E. V. Aparina

Effect of the humidity on the uptake of NO3 on coatings composed of MgCl2 · 6H2O and MgBr2 · 6H2O and mixtures thereof with NaCl

The reactive uptake of NO3 radicals on the surface of wetted individual X salts and of wetted X-NaCl salts (X = MgCl2 · 6H2O and MgBr2 · 6H2O) at [H2O] = 2 × 1012−2 × 1015 cm−3 and NO3 (4.8 × 1012 cm−3) was studied using a reactor with a movable insert covered with a salt coating in combination with a mass spectrometer for monitoring the initial reactant and products. The probabilities of NO3 uptake γ on X-NaCl binary salts as functions of the content of doping salt were determined. A parametric approximation of the experimental data was proposed, which makes it possible to quantitatively predict the extent of surface enrichment of a wetted binary salt coating in doping salt and its dependence on the humidity and the content of this salt in the binary mixture. It was established that the relative surface density σX of X doping salt depends on its mole fraction μX in the X-NaCl binary salt as σX = aμX (a = 2.2 for MgBr2 and 13.1 for MgCl2) over the entire humidity range covered. The contributions of the X salts to the overall uptake of NO3 at NO3 concentration typical of the tropospheric conditions ([NO3] ∼ 107 cm−3 and relative humidities of RH ≤ 20%) were estimated.

Initial uptake of NO2 on methane flame soot

A thermostated flow reactor with a movable soot-coated insert coupled to a high-resolution mass spectrometer with low-energy electron ionization is used to study the uptake of NO2 reagent gas at [NO2] = 1 × 1012−2 × 1013 cm−3 and a humidity of [H2O] = 5 × 1012−1.8 × 1015 cm−3. The BET (Brunnauer-Emmett-Teller) method is used to determine the specific surface area of the soot coating: (40 ± 10 (2σ)) m2/g. A set of time-dependent uptake coefficients of NO2 on fresh soot coatings in this range of reactant gas concentrations is determined. An analysis of the experimental data shows that the uptake coefficient depends on the time as 1/γ(t) = 1/γ0 + c1t and yields the dependences of the parameters γ0 and c1 on the NO2 concentration: 1/γ0 = c2 + c3[NO2] and c1 = k[NO2] with constants c2 = (6.5 ± 1.3) × 103, c3 = (5.6 ± 1.3) × 10−10 cm3 molecule−1, and k = (2.4 ± 0.2) × 10−10 cm3 molecule−1 s−1. The gas-phase products of NO2 uptake on soot are NO and HONO, with the NO yield constituting ∼50% of the reacted NO2. It is experimentally demonstrated that an increase in the humidity causes no changes in the uptake coefficient and in the composition and ratio of the products. The initial stage of NO2 uptake on a methane soot coating is described using the Langmuir adsorption model, according to which the process of uptake consists of a sequence of elementary steps, such as reversible adsorption, surface complex formation, and its subsequent unimolecular decomposition to form products. Interpretation of the experimental dependence γ0 = f([NO2]) enables to estimate the Langmuir constant for the NO2-methane soot pair, KL = (8.6 ± 2.6) × 10−14 cm3 molecule−1, the rate constant for NO2 desorption from the soot coating, kd = (530 ± 160) s−1, and the rate constant for the monomolecular decomposition of the surface complex, kr = (8.2 ± 2.5) × 10−2 s−1.

The Kinetic Mechanisms of the Trapping of Atmospheric Gases by Marine Salt Surfaces: IX. The Heat of Adsorption of Cl Atoms on the Surface of NaCl

