Mechanism and Rate Constants of Ion–Molecule Reactions Leading to Formation of H+(H2O)n in Moist Oxygen and Air

Computational analysis of an electrostatic separator design for removal of volatile organic compounds from indoor air

Concentrations of volatile organic compounds (VOCs) in air can be reduced in electrostatic separators where VOCs are ionized using ion-molecule reactions, extracted using electric fields, and eliminated in a waste flow. Embodiments for such separator technology have been explored in only a few studies, despite the possible advantage of purification without adsorbent filters. In one design, based on ionization of VOCs in positive polarity with hydrated protons as reactant ions, efficiencies for removal were measured as 30-40% . The results were fitted to a one-dimensional convective diffusion model requiring an unexpectedly high production rate of reactant ions to match both the model and data. A realistic rate of reactant ion production was used in finite element method simulations (COMSOL) and demonstrated that low removal efficiency could be attributed to non-uniform patterns of sample flow and to incomplete mixing of VOCs with reactant ions. In analysis of complex systems, such as this model, even limited computational modeling can outperform a pure analytical approach and bring insights into limiting factors or system bottlenecks.Implications: In this work, we applied modern computational methods to understand the performance of an air purifier based on electrostatics and ionized volatile organic compounds (VOCs). These were described in the publication early 2000s. The model presented was one-dimensional and did not account for the effects of flow. In our multiphysics finite element models, the efficiency and operation of the filter is better explained by the patterns of flow and flow influences on ion distributions in electric fields. In general, this work helps using and applying computational modelling to understand and improve the performance bottlenecks in air purification system designs.

Evidence for the co-existence of isomers of water dimer radical cations and their inter-conversion in a linear ion trap

Water dimer radical cations are regarded as key intermediates in many aqueous reactions and biochemical processes. However, the structure of the water dimer radical cations, and particularly the inter-conversion between their isomers, remain difficult to investigate experimentally due to their short lifetime and low abundance under ambient conditions. Furthermore, the isomers cannot be distinguished in a full mass spectra. In this study, we report the experimental evidence for the hemi-bonded and proton-transferred isomers of gas-phase water dimer radical cations, and the inter-conversion process between them in a linear ion trap at low pressure and near room temperature. Multiple collisions of isolated water dimer radical cations with He inside the ion trap were systematically investigated; first, under different trapping times (i.e., reaction times) ranging from 0.03 to 800 ms, and then at a very low collision energies ranging from 0.1% to 10% normalized collision energy. The proton-transferred isomers were dominant at shorter trapping times (≤250 ms), while the hemi-bonded isomers were dominant at longer trapping times (250-800 ms). Moreover, the difference in symmetry of the shapes of the H₂O^•+ signal profiles and the H₃O⁺ signal profiles implied the existence of two kinds of isomers and there were small potential differences between them. Our results also suggested that by tuning the experimental parameters the hemi-bonded isomers would become dominant, which could allow the study of novel chemical reactions involving the hemi-bonded two-center-three-electron (2c-3e) structure in a linear ion trap.

Photodissociation Dynamics of the [O2-H2O]+ Ionic Complex

We present an experimental study on the photodissociation dynamics of [O₂-H₂O]⁺ in the 580-266 nm wavelength range using a cryogenic ion trap velocity map imaging spectrometer. The cryogenic ion trap produces mass selected and internally cold [O2-H2O]+ ions for photodissociation. By detecting both the O₂⁺ and H₂O⁺ photofragments using the time-of-flight mass spectrometry and velocity map imaging techniques, branching ratios and total kinetic energy release distributions of the O₂⁺ + H₂O and H₂O⁺ + O₂ product channels are experimentally measured at 16 different excitation energies. State-resolved photodissociation mechanisms of the parent [O₂-H₂O]⁺ are interpreted as (1) the O₂(X³Σ_g^-) + H₂O⁺(X~2B1), O₂(a¹Δ_g) + H₂O⁺(X~2B1), and O₂(X³Σ_g^-) + H₂O⁺(A~2A1) channels are produced from direct dissociation of [O₂-H₂O]⁺ in its excited B~2A″, D~2A″, and F~2A″ states, respectively; (2) the O₂⁺(X²Π_g) + H2O(X~A11) channel is produced from nonadiabatic relaxations of the excited B~2A″, D~2A″, and F~2A″ states to the X~2A″ ground state with subsequent dissociation. The latter nonadiabatic processes involve charge-transfer on the potential energy surfaces, and the charge-transfer probabilities are determined from experimental results. The dissociation energy of the ground state to the lowest dissociation limit is experimentally refined as D₀ = 1.05 ± 0.05 eV. This work provides important information to understand the charge-transfer dynamics in the photochemistry of [O₂-H₂O]⁺ and in the ion-molecule reaction O₂ + H₂O⁺ → O₂⁺ + H₂O.

