15N Analysis of nitric oxide and nitrous oxide by cryotrap enrichment using a gas chromatograph quadrupole mass spectrometer and its application to (15)N-tracer investigations of NO/N(2)O formation in soil

Nitric oxide (NO) and nitrous oxide (N(2)O) are two important trace gases in the atmosphere. Determining the concentration and (15)N abundance of NO and N(2)O in air is difficult owing to their very low concentration in the atmosphere (NO < 1 ppb(v); N(2)O approximately 0.32 ppm(v)). Although (15)N analysis of N(2)O in ambient concentrations can be carried out using a gas chromatograph quadrupole mass spectrometer system (GC-QMS) and a dosage of 2 mL of air by means of a sample loop, this system is not sensitive enough to measure the ambient concentration of NO and its (15)N abundance. Therefore the concentration of NO must be enriched by cryotrapping (cooling with liquid nitrogen). The (15)N analytical method developed enables the sensitive and sufficiently precise measurement of (15)N-enriched NO in air. Furthermore, the analytical equipment developed greatly improves existing (15)N(2)O analysis using the GC-QMS technique. An application of the (15)N analysis method will be shown for an investigation on the NO and N(2)O formation in black earth soil after (15)NH(4)(+), (15)NO(3)(-) and (15)NO(2)(-) labelling. Copyright 1999 John Wiley & Sons, Ltd.     

Nitrogen-15 fractionation in the thermal decomposition of nitrous oxide of natural isotopic composition

The 15N fractionation in the thermal decomposition of nitrous oxide (N2O) of natural isotopic composition has been investigated in quartz reaction vessel in the temperature interval 888-1073 K. The formulas relating the observed experimentally 15N fractionations with the primary 15N kinetic isotope effect, (k14/k15)p for 14N15N16O, and secondary 15N kinetic isotope effect, (k14/k15)s for 15N14N16O, have been derived. The experimentally estimated 15N kinetic isotope effects have been compared with the primary and secondary 15N kinetic isotope effects calculated with the absolute rate theory formulations applied to linear three atom molecules. A good agreement was found for the primary 15N kinetic isotope effect, (k14/k15)p, in the temperature interval 888-1007 K. But at 1073 K the decompositions of N2O, accompanied by NO (nitric oxide) formation proceed with a twice times smaller primary kinetic isotope effect, (k14/k15)p of 1.0251 +/- 0.0009, only, suggesting the nonlinear transition state structures with participation of the fourth external atom at high temperature decompositions of nitrous oxide. The nitrogen isotope effects determined in this study correlate well with nitrogen isotope fractionations observed in the natural biological, earth and atmospheric processes.     

The reaction of nitrous oxide with rhodium

A clean rhodium filament at room temperature is highly reactive towards nitrous oxide. The oxygen atom of the N₂O molecule is adsorbed with a sticking probability of 0.45 whilst the nitrogen atoms appear in the gas phase as molecular nitrogen. The room temperature uptake of oxygen is about 5 × 10¹⁴ atom cm^?2 and is independent of nitrous oxide pressure in the range 3.5 × 10^?8 to 1.1 × 10^?6 torr. The adsorption curve is of typical form with an initial region of essentially constant sticking probability. For the first 80% of adsorption at room temperature the shape is satisfactorily accounted for if molecules are able to visit 4–5 adsorption sites whilst held in a weakly-bonded precursor state.     