The heterogeneous trapping of chlorine atoms on the surface of NaCl was studied using two coaxial stream reactors connected to an EPR cavity or a mass spectrometer. The kinetics of trapping was measured by the EPR method over a wide range of chlorine atom concentrations (1010–1013 cm−3) at temperatures of 250–330 K. At [Cl] ≥ 1012 cm−3, chlorine atoms were recorded by the EPR method in the gas phase. At lower concentrations (~1010−3 × 1011 cm−3), Cl atoms were replaced with RO2 radicals by adding hexane RH and O2 at the entrance of the EPR cavity. This was followed by the matrix isolation of RO2 in the cavity at liquid nitrogen temperature. The probability of the trapping of chlorine atoms on the chemically inactive surface of NaCl was found to increase as the concentration of Cl grew. The temperature dependence of the trapping coefficient γ was pronounced at a concentration of chlorine atoms of ~3 × 1010 cm−3, whereas no such dependence was observed at a chlorine concentration of ~ 1013 cm−3. The recombination of Cl atoms was well described by the Rideal-Eley mechanism, and the heat of adsorption of chlorine atoms on the inactive surface of NaCl was estimated at Q = 17 ± 0.6 kcal/mol. It was shown mass spectrometrically that the trapping coefficient γ of Cl atoms decreased with the time of measurements, like the partial coefficient of the formation of the HCl product, whereas the partial coefficient of the formation of the Cl2 product, conversely, increased with the time. The characteristic time of the attainment of stationary values by all the γ coefficients weakly depended on the initial concentration of Cl and equaled several dozen seconds. Reactions of adsorbed Cl atoms formed in the trapping of NO3 radicals by the surface of marine salt NaCl in coastal troposphere are discussed.

Mass Spectrometric Studies of Physical, Thermochemical and Reactive Properties of Xenon Fluorides, Xenon Oxides and Xenon Oxyfluorides

Using a reactor with a flowing diffusion cloud coupled to a high-resolution, low-energy electron-impact ionization mass spectrometer, mechanistic, kinetic and thermochemical characteristics of gas-phase reactions with the participation of charged and neutral xenon oxides, xenon fluorides and xenon oxyfluorides have been investigated. Ionization energies for XeF, XeF2, XeF4, XeO3, XeO4, XeOF4 molecules and appearance energies for the ions formed from these molecules were obtained. Based on experimental and reference data, the enthalpies of XeO3 and XeOF4 formation were refined and a number of binding energies in the parent and fragment ions were calculated. For electron-impact ionization, the ionization cross-sections for Xe, XeF2, XeF4 and XeOF4 proved to correlate with a semi-empirical principle of full ionization. Based on the temperature dependencies of saturated vapor pressures for XeO4, XeOF4 and XeO2F2, their enthalpies of evaporation, sublimation and melting were determined. The mechanisms of gas-phase reactions between H atoms and neutral XeF2, XeF4, XeF6, XeO4 and XeOF4 were studied.

Kinetic mechanisms of the uptake of atmospheric gases on the surface of sea salts. ClNO3 uptake on MgCl2 · 6H2O/NaCl

The uptake coefficients γ of chlorine nitrate on MgCl2 · 6H2O crystallites and a MgCl2 · 6H2O-NaCl mixture deposited from an aqueous solution are measured using a flow reactor with a movable salt-substrate-coated insert equipped in combination with a mass spectrometer at 295–428 K, [ClNO3] = (0.2−12) × 1012 cm−3, and [H2O] = 1.0 × 1012 − 4.3 × 1015 cm−3. Immediately after the exposure of the salt substrate to a ClNO3 flow, γ(t) decreases exponentially with time, γ(t) = γ0 × exp(−t/τ) + γs, to a steady-state level, γs, which depends on the temperature and the ClNO3 and H2O concentrations. The main gas-phase product is Cl2, HOCl appears only when water vapor is admitted into the reactor. The coefficient of steady-state uptake on wetted MgCl2 · 6H2O at 295 K can be described by the approximation γ = a + b [H2O] with a = 3.5 × 10−3 cm3 and b = 3.2 × 10−18 cm3. The mechanism of the uptake of ClNO3 on MgCl2 · 6H2O is discussed. The experimental data are treated within the framework of a steady-state uptake model to estimate the heat of adsorption of ClNO3 on MgCl2 · 6H2O (Qad = 62 kJ/mol) and the activation energy of the bimolecular heterogeneous reaction ClNO3 + Zs = 2Cl2 + Mg(NO3)2 · 6H2O (Ea = 21.8 kJ/mol; Zs denotes a ClNO3-MgCl2 · 6H2O surface complex). When the MgCl2 · 6H2O: NaCl is varied from 0 to ∼3 wt %, the steady-state uptake coefficient changes from the value corresponding to uptake on pure NaCl to that characteristic of uptake on pure MgCl2 · 6H2O.