An atmospheric pressure field effect ionisation source for ion mobility spectrometry

An atmospheric Pressure Field Effect (APFE) ionisation source for drift tube ion mobility spectrometry has been developed for operation in positive and negative polarities. The formation of negative and positive ions in synthetic air was studied and compared with the Atmospheric Pressure Corona Discharge (APCD) ionisation source. The APFE ionisation source is of point-to-plane geometry with a 10 μm Pt point electrode, a stainless steel plate electrode and ultra-high resistance (20 GΩ) current limiters. In the case of negative polarity, the ionisation source was able to generate Reactant Ions (RIs) O₂^-(H₂O)_n and O₂^-CO₂(H₂O)_n, and in the case of positive polarity, stable production of H⁺(H₂O)_n RI was achieved in two different gas flow regimes of the IMS. RIs formed in the APFE in both polarities have made it a reliable chemical ionisation source at atmospheric pressure. The identification of the ions generated in the APFE was performed using an Ion Mobility Spectrometer coupled with an orthogonal acceleration Time of Flight Mass Spectrometer (IMS-oa-TOF-MS). The chemical ionisation of molecules was demonstrated for the APFE ionisation source in positive (2,6-di-tert-butyl-pyridine) and negative (tetrachloromethane) polarities.

Reaction Dynamics of NO+ with Water Clusters

The reaction of NO⁺ with water molecules plays a crucial role in the D-region of the atmosphere because the reaction provides nitrous acid (HONO) and protonated water species (H₃O⁺). In this study, the reaction of NO⁺ with water clusters, NO⁺ + (H₂O)_n (n = 1-7), was investigated by means of the direct ab initio molecular dynamics method to elucidate the reaction mechanism of NO⁺ in the atmosphere from a theoretical viewpoint. At n = 1 and 2, the reaction of NO⁺ with (H₂O)_n led to the formation of a complex: NO⁺ + (H₂O)_n → NO⁺(H₂O)_n (n = 1 and 2). At n = 3, the formation channel of HONO was open, and HONO was formed according to NO⁺ + (H₂O)_n → HONO---H⁺(H₂O)_n-1 (n = 3), through which H₃O⁺ was also formed as H⁺(H₂O)₂. However, the HONO formation efficiency was significantly low for n = 3. In large clusters with n = 4-7, the HONO formation channel became the main channel, and the dissociation of HONO from the HONO--H⁺(H₂O)_n-1 complex occurred in part: NO⁺ + (H₂O)_n → HONO---H⁺(H₂O)_n-1 → HONO + H⁺(H₂O)_n-1. The energetics and reaction mechanism were discussed on the basis of theoretical results.

Water Radical Cations in the Gas Phase: Methods and Mechanisms of Formation, Structure and Chemical Properties

Water radical cations, (H₂O)_n^+•, are of great research interest in both fundamental and applied sciences. Fundamental studies of water radical reactions are important to better understand the mechanisms of natural processes, such as proton transfer in aqueous solutions, the formation of hydrogen bonds and DNA damage, as well as for the discovery of new gas-phase reactions and products. In applied science, the interest in water radicals is prompted by their potential in radiobiology and as a source of primary ions for selective and sensitive chemical ionization. However, in contrast to protonated water clusters, (H₂O)_nH⁺, which are relatively easy to generate and isolate in experiments, the generation and isolation of radical water clusters, (H₂O)_n^+•, is tremendously difficult due to their ultra-high reactivity. This review focuses on the current knowledge and unknowns regarding (H₂O)_n^+• species, including the methods and mechanisms of their formation, structure and chemical properties.