The structure and extinction of nonpremixed methane/nitrous oxide and ethane/nitrous oxide flames

Knowledge of combustion of hydrocarbon fuels with nitrogen-containing oxidizers is a first step in understanding key aspects of combustion of hypergolic and gun propellants. Here an experimental and kinetic-modeling study is carried out to elucidate aspects of nonpremixed combustion of methane (CH₄) and nitrous oxide (N₂O), and ethane (C₂H₆) and N₂O. Experiments are conducted, at a pressure of 1 atm, on flames stabilized between two opposing streams. One stream is a mixture of oxygen (O₂), nitrogen (N₂), and N₂O, and the other a mixture of CH₄ and N₂ or C₂H₆ and N₂. Critical conditions for extinction are measured. Kinetic-modeling studies are performed with the San Diego Mechanism. Experimental data and results of kinetic-modeling show that N₂O inhibits the flame by promoting extinction. Analysis of the flame structure shows that H radicals are produced in the overall chain-branching step 3H₂ + O₂ ? 2H₂O + 2H, in which molecular hydrogen is consumed. Hydrogen is also consumed in the overall step N₂O + H₂ ? N₂ + H₂O where stable products are formed. Inhibition of the flames by N₂O is attributed to competition between these two overall steps.     

Flame propagation in gaseous nitrous oxide

The characteristics of the propagation of a nitrous oxide decomposition flame in a tube with an internal diameter of 70 mm were measured. It was demonstrated that the pattern of flame propagation and the extent of burnout are determined by the convective motion of the flame kernel because of a very slow burning of nitrous oxide. The laminar flame speed estimated from pressure oscillograms and calculated using thermal theory of flame propagation was found to be ～1 cm/s. The critical diameter of flame quenching in channels were measured to decrease from 10 to 4 mm as the pressure was increased from 15 to 20 atm. Because of the possibility of reignition of the fresh mixture behind the flame arrester by the outflowing combustion products, the channel should be significantly longer than 200 mm.     

Sextic force field of nitrous oxide

The sextic force field in the curvilinear internal coordinates has been studied for the nitrous oxide molecule from the spectroscopic data of ¹⁴N₂¹⁶O, ¹⁴N¹⁵N¹⁶O, and ¹⁵N¹⁴N¹⁶O. The bands below 6600 cm⁻¹ have been used. The force constants in the internal coordinates are converted to those in dimensionless normal coordinates by two successive transformations. The vibration Hamiltonian matrix for each symmetry species of a given isotopic species has been constructed from the harmonic oscillator basis functions, and it is then diagonalized numerically to give the vibrational energy levels and the wavefunctions. The latter have been used for the evaluation of ratational constants. The least-squares refinement has been very successful in the present study, and it is shown that the general quartic force field supplemented by the quintic and sextic stretching diagonal force constants estimated from the Morse function, provided that the terms up to sextic are kept in the dimensionless normal coordinate space, well reproduces the spectroscopic constants such as the vibrational levels, rotational constants, l-type doubling constants, and centrifugal distortion constants. The spectroscopic constants of the isotopic molecules which are excluded from the refinement process are also in good agreement with the computed ones. The bond dissociation energies of the NN and NO bonds estimated from the present results have been critically examined.     

The potential energy function of the nitrous oxide molecule using pure vibrational data

Using a very large and partly new set of vibrational data, a preliminary determination of the potential energy function of the nitrous oxide molecule has been undertaken and the results are presented in this paper. These results are discussed.     

15N Distribution in Wheat and Chemical Fractionation of Root-Borne 15N in the Soil

Abstract

Spring wheat plants were grown in split-root containers and labelled with ¹⁵N by fertilizing one compartment of the container with ¹⁵NH₄ ¹⁵NO₃ (95 at.-% ¹⁵N exc.). After the harvest, approx. 90% of the ¹⁵N incorporated by the plants were found in the shoots and 3% in the roots; approx. 7% had been released into the soil of the unlabelled compartment. The ¹⁵N in the soil of the unlabelled compartment was extracted with KCl and hydrolysed with HCl. Approx. 60% of the ¹⁵N was found in the hydrolysable organic N pool of the soil and 9% in the fraction of the soluble and exchangeable inorganic nitrogen.     