Kinetics of NO2 uptake on methane flame soot

The dependences of the initial NO2 uptake coefficient γ on a methane soot coating at 255 K on the NO2 concentration (1.3 × 1012–3.3 × 1013 cm–3) and time t were studied using a flow reactor with a mobile insert and mass spectrometric recording of gaseous reagents and products: 1/γ(t) = 1/γ0 + at, where γ0 and a are the parameters that depend on the NO2 concentration; γ0 = γ0ini/(1 + kL [NO2], a= k [NO2], with the constants γ0ini = (4.8 ± 2) × 10–4, KL = (8.2 ± 3) × 10–13 cm3, and k = (2.3 ± 0.1) × 10–10 cm3 s–1. The elementary parameters that determine the uptake were evaluated on the basis of the Langmuir adsorption model: desorption rate constant kd = (52 ± 20) s–1, adsorption heat Qad = (38 ± 8) kJ mol–1, rate constant of the unimolecular heterogeneous conversion of NO2 into the product kr = (2.5 ± 1.3) × 10–2 s–1, and its activation energy Ea = (19 ± 10) kJ mol–1. The laboratory data were extrapolated, based on the uptake coefficient, to the limiting tropospheric NO2 concentrations in remote marine (1 ppb) and polluted industrial regions (40 ppb).

Kinetics of NO3 Uptake on Pyrene as a Representative Organic Aerosols

The uptake of NO3 on a pyrene coating at [NO3] = 2.5 × 1012–1.2 × 1013 cm−3 is studied using a coated-insert flow tube reactor coupled to a mass spectrometer. It is established that, in this concentration range, the uptake occurs by the impact recombination mechanism, whereas the consumption of surface sites involves the unimolecular decomposition of stabilized surface complexes. Several hundreds of NO3 radicals are consumed per surface site destroyed. The uptake coefficient depends on the exposure time, being described by the expression γ(t) = γ0exp(−t/τ), where γ0 and τ are parameters dependent on the NO3 concentration. Based on the Langmuir adsorption concept, the following elementary parameters determining the uptake process are estimated: the Langmuir coefficient, KL = 7.1 × 10−13 cm3; the desorption rate constant, kd = 44 s−1; and the rate constant for the unimolecular heterogeneous conversion of surface complexes, kr = 6.6 × 10−2 s−1. Gas chromatography–mass spectrometry measurements showed that the main products of the reaction are phthalates. The nitropyrene yield is found to be ~0.6%.

Possibilities for Energy Transport into a Radio-Frequency Quadrupole by a Shifted Supersonic Gas Jet. Part II. Accumulation and Heating of Ions

The aim of this work was the assessment of the ability of a supersonic jet to accumulate sufficiently dense ion clouds inside the quadrupole, the ion cloud being “heated” to a relatively high temperature under a relatively low density of the residual gas (pressure lower than 10–4 Torr). Kinetic measurements gave an estimate of the number of accumulated ions at the beginning of the quadrupole of about 2 × 107 and their internal temperature of 6000 K.

Possibilities for Energy Pumping in a Radio-Frequency Quadrupole by Shifted Supersonic Gas Jet. Part I: Accelerated and Excited Atom Transmission

Generation of an ion beam and its transmission into a mass analyzer is one of central problems in mass spectrometry. The use of a narrowly directed supersonic gas jet has a number of advantages in comparison with other sampling methods. The aim of this work was to confirm the declared earlier properties of the jet formed at the outlet of a cylindrical channel when the free path length of gaseous atoms at the beginning of the channel is comparable with the channel diameter. The paper describes the ability of such a supersonic jet to conserve an additional energy of jet gas atoms. A significant influence of the temperature of the gas flow on the yield of cyclohexane fragment ions was found, cyclohexane being an admixture in the noble gas jet passing through an electron ionization ion source. A possibility of obtaining a flow of metastable electronically excited atoms inside the jet is also shown. The results of the work confirm the availability of the supersonic gas jet for the design of a high efficiency ion source inside the radio-frequency quadrupole at the input of the mass analyzer.

Two possible improvements for mass spectrometry analysis of intact biomolecules.

The goals of our study were to investigate abilities of two approaches to eliminate possible errors in electrospray mass spectrometry measurements of biomolecules. Passing of a relatively dense supersonic gas jet through ionization region followed by its hitting the spray of the analyzed solution in low vacuum may be effective to keep an initial biomolecule structure in solution. Provided that retention of charge carriers for some sites in the biomolecule cannot be changed noticeably in electrospray ion source, decomposition and separation of charge-state distributions of electrosprayed ions may give additional information about native structure of biomolecules in solution.