Positive Reactant Ion Formation in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

In contrast to classical Ion Mobility Spectrometers (IMS) operating at ambient pressure, the High Kinetic Energy Ion Mobility Spectrometer (HiKE-IMS) is operated at reduced pressures of between 10 and 40 mbar and higher reduced electric field strengths of up to 120 Td. Thus, the ion-molecule reactions occurring in the HiKE-IMS can significantly differ from those in classical ambient pressure IMS. In order to predict the ionization pathways of specific analyte molecules, profound knowledge of the reactant ion species generated in HiKE-IMS and their dependence on the ionization conditions is essential. In this work, the formation of positive reactant ions in HiKE-IMS is investigated in detail. On the basis of kinetic and thermodynamic data from the literature, the ion-molecule reactions are kinetically modeled. To verify the model, we present measurements of the reactant ion population and its dependence on the reduced electric field strength, the operating pressure, and the water concentration in the sample gas. All of these parameters significantly affect the reactant ion population formed in HiKE-IMS.

Analyzing Positive Reactant Ions in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS) by HiKE-IMS-MS

In contrast to classical ion mobility spectrometers (IMS) operating at ambient pressure, the high kinetic energy ion mobility spectrometer (HiKE-IMS) is operated at reduced pressures between 10-40 mbar. In HiKE-IMS, ions are generated in a reaction region before they are separated in a drift region. Due to the operation at reduced pressure, it is possible to reach high reduced electric field strengths up to 120 Td in both the reaction as well as drift region, resulting in a pronounced decrease in chemical cross sensitivities and a significant enhancement of the dynamic range. Until now though, only limited knowledge about the ionization pathways in HiKE-IMS is available. Typically, proton bound water clusters, H⁺(H₂O)_n, are the most abundant positive reactant ion species in classical IMS with atmospheric chemical ionization sources. However, at reduced pressure and increased effective ion temperature, the reactant ion population significantly changes. As the ionization efficiency of analyte molecules in HiKE-IMS strongly depends on the reactant ion population, a detailed knowledge of the reactant ion population generated in HiKE-IMS is essential. Here, we present a coupling stage of the HiKE-IMS to a mass spectrometer enabling the identification of ion species and the investigation of ion molecule reactions prevailing in HiKE-IMS. In the present study, the HiKE-IMS-MS is used to identify positive reactant ion populations in both, purified air and nitrogen, respectively. The experimental data suggest the generation of systems of clustered primary ions (H⁺(H₂O)_n, NO⁺(H₂O)_m, and O₂⁺(H₂O)_p), which most probably serve as reactant ions. Their relative abundances highly depend on the reduced electric field strength in the reaction region. Furthermore, their effective mobilities are studied as a function of the reduced electric field strength in the drift region.

Interpretation of Ionization Mechanism Responsible for Reagent Ion and Analyte Formation in Microwave-Induced Plasma Desorption Ionization Mass Spectrometry

Ambient desorption/ionization (ADI) sources coupled to mass spectrometer have gained increasing interest in the field of analytical chemistry for its fast and direct analysis of samples. Among many ADI sources, plasma-based ADI sources are an important branch. Despite its extensive use in mass spectrometry analysis, the ionization mechanism of these sources still remain uncertain. The study on ionization mechanism is of great significance to optimize the design of ion sources and to improve ionization efficiency. In this study, targeted research on a better understanding of afterglow distance effects on ionization process was conducted. Based on the quantified signal expression of reagent ions in mass spectrum, the concept that optimal atmospheric analysis distance of plasma ADI source is defined for the first time. From the perspective of mutual restriction effect between atmospheric components, the formation progress of reagent ions was visually revealed in detail, which involved the initial step of forming precursor reagent ions, the clusters reaction for increasing production of reagent ions, and the matrix effect results in reagent ion depletion. The formation mechanism of reagent ions further clarified the explicit reason for abundant reagent ions generated at an optimal distance. Most importantly, the analyte analysis results verified the significant impact of appropriate distance on ionization efficiency in afterglow region. It was confirmed that the quantity and type of reagent ions intimately influenced the status of analyte ions in mass spectrum.

