of multiphase ozonolysis, an unsaturated triglyceride, triolein, of the type present in skin oil, biological membranes, and most cooking oils was oxidized by gas-phase ozone on a surface. A high-performance liquid chromatography/electrospray ionization mass spectrometry (HPLC-ESI-MS) method was developed for analyzing triolein and its oxidized products. Upon exposure to ozone, the decay of thin coatings of triolein was observed, accompanied by the formation of functionalized condensed-phase products including secondary ozonides (SOZ), acids, and aldehydes. By studying the reaction kinetics as a function of average coating thickness and ozone mixing ratio, we determined that the reactive uptake coefficient (γ) is on the order of 10^-6 to 10^-5. It is also concluded that the reaction occurs in the bulk without a major interfacial component, and the reacto-diffusive depth of ozone in the triolein coating is estimated to be between 8 and 40 nm. The specific nature of the reaction products is affected by the reactions of the Criegee intermediate formed during ozonolysis. In particular, although an increase in the relative humidity to 50% from dry conditions has no effect on the kinetics of triolein decay, the yield of SOZs is significantly depressed, indicating reactions of the Criegee intermediates to form hydroperoxides. Once formed, the SOZ products are thermally stable over periods of at least 48 h at room temperature but decomposition was observed under simulated outdoor sunlight, likely forming organic acids. From an environmental perspective, this chemistry indicates that SOZs and other oxygenates will form via ozonolysis of oily indoor surfaces and skin oil.

The presence of nitroarenes formed by secondary atmospheric processes in the Japanese freshwater environment

In this study, the concentrations and distributions of nitrated polycyclic aromatic hydrocarbons (NPAHs) were characterized in the freshwater environment of a Japanese city. While the NPAHs were few in number, they were found in pg/L concentrations and the specific isomers suggested the deposition of NPAHs formed via the atmospheric transformation of PAHs. The absence of NPAHs formed via primary combustion processes such as automobile exhaust, suggests that improvements in emission standards are being reflected in the environment, though the NPAHs formed by secondary atmospheric processes are still a significant ecotoxicological threat. The stability of the NPAHs was also examined in spiked freshwater matrices. There was a significant decrease in spiked NPAHs over this period, suggesting that they were either being sorbed or transformed and are therefore not long lived in the freshwater environment. This indicates that the NPAHs found in freshwater samples are from recent deposition.

Multiphase reactivity of polycyclic aromatic hydrocarbons is driven by phase separation and diffusion limitations

Polycyclic aromatic hydrocarbons (PAHs) are among the most prominent toxic compounds in the air. Heterogeneous reactions involving O₃ can change the toxicity of PAHs, but the reaction mechanism and kinetics remain to be elucidated. Based on new experiments combined with state-of-the-art kinetic and thermodynamic models, we show that phase separation plays a critical role in the ozonolysis of PAHs mixed with secondary organic aerosols and organic oils. Ozonolysis products of PAHs phase separate to form viscous surface crusts, which protect underlying PAHs from ozonolysis to prolong their chemical lifetime. These results have significant implications for outdoor and indoor air quality by affecting PAH long-range transport and fate in indoor environments.

Benzo[a]pyrene (BaP), a key polycyclic aromatic hydrocarbon (PAH) often associated with soot particles coated by organic compounds, is a known carcinogen and mutagen. When mixed with organics, the kinetics and mechanisms of chemical transformations of BaP by ozone in indoor and outdoor environments are still not fully elucidated. Using direct analysis in real-time mass spectrometry (DART-MS), kinetics studies of the ozonolysis of BaP in thin films exhibited fast initial loss of BaP followed by a slower decay at long exposure times. Kinetic multilayer modeling demonstrates that the slow decay of BaP over long times can be simulated if there is slow diffusion of BaP from the film interior to the surface, resolving long-standing unresolved observations of incomplete PAH decay upon prolonged ozone exposure. Phase separation drives the slow diffusion time scales in multicomponent systems. Specifically, thermodynamic modeling predicts that BaP phase separates from secondary organic aerosol material so that the BaP-rich layer at the surface shields the inner BaP from ozone. Also, BaP is miscible with organic oils such as squalane, linoleic acid, and cooking oil, but its oxidation products are virtually immiscible, resulting in the formation of a viscous surface crust that hinders diffusion of BaP from the film interior to the surface. These findings imply that phase separation and slow diffusion significantly prolong the chemical lifetime of PAHs, affecting long-range transport of PAHs in the atmosphere and their fates in indoor environments.