The thermal conductivity of argon,carbon dioxide and nitrous oxide

The paper presents new, absolute measurements of the thermal conductivity of three gases: argon (Ar), carbon dioxide (CO₂) and nitrous oxide (N₂O). The measurements have been carried out with a transient hot-wire instrument in the temperature range 308 to 430 K and at pressures up to 11 MPa. For most of the range of thermodynamic states covered by the measurements it is estimated that the accuracy of the thermal conductivity data is one of ±0.3%. However, for carbon dioxide and nitrous oxide near their critical conditions the accuracy is degraded and the uncertainty may be as much as ±2%. The new experimental data for argon confirm the accuracy claimed for the thermal conductivity in the limit of zero-density.The thermal conductivity of the polyatomic gases in the limit of zero-density is used in conjunction with information on other transport cross-sections for the same systems, to extract a consistent set of cross-sections sensitive to the anisotropy of the intermolecular pair potential for use in the testing of proposed potential surfaces. Cross-sections for both of the available formulations of the thermal conductivity of a polyatomic gas due to Wang Chang and Uhlenbeck and Thijsse et al. are derived. For both gases it is shown that the Mason-Monchick approximation breaks down in either formulation. However, the effect of the failure on the formulation of Thijsse et al. is smaller and it is possible to represent the data with the aid of a single cross-section which has a very simple temperature dependence. The analysis also demonstrates that all available high-temperature thermal conductivity data for carbon dioxide are in substantial error.In the moderately dense gas the concept of a temperature-independent excess thermal conductivity is confirmed to a high degree of precision for carbon dioxide and argon when due allowance is made for the critical enhancement. A generalized correlation of the temperature dependence of the first density coefficient of thermal conductivity is broadly confirmed.     

The 14N2/15N2 and 14N2/14N15N liquid-vapour isotope separation factor

A theoretical calculation of the ¹⁴N₂/¹⁵N₂ and ¹⁴N₂/¹⁴N¹⁵N liquid-vapour isotope separation factors using a diatomic, or atom-atom potential is undertaken. The results show that this potential is capable of representing the isotope separation factors with some permissible variation in the parameter set.     

Decay pathways of superexcited states of nitrous oxide

The ionization and ionic dissociation of the superexcited state of N20 are studied by using electron energy loss spectroscopy and positive ion time-of-flight mass spectroscopy at different momentum transfers; that is, 0 and 0.23 a.u. （atomic unit） . The transitions at 13.8 eV and 14.0 eV are reassigned as 3pπ（000） and 3pσ（000） converging to A^2∑＋, respectively. The competition between the main decay pathways of superexcited states at different momentum transfers is revealed. It is found that 3dσ converging to C^2∑＋ mainly decays into N2O^＋ while 4dσ can decay into both N2^O＋ and NO^＋.     

Bands of nitrous oxide at 5.3 μm

Several bands of nitrous oxide occurring in the region of about 5.3 μm have been measured by using a high resolution vacuum infrared spectrograph. The molecular constants derived from the data have been given along with the basic data.     

The Position Dependent 15N Enrichment of Nitrous Oxide in the Stratosphere

Abstract

The position dependent ¹⁵N fractionation of nitrous oxide (N₂O), which cannot be obtained from mass spectrometric analysis on molecular N₂O itself, can be determined with high precision using isotope ratio mass spectrometry on the NO⁺ fragment that is formed on electron impact in the source of an isotope ratio mass spectrometer. Laboratory UV photolysis experiments show that strong position dependent ¹⁵N fractionations occur in the photolysis of N₂O in the stratosphere, its major atmospheric sink. Measurements on the isotopic composition of stratospheric N₂O indeed confirm the presence of strong isotope enrichments, in particular the difference in the fractionation constants for ¹⁵N¹⁴NO and ¹⁴N¹⁵NO. The absolute magnitudes of the fractionation constants found in the stratosphere are much smaller, however, than those found in the lab experiments, demonstrating the importance of dynamical and also additional chemical processes like the reaction of N₂O with O(¹D).     