Photodissociation processes of a water–oxygen complex cation studied by an ion imaging technique

Photodissociation dynamics of O₂⁺–H₂O in the visible and ultraviolet regions was studied by ion imaging experiments and theoretical calculations.

Generation of nanobubbles by ceramic membrane filters: The dependence of bubble size and zeta potential on surface coating, pore size and injected gas pressure

Generation of gaseous nanobubbles (NBs) by simple, efficient, and scalable methods is critical for industrialization and applications of nanobubbles. Traditional generation methods mainly rely on hydrodynamic, acoustic, particle, and optical cavitation. These generation processes render issues such as high energy consumption, non-flexibility, and complexity. This research investigated the use of tubular ceramic nanofiltration membranes to generate NBs in water with air, nitrogen and oxygen gases. This system injects pressurized gases through a tubular ceramic membrane with nanopores to create NBs. The effects of membrane pores size, surface energy, and the injected gas pressures on the bubble size and zeta potential were examined. The results show that the gas injection pressure had considerable effects on the bubble size, zeta potential, pH, and dissolved oxygen of the produced NBs. For example, increasing the injection air pressure from 69 kPa to 414 kPa, the air bubble size was reduced from 600 to 340 nm respectively. Membrane pores size and surface energy also had significant effects on sizes and zeta potentials of NBs. The results presented here aim to fill out the gaps of fundamental knowledge about NBs and development of efficient generation methods.

Reactivity of the O2+·(H2O)n and NO+·(H2O)n cluster ions in the D-region of the ionosphere

Ab initio molecular dynamics simulations reveal different reactivities of NO⁺·(H₂O)_n and O₂⁺·(H₂O)_n cluster ions in the D-region of the ionosphere.

Correlation between experimental data of protonation of aromatic compounds at (+) atmospheric pressure photoionization and theoretically calculated enthalpies

RATIONALE

The theoretical enthalpy calculated from the overall protonation reaction (electron transfer plus hydrogen transfer) in positive-mode (+) atmospheric-pressure photoionization (APPI) was compared with experimental results for 49 aromatic compounds. A linear relationship was observed between the calculated ΔH and the relative abundance of the protonated peak. The parameter gives reasonable predictions for all the aromatic hydrocarbon compounds used in this study.

METHODS

A parameter is devised by combining experimental MS data and high-level theoretical calculations. A (+) APPI Q Exactive Orbitrap mass spectrometer was used to obtain MS data for each solution. B3LYP exchange-correlation functions with the standard 6-311+G(df,2p) basis set was used to perform density functional theory (DFT) calculations.

RESULTS

All the molecules with ΔH <0 kcal/mol for the overall protonation reaction with toluene clusters produced protonated ions, regardless of the desolvation temperature. For molecules with ΔH >0, molecular ions were more abundant at typical APPI desolvation temperatures (300°C), while the protonated ions became comparable or dominant at higher temperatures (400°C). The toluene cluster size was an important factor when predicting the ionization behavior of aromatic hydrocarbon ions in (+) APPI.

CONCLUSIONS

The data used in this study clearly show that the theoretically calculated reaction enthalpy (ΔH) of protonation with toluene dimers can be used to predict the protonation behavior of aromatic compounds. When compounds have a negative ΔH value, the types of ions generated for aromatic compounds could be very well predicted based on the ΔH value. The ΔH can explain overall protonation behavior of compounds with ΔH values >0. Copyright © 2017 John Wiley & Sons, Ltd.