Heterogeneous Chlorination of Squalene and Oleic Acid

Washing with chlorine bleach leads to high mixing ratios of gas-phase HOCl. Using two methods that are sensitive to surface film composition-attenuated total reflection fourier transform infrared (ATR-FTIR) spectroscopy and direct analysis in real time mass spectrometry (DART-MS)-we present the first study of the chlorination chemistry that occurs when gaseous HOCl reacts with thin films of squalene and oleic acid. At mixing ratios of 600 ppbv, HOCl forms chlorohydrins by adding across carbon-carbon double bonds without breaking the carbon backbone. The initial uptake of one HOCl molecule occurs on the time scale of a few minutes at these mixing ratios. For oleic acid, ester formation proceeds immediately thereafter, leading to dimeric and trimeric chlorinated products. For squalene, subsequent HOCl uptake occurs until all six of its carbon-carbon double bonds become chlorinated within 1-2 h. These results indicate that chlorination of skin oil, which contains substantial carbon unsaturation, is likely to occur rapidly under common cleaning conditions, potentially leading to the irritation associated with chlorinated bleach. This chemistry will likely also proceed with cooking oils, in the human respiratory system which has unsaturated surfactants as important components of lung fluid, and with organic components of the sea surface microlayer.

Quantitative and Orthogonal Formation and Reactivity of SuFEx Platforms

The constraints of minute reactant amounts and the impossibility to remove any undesired surface‐bound products during monolayer functionalization of a surface necessitate the selection of efficient, modular and orthogonal reactions that lead to quantitative conversions. Herein, we explore the character of sulfur–fluoride exchange (SuFEx) reactions on a surface, and explore the applicability for quantitative and orthogonal surface functionalization. To this end, we demonstrate the use of ethenesulfonyl fluoride (ESF) as an efficient SuFEx linker for creating “SuFEx‐able” monolayer surfaces, enabling three distinct approaches to utilize SuFEx chemistry on a surface. The first approach relies on a di‐SuFEx loading allowing dual functionalization with a nucleophile, while the two latter approaches focus on dual (CuAAC–SuFEx/SPOCQ–SuFEx) click platforms. The resultant strategies allow facile attachment of two different substrates sequentially on the same platform. Along the way we also demonstrate the Michael addition of ethenesulfonyl fluoride to be a quantitative surface‐bound reaction, indicating significant promise in materials science for this reaction.

Heterogeneous oxidation of indoor surfaces by gas-phase hydroxyl radicals

We investigate heterogeneous oxidation kinetics of monolayer-thick, surface-sorbed organics, namely di-n-octyl phthalate (DnOP) and palmitic acid (PA), with gas-phase OH. The pseudo-first order rate constants for organic loss at OH concentrations of 1.6 × 108  molecules/cm3 are: (2.3 ± 0.1) × 10-4 to (4.8 ± 0.8) × 10-4  s-1 , and (1.3 ± 0.5) × 10-4  s-1 for DnOP and PA, respectively. Films developed in indoor office environments over a few weeks are also oxidized using the same OH concentration. Heterogeneous decay rate constants of mass signals from these films, attributed to phthalates (MW = 390.6) and to PA, are similar to those for the single-component films, ie, (1.9 ± 0.4) × 10-4 to (3.4 ± 0.5) × 10-4  s-1 , and (1.1 ± 0.4) × 10-4  s-1 , respectively. These results suggest that the lifetimes for OH heterogeneous oxidation of monolayer-thick indoor organic films will be on the timescale of weeks to months. To support this argument, we present the first analysis of the mass transfer processes that occur when short-lived gas-phase molecules, such as OH, are taken up by reactive indoor surfaces. Due to rapid chemical production, the diffusion limitation to mass transfer is less important for short-lived molecules than for molecules with little chemical production, such as ozone.

Reactive indoor air chemistry and health-A workshop summary

The chemical composition of indoor air changes due to the reactive nature of the indoor environment. Historically, only the stable parent compounds were investigated due to their ease of measurement by conventional methods. Today, however, scientists can better characterize oxidation products (gas and particulate-phase) formed by indoor chemistry. An understanding of occupant exposure can be developed through the investigation of indoor oxidants, the use of derivatization techniques, atmospheric pressure detection, the development of real-time technologies, and improved complex modeling techniques. Moreover, the connection between exposure and health effects is now receiving more attention from the research community. Nevertheless, a need still exists for improved understanding of the possible link between indoor air chemistry and observed acute or chronic health effects and long-term effects such as work-related asthma.