Dipole moment functions of carbon dioxide and nitrous oxide

The vibrational wavefunctions obtained from the direct numerical diagonalization of the vibrational Hamiltonian matrix are used to compute the matrix elements of transition dipole moments for carbon dioxide and nitrous oxide. The computed matrix elements are found very close to those obtained previously by the contact transformation method. The least-squares fit of the dipole moment derivatives up to the second order has been attempted. It is shown that the values as well as the signs of the second derivatives can be determined if the mechanical anharmonicity contributes considerably to the corresponding combination and/or difference bands. The bands selected for N₂O include the fundamentals, first overtones, binary combination, and difference bands as well as the bands coupled with the above by Fermi resonances. The converged set of the dipole moment derivatives of N₂O gives the computed matrix elements very close to the observed. This also demonstrates the wider applicability of the direct diagonalization method.     

Vibration-rotation bands of nitrous oxide: 4.1 micron region

The infrared spectrum of nitrous oxide has been measured and analyzed from 2265 cm^?1 to 2615 cm^?1. Newly refined effective rotational constants for twenty-one vibrational states of ¹⁴N₂O, three vibrational states each of ¹⁴N₂¹⁸O and ¹⁵N¹⁴N¹⁶O, two states of ¹⁴N¹⁵N¹⁶O and one state of ¹⁴N₂¹⁷O have been calculated.The most interesting features observed are two Δ-Σ “forbidden” bands, 04^2c0-00⁰0 and 12^2c0-00⁰0. These bands occur because of Coriolis interaction between unperturbed vibrational states having l = 0 and l = 2.     

Detection of small traces of 15N2 and 14N2 by Faraday LMR spectroscopy of the corresponding isotopomers of nitric oxide

15 N₂ in a gas phase sample is described. The nitrogen is transformed by a microwave discharge into nitric oxide NO. The latter is analyzed by recording both a ¹⁵NO and a ¹⁴NO Faraday LMR signal. The determination of the transformation rate from N₂ into NO is described. The method of measurement and the achieved sensitivity (∼0.1 ppm ¹⁵N₂≈4 nanomoles ¹⁵N₂/litergas) of the spectrometer are discussed. An application in pharmacology, where ¹⁵N is used as a tracer for metabolism is indicated. First experiments with the exhalation of a rat show that the apparatus is useful to give a new quality of results. Received: 30 April 1996/Revised version: 29 July 1996     

A shell-model calculation in the continuum for the reaction N15(n,n′) N15

Using the shell-model approach to nuclear reactions, the Lippmann-Schwinger equation for elastic and inelastic nucléon scattering is solved numerically. The procedure is based upon an adaptation of a method due toWeinberg. The differential and total elastic and inelastic cross-sections and the asymmetry of the scattering of a transversally polarised beam of neutrons are calculated. The positions (energies and total widths) and residues (partial widths) of the scattering matrix in the complex plane are determined. The results are discussed and compared with previous treatments.     

The role of (15)N CSA and CSA/dipole cross-correlation in (15)N relaxation in solid proteins

The influence of the (15)N CSA on (15)N longitudinal relaxation is investigated for an amide group in solid proteins in powder form under MAS. This contribution is determined to be typically 20-33% of the overall longitudinal relaxation rate, at 11.74 and 16.45 T, respectively. The improved treatment is used to analyze the internal dynamics in the protein Crh, in the frame of a motional model of diffusion in a cone, using the explicit average sum approach. Significant variations with respect to the determined dynamics parameters are observed when properly accounting for the contribution of (15)N CSA fluctuations. In general, the fit of experimental data including CSA led to the determination of diffusion times (tau(w)) which are longer than when considering only an (15)N-(1)H dipolar relaxation mechanism. CSA-Dipole cross-correlation is shown to play little or no role in protonated solids, in direct contrast to the liquid state case.     

Emissivity and band-model parameters for infrared bands of nitrous oxide

Narrow band-model parameters S/d, γ_N/d and B have been obtained experimentally for the v₃, v₁ and 2v₂ bands of N₂O in the temperature range from 200°K to 500°K. The exponential-wide band-model parameters have been determined for all of the contributing bands and have been found to differ greatly from the values of Tien et al. Data of revised total gas emissivities are presented.