Formation of Pyrylium from Aromatic Systems with a Helium:Oxygen Flowing Atmospheric Pressure Afterglow (FAPA) Plasma Source

The effects of oxygen addition on a helium-based flowing atmospheric pressure afterglow (FAPA) ionization source are explored. Small amounts of oxygen doped into the helium discharge gas resulted in an increase in abundance of protonated water clusters by at least three times. A corresponding increase in protonated analyte signal was also observed for small polar analytes, such as methanol and acetone. Meanwhile, most other reagent ions (e.g., O₂^+·, NO⁺, etc.) significantly decrease in abundance with even 0.1% v/v oxygen in the discharge gas. Interestingly, when analytes that contained aromatic constituents were subjected to a He:O₂-FAPA, a unique (M + 3)⁺ ion resulted, while molecular or protonated molecular ions were rarely detected. Exact-mass measurements revealed that these (M + 3)⁺ ions correspond to (M - CH + O)⁺, with the most likely structure being pyrylium. Presence of pyrylium-based ions was further confirmed by tandem mass spectrometry of the (M + 3)⁺ ion compared with that of a commercially available salt. Lastly, rapid and efficient production of pyrylium in the gas phase was used to convert benzene into pyridine. Though this pyrylium-formation reaction has not been shown before, the reaction is rapid and efficient. Potential reactant species, which could lead to pyrylium formation, were determined from reagent-ion mass spectra. Thermodynamic evaluation of reaction pathways was aided by calculation of the formation enthalpy for pyrylium, which was found to be 689.8 kJ/mol. Based on these results, we propose that this reaction is initiated by ionized ozone (O₃^+·), proceeds similarly to ozonolysis, and results in the neutral loss of the stable CHO₂^· radical. Graphical Abstract ᅟ.

Mechanistic study on lowering the sensitivity of positive atmospheric pressure photoionization mass spectrometric analyses: size-dependent reactivity of solvent clusters

RATIONALE

Understanding the mechanism of atmospheric pressure photoionization (APPI) is important for studies employing APPI liquid chromatography/mass spectrometry (LC/MS). In this study, the APPI mechanism for polyaromatic hydrocarbon (PAH) compounds dissolved in toluene and methanol or water mixture was investigated by use of MS analysis and quantum mechanical simulation. In particular, four different mechanisms that could contribute to the signal reduction were considered based on a combination of MS data and quantum mechanical calculations.

METHODS

The APPI mechanism is clarified by combining MS data and density functional theory (DFT) calculations. To obtain MS data, a positive-mode (+) APPI Q Exactive Orbitrap mass spectrometer was used to analyze each solution. DFT calculations were performed using the general atomic and molecular electronic structure system (GAMESS).

RESULTS

The experimental results indicated that methanol significantly reduced the signal in (+) APPI, but no significative signal reduction was observed when water was used as a co-solvent with toluene. The signal reduction is more significant especially for molecular ions than for protonated ions. Therefore, important information about the mechanism of methanol-induced signal reduction in (+) APPI-MS can be gained due its negative impact on APPI efficiency.

CONCLUSIONS

The size-dependent reactivity of methanol clusters ((CH3 OH)n , n = 1-8) is an important factor in determining the sensitivity of (+) APPI-MS analyses. Clusters can compete with toluene radical ions for electrons. The reactivity increases as the sizes of the methanol clusters increase and this effect can be caused by the size-dependent ionization energy of the solvent clusters. The resulting increase in cluster reactivity explains the flow rate and temperature-dependent signal reduction observed in the analytes. Based on the results presented here, minimizing the sizes of methanol clusters can improve the sensitivity of LC/(+)-APPI-MS.

Review on ion mobility spectrometry. Part 2: hyphenated methods and effects of experimental parameters

Ion Mobility Spectrometry (IMS) is a widely used and 'well-known' technique of ion separation in the gaseous phase based on the differences of ion mobilities under an electric field. This technique has received increased interest over the last several decades as evidenced by the pace and advances of new IMS devices available. In this review we explore the hyphenated techniques that are used with IMS, specifically mass spectrometry as an identification approach and a multi-capillary column as a pre-separation approach. Also, we will pay special attention to the key figures of merit of the ion mobility spectrum and how data sets are treated, and the influences of the experimental parameters on both conventional drift time IMS (DTIMS) and miniaturized IMS also known as high Field Asymmetric IMS (FAIMS) in the planar configuration. The present review article is preceded by a companion review article which details the current instrumentation and contains the sections that configure both conventional DTIMS and FAIMS devices. These reviews will give the reader an insightful view of the main characteristics and aspects of the IMS technique.