Carbon Nanotube Fiber Ionization Mass Spectrometry: A Fundamental Study of a Multi-Walled Carbon Nanotube Functionalized Corona Discharge Pin for Polycyclic Aromatic Hydrocarbons Analysis

Mass spectrometry continues to tackle many complicated tasks, and ongoing research seeks to simplify its instrumentation as well as sampling. The desorption electrospray ionization (DESI) source was the first ambient ionization source to function without extensive gas requirements and chromatography. Electrospray techniques generally have low efficiency for ionization of nonpolar analytes and some researchers have resorted to methods such as direct analysis in real time (DART) or desorption atmospheric pressure chemical ionization (DAPCI) for their analysis. In this work, a carbon nanotube fiber ionization (nanoCFI) source was developed and was found to be capable of solid phase microextraction (SPME) of nonpolar analytes as well as ionization and sampling similar to that of direct probe atmospheric pressure chemical ionization (DP-APCI). Conductivity and adsorption were maintained by utilizing a corona pin functionalized with a multi-walled carbon nanotube (MWCNT) thread. Quantitative work with the nanoCFI source with a designed corona discharge pin insert demonstrated linearity up to 0.97 (R²) of three target PAHs with phenanthrene internal standard. Graphical Abstract ᅟ.

Epoxide formation from heterogeneous oxidation of benzo[a]pyrene with gas-phase ozone and indoor air

The formation of two classes of epoxide products from the heterogeneous reaction of benzo[a]pyrene (BaP) with gas-phase ozone was demonstrated. BaP was coated on a Pyrex glass tube and oxidized with different concentrations of ozone. After oxidation, the epoxide products were derivatized by N-acetylcystein (NAC) and then analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The results show that in addition to mono-epoxides, diol-epoxides were also formed. BaP exposed to genuine indoor air also produces mono- and diol-epoxides, having similar chromatograms to those produced by oxidation of BaP by low concentrations of ozone. Although it is well recognized that diol-epoxides are formed from BaP oxidation in the human body and that they exhibit carcinogenicity via formation of adducts with DNA, this is the first demonstration that such classes of compounds can be formed by abiotic heterogeneous oxidation.

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.

Kinetics and Products from Heterogeneous Oxidation of Squalene with Ozone

Motivated by the importance of the heterogeneous chemistry of squalene contained within skin oil to indoor air chemistry, the surface reaction of squalene with gas-phase ozone has been investigated. Using direct analysis in real time mass spectrometry (DART-MS) to monitor squalene, the reactive uptake coefficients were determined to be (4.3 ± 2.2) × 10^-4 and (4.0 ± 2.2) × 10^-4 for ozone mixing ratios (MR_O3) of 50 and 25 ppb, respectively, on squalene films deposited on glass surfaces. At an MR_O3 of 25 ppb, the lifetime for oxidation was the same as that in an indoor office with an MR_O3 between 22 and 32 ppb, suggesting that O₃ was the dominant oxidant in this indoor setting. While the heterogeneous kinetics of squalene and O₃ were independent of relative humidity (RH), the RH significantly affected the reaction products. Under dry conditions (<5% RH), in addition to several products between m/z 300 and 350, the major condensed-phase end products were levulinic acid (LLA) and succinic acid (SCA). Under humid conditions (50% RH), the major end products were 4-oxopentanal, 4-oxobutanoic acid, and LLA. The molar yields of LLA and SCA were quantified as 230 ± 43% and 110 ± 31%, respectively, under dry conditions and 91 ± 15% and <5%, respectively, at 50% RH. Moreover, high-molecular weight (molecular weight of >450 Da) products were observed under dry conditions with indications that LLA was involved in their formation. The mechanism of squalene oxidation is discussed in light of these observations, with indications of an important role played by Criegee intermediates.

Use of Ambient Ionization High-Resolution Mass Spectrometry for the Kinetic Analysis of Organic Surface Reactions

In contrast to homogeneous systems, studying the kinetics of organic reactions on solid surfaces remains a difficult task due to the limited availability of appropriate analysis techniques that are general, high-throughput, and capable of offering quantitative, structural surface information. Here, we demonstrate how direct analysis in real time mass spectrometry (DART-MS) complies with above considerations and can be used for determining interfacial kinetic parameters. The presented approach is based on the use of a MS tag that--in principle--allows application to other reactions. To show the potential of DART-MS, we selected the widely applied strain-promoted alkyne-azide cycloaddition (SPAAC) as a model reaction to elucidate the effects of the nanoenvironment on the interfacial reaction rate.

Monolith dip-it: a bifunctional device for improving the sensitivity of direct analysis in real time mass spectrometry

A bifunctional monolith dip-it was fabricated and applied for improving the sensitivity of direct analysis in real time mass spectrometry (DART-MS).