Real-time monitoring of exhaled drugs by mass spectrometry

Future individualized patient treatment will need tools to monitor the dose and effects of administrated drugs. Mass spectrometry may become the method of choice to monitor drugs in real time by analyzing exhaled breath. This review describes the monitoring of exhaled drugs in real time by mass spectrometry. The biological background as well as the relevant physical properties of exhaled drugs are delineated. The feasibility of detecting and monitoring exhaled drugs is discussed in several examples. The mass spectrometric tools that are currently available to analyze breath in real time are reviewed. The technical needs and state of the art for on-site measurements by mass spectrometry are also discussed in detail. Off-line methods, which give support and are an important source of information for real-time measurements, are also discussed. Finally, some examples of drugs that have already been successfully detected in exhaled breath, including propofol, fentanyl, methadone, nicotine, and valproic acid are presented. Real-time monitoring of exhaled drugs by mass spectrometry is a relatively new field, which is still in the early stages of development. New technologies promise substantial benefit for future patient monitoring and treatment.

Gas chromatography connected to multiple channel electrospray ionization mass spectrometry for the detection of volatile organic compounds

This work successfully connected gas chromatography (GC) to seven-channel electrospray ionization (ESI) mass spectrometry to separate and detect a mixture of volatile organic compounds. Gaseous analyte was eluted separately from a GC column and directed into the central channel of the ESI source. The analyte was protonated by ion-molecule reactions between the analyte and the ions which were generated by electrospraying the acidic solution through the outside six channels surrounding the central channel. Real-time analysis of the organic reaction involving volatile and thermally unstable compounds (dimethylhydrazine ⇌ azomethane + H(2)) was also achieved by continuously purging the air in the reaction vessel to the seven-channel ESI source.

Detection of microbial volatile organic compounds (MVOCs) by ion-mobility spectrometry

Traces of microbial volatile organic compounds (MVOCs) in air can indicate the presence of growth of moulds in the indoor environment. Ion-mobility spectrometry is a very promising method for detection of these MVOCs, because of its high sensitivity. For development of an in-situ method for detection of MVOCs, a portable ion-mobility spectrometer (IMS) was used and test gases of 14 MVOCs and their respective mixtures were investigated. IMS spectra were recorded as a function of concentration of MVOCs in air. Drift time and mobility of reactant ions formed in positive polarity mode were determined and correlated with the mass-to-charge ratio (m/z) of the MVOCs investigated. The estimated detection limit has a specific value for each MVOC and is in the range 3 to 96 microg m(-3) (1 to 52 ppb(V)). Indoor trials show that IMS can indicate hidden mould growth.

Measurements of volatile organic compounds in the earth's atmosphere using proton-transfer-reaction mass spectrometry

Proton-transfer-reaction mass spectrometry (PTR-MS) allows real-time measurements of volatile organic compounds (VOCs) in air with a high sensitivity and a fast time response. The use of PTR-MS in atmospheric research has expanded rapidly in recent years, and much has been learned about the instrument response and specificity of the technique in the analysis of air from different regions of the atmosphere. This paper aims to review the progress that has been made. The theory of operation is described and allows the response of the instrument to be described for different operating conditions. More accurate determinations of the instrument response involve calibrations using standard mixtures, and some results are shown. Much has been learned about the specificity of PTR-MS from inter-comparison studies as well the coupling of PTR-MS with a gas chromatographic interface. The literature on this issue is reviewed and summarized for many VOCs of atmospheric interest. Some highlights of airborne measurements by PTR-MS are presented, including the results obtained in fresh and aged forest-fire and urban plumes. Finally, the recent work that is focused on improving the technique is discussed.

The intriguing case of organic impurities contained in synthetic methanol: a mass spectrometry based investigation

The role of organic impurities in the methanol-to-olefin (MTO) industrial process catalyzed by zeolites is the subject of ongoing debate. We have found that methanol (HPLC and RPE grade) purchased from different chemical companies may contain organic impurities, whose ionization is the dominant process in the positive ion atmospheric pressure chemical ionization (APCI) spectrum of commercial CH(3)OH. Such impurities produce ions with elemental formulae C(n)H(2n+1)O(+) (n = 4, 5, 6); likewise, ionization of tetradeuterated methanol (CD(3)OD) leads to the corresponding fully deuterated series C(n)D(2n+1)O(+) (n = 4, 5, 6), an outcome which represents a clear evidence of their widespread diffusion. We suggest that their formation might be inherent to the chemical process whereby methanol is synthesized on an industrial scale. Mass spectrometry (MS) experiments, gas chromatography/mass spectrometry (GC/MS) analysis and nuclear magnetic resonance (NMR) measurements allowed us to establish that commercial methanol contains dimethyl acetals of simple alkyl ketones, such as propanone, butanone and pentanone. Ab initio calculations (DFT/B3LYP) proved useful to understanding the ionization mechanisms of such impurities.

Much improved upper limit for the rate constant for the reaction of O2+ with N2

The rate constant for the reaction of O2+ with N2 to produce NO+ plus NO has been measured at 423, 523, and 623 K in a turbulent ion flow tube. Much improved upper limits for this reaction at the three temperatures are 2, 4, and 10x10(-21) cm3 s-1, respectively. These results should render this reaction irrelevant when modeling all plasmas involving atmospheric gases.

Benzene-assisted atmospheric-pressure chemical ionization: a new liquid chromatography/mass spectrometry approach to the analysis of selected hydrophobic compounds

Charge-exchange reactions involving benzene have been successfully exploited to increase the sensitivity of atmospheric-pressure chemical ionization mass spectrometry (APCI-MS) towards hydrophobic compounds of significant environmental relevance which are not detectable with the ordinary APCI techniques. Among them, good sensitivity have been found for (a) highly chlorinated biphenyl derivatives such as dichlorodiphenyltrichloroethane (DDT), dichlorodiphenyldichloroethane (DDD) and dichlorodiphenyldichloroethene (DDE); (b) cyclopentadienes such as Aldrin and its epoxy derivatives Dieldrin and Endrin; and (c) dibenzofurans and dibenzo-para-dioxins such as 2,3,7,8-tetrachlorodibenzofuran (2,3,7,8-TCDF) and 2,3,7,8-tetrachloro-dibenzo-para-dioxin (2,3,7,8-TCDD). The reactant benzene molecules were introduced into the source either through the nebulizer gas or by direct post-column addition of neat liquid, whereas the targeted compounds were analyzed using a high-performance liquid chromatography (HPLC) RP-18 column using methanol/water solutions as mobile phase. By using benzene as post-column reagent, positive ion mode detection was proven to be significantly enhanced as compared with APCI measurements carried out without benzene assistance.

Factors influencing the analytical performance of an atmospheric sampling glow discharge ionization source as revealed via ionization dynamics modeling

A kinetic model is developed for the dynamic events occurring within an atmospheric sampling glow discharge that affect its performance as an ion source for analytical mass spectrometry. The differential equations incorporate secondary electron generation and thermalization, reagent and analyte ion formation via electron capture and ion-molecule reactions, ion loss via recombination processes, diffusion, and ion-molecule reactions with matrix components, and the sampling and pumping parameters of the source. Because the ion source has a flow-through configuration, the number densities of selected species can be estimated by applying the steady-state assumption. However, understanding of its operation is aided by knowledge of the dynamic behavior, so numerical methods are applied to examine the time dependence of those species as well. As in other plasma ionization sources, the ionization efficiency is essentially determined by the ratio of the relevant ion formation and recombination rates. Although thermal electron and positive reagent ion number densities are comparable, the electron capture/ion-molecule reaction rate coefficient ratio is normally quite large and the ion-electron recombination rate coefficient is about an order of magnitude greater than that for ion-ion recombination. Consequently, the efficiency for negative analyte ion formation via electron capture is generally superior to that for positive analyte ion generation via ion-molecule reaction. However, the efficiency for positive analyte ion formation should be equal to or better than that for negative analyte ions when both ionization processes occur via ion-molecule reaction processes (with comparable rate coefficients), since the negative reagent ion density is considerably less than that for positive reagent ions. Furthermore, the particularly high number densities of thermal electrons and reagent ions leads to a large dynamic range of linear response for the source. Simulation results also suggest that analyte ion number densities might be enhanced by modification of the standard physical and operating parameters of the source.

Protonation in electrospray mass spectrometry: wrong-way-round or right-way-round?

The term "wrong-way-round ionization" has been used in studies of electrospray ionization to describe the observation of protonated or deprotonated ions when sampling strongly basic or acidic solutions (respectively) where such ions are not expected to exist in appreciable concentrations in solution. Study of the dependence of ionization of the weak base caffeine on the electrospray capillary potential reveals three distinct contributors to wrong-way-round ionization. At near-neutral pH in solutions of low ionic strength, protonation of caffeine results from the surface enrichment of electrolytically produced protons in the surface layer of the droplets from which ions are desorbed. For solutions made strongly basic with ammonia, gas-phase proton transfer from ammonium ions can create protonated caffeine. These two mechanisms have been discussed previously elsewhere. For solutions of high ionic strength at neutral or high pH, the data suggest that discharge-induced ionization is responsible for the production of protonated caffeine. This mechanism probably accounts for some of the wrong-way-round ionization reported elsewhere.

Importance of gas-phase proton affinities in determining the electrospray ionization response for analytes and solvents

The effect of gas-phase proton transfer reactions on the mass spectral response of solvents and analytes with known gas-phase proton affinities was evaluated. Methanol, ethanol, propanol and water mixtures were employed to probe the effect of gas-phase proton transfer reactions on the abundance of protonated solvent ions. Ion-molecule reactions were carried out either in an atmospheric pressure electrospray ionization source or in the central quadrupole of a triple-quadrupole mass spectrometer. The introduction of solvent vapor with higher gas-phase proton affinity than the solvent being electrosprayed caused protons to transfer to the gas-phase solvent molecules. In mixed solvents, protonated solvent clusters of the solvent with higher gas-phase proton affinity dominated the resulting mass spectra. The effect of solvent gas-phase proton affinity on analyte response was also investigated, and the analyte response was suppressed or eliminated in solvents with gas-phase proton affinities higher than that of the analyte.

Airborne mass spectrometers: four decades of atmospheric and space research at the Air Force research laboratory

Mass spectrometry is a versatile research tool that has proved to be extremely useful for exploring the fundamental nature of the earth's atmosphere and ionosphere and in helping to solve operational problems facing the Air Force and the Department of Defense. In the past 40 years, our research group at the Air Force Research Laboratory has flown quadrupole mass spectrometers of many designs on nearly 100 sounding rockets, nine satellites, three Space Shuttles and many missions of high-altitude research aircraft and balloons. We have also used our instruments in ground-based investigations of rocket and jet engine exhaust, combustion chemistry and microwave breakdown chemistry. This paper is a review of the instrumentation and techniques needed for space research, a summary of the results from many of the experiments, and an introduction to the broad field of atmospheric and space mass spectrometry in general.

Hydration of gas-phase ions formed by electrospray ionization

The hydration of gas-phase ions produced by electrospray ionization was investigated. Evidence that the hydrated ions are formed by two mechanisms is presented. First, solvent condensation during the expansion inside the electrospray source clearly occurs. Second, some solvent evaporation from more extensively solvated ions or droplets is apparent. To the extent that these highly solvated ions have solution-phase structures, then the final isolated gas-phase structure of the ion will be determined by the solvent evaporation process. This process was investigated for hydrated gramicidin S in a Fourier-transform mass spectrometer. Unimolecular dissociation rate constants of isolated gramicidin S ions with between 2 and 14 associated water molecules were measured. These rate constants increased from 16 to 230 s-1 with increasing hydration, with smaller values corresponding to magic numbers.