Gas chromatography for in situ analysis of a cometary nucleus: characterization and optimization of diphenyl/dimethylpolysiloxane stationary phases

The development of a gas chromatograph for the cometary sampling and composition (COSAC) experiment is described in the context of the preparation for the European Space Agency (ESA) Mission Rosetta for investigation of a cometary nucleus. COSAC is one out of ten experiments on the Rosetta Lander. Its scientific goal is to analyze in situ the chemical composition of the volatile constituents of the nucleus of the target comet P/Wirtanen. Constituted of several (up to eight) capillary wall-coated and porous-layer open tubular columns operating in parallel, the GC system is designed to separate and identify both organic and inorganic compounds which evolve from the comet naturally or are obtained from cometary samples through stepwise heating in a miniaturized pyrolizer. In this first part of our study, dimethylpolysiloxane (DMPS) stationary phases with increasing percentages of diphenyl substituted group (DP) have been investigated. A coupled experimental and theoretical approach has been taken in order to predict chromatographic data. By the use of a four-point experimental calibration (0 to 65% diphenyl group) in conjunction with Pro ezGC modeling software, results in prediction of multicomponent chromatograms with a mean error less than 5% for each compound retention time were obtained, irrespective of the stationary phase's diphenyl content and column physical parameters. The possibility to associate such phases is illustrated by the evolution of coelutions obtained on a non-polar (100% DMPS) and a medium polar (65% DP-DMPS) stationary phase, respectively. This study showed that with a small number of well tuned DP-DMPS columns, the separation and identification of most of the targeted compounds can be achieved with a minimum amount of coelutions and within the experiment requirements.     

Quasi-Neutral Limit for Euler-Poisson System

Summary. This paper provides a rigorous proof of the quasi-neutral limit for the Euler-Poisson system on a bounded domain in one space dimension. The most general case is being considered when the plasma is sustained by ionization. A wide range of plasmas, from collisionless to highly collisional, is permitted. At the plasma center, the ions are assumed to be at rest, and essentially quasi-neutral initial data are prescribed. The theorem asserts that the quasi-neutral limit is obtained until the ion velocity reaches the ion-sound speed. In addition, formal matched asymptotic expansions are given which describe the solution in its passage from the plasma center to the wall. Received June 8, 2000; accepted February 10, 2001 Online publication April 20, 2001     

In-situ chemical investigation of a comet nucleus by gas chromatography: Porous layer open tubular columns for the separation of light, volatile compounds

Summary In the framework of the cometary sampling and composition (COSAC) experiment of the European Space Agency (ESA) Rosetta space mission to a comet, a new generation of porous polymer porous-layer open tubular (PLOT) columns has been investigated with the aim of separating the light organic compounds targeted by the experiment within the constraints of the operating conditions used in space. Both styrene-divinylbenzene and ethylene glycol-dimethyl acrylate-divinylbenzene porous polymers can be used to separate most of the target compounds, but the latter was selected because of its better separating properties. The most appropriate column characteristics were found to be 10 m length, 0.18 mm internal diameter, and 1–2 μm layer thickness, despite the low sample-loading capacity of such a column, which impairs its chromatographic performance. The influence of column overloading on retention properties and efficiency was, therefore, studied. To achieve the separation with optimum sensitivity, and within the in-situ time allowed for the analysis, it was also demonstrated that the maximum allowed temperature and reduced outlet pressure are required. Finally, it was shown that in the presence of water, the most abundant volatile compound in comets the separating properties of the studied columns are conserved. This paper thus demonstrates the suitability of the porous polymer PLOT column selected for the COSAC experiment and requirements for use in space; it is the first porous polymer PLOT column to be used in space exploration. Presented at: 23^rd International Symposium on Chromatography, London, UK, October 1–5, 2000     

Calculation of the susceptibility of aromatic compounds with a localized MO model

Using previously derived equations for the susceptibility [8] we calculated the components of the susceptibility tensor for ethene, benzene and naphthalene. Starting from PCILO bond orbitals and energy localized orbitals we evaluated theoretical increments for the susceptibility of the bonds building up these molecules. It was demonstrated that only with banana bonds was it possible to obtain satisfactory mean values and the right sign of the anisotropy, whereas a bond scheme with ∝ and π bonds yielded considerably worse values. The benzene value of X _m=?712·6×10^?12 m³ mol^?1 resulted from a Kekulé structure with three a bonds and three pairs of r bonds and the naphthalene value of X _m=?1137·4×10^?12 m³ mol^?1 from a similar bond scheme.     

Substitutional boron in nanodiamond, bucky-diamond, and nanocrystalline diamond grain boundaries

Although boron has been known for many years to be a successful dopant in bulk diamond, efficient doping of nanocrystalline diamond with boron is still being developed. In general, the location, configuration, and bonding structure of boron in nanodiamond is still unknown, including the fundamental question of whether it is located within grains or grain boundaries of thin films and whether it is within the core or at the surface of nanoparticles. Presented here are density functional tight-binding simulations examining the configuration, potential energy surface, and electronic charge of substitutional boron in various types of nanocrystalline diamond. The results predict that boron is likely to be positioned at the surface of isolated particles and at the grain boundary of thin-film samples.     

Gamma-convergence and the emergence of vortices for Ginzburg–Landau on thin shells and manifolds

We analyze the Ginzburg–Landau energy in the presence of an applied magnetic field when the superconducting sample occupies a thin neighborhood of a bounded, closed manifold in ${\mathbb R^3}$ . We establish Γ-convergence to a reduced Ginzburg–Landau model posed on the manifold in which the magnetic potential is replaced in the limit by the tangential component of the applied magnetic potential. We then study the limiting problem, constructing two-vortex critical points when the manifold ${\mathcal{M}}$ is a simply connected surface of revolution and the applied field is constant and vertical. Finally, we calculate that the exact asymptotic value of the first critical field H _c1 is simply (4π/(area of ${\mathcal{M}}$ )) ln κ for large values of the Ginzburg–Landau parameter κ. Merging this with the Γ-convergence result, we also obtain the same asymptotic value for H _c1 in 3d valid for large κ and sufficiently thin shells.     

Reverse dependences of the permittivity of xPZN − (1 − x)PSN ferroelectric ceramics on the bias electric field

The effect of bias electric fields E ₌ on the dielectric response of xPZN ? (1 ? x)PSN ferroelectric solid solutions with PZN contents x = 0, 0.05, 0.20, and 0.50 is investigated in the temperature ranges corresponding to the smeared phase transitions of the studied compounds. It is concluded that an increase in the PZN content in this system leads to a narrowing of the temperature range of existence of the relaxor phase.     

Elliptic problems on networks with constrictions

We investigate the asymptotic behavior of minimizers to sequences of elliptic variational problems posed on thin three-dimensional domains. These domains arise as thin neighborhoods of artibrary graphs that contain severe constrictions near the graph nodes. We characterize an appropriate limit of minimizers as a function of one variable defined on the graph that necessarily minimizes a one-dimensional variational problem. The most salient feature of these limits of minimizers is the emergence of jump discontinuities across the graph nodes. While the approach can handle quite general elliptic problems, we pay particular attention to an application to generalized Josephson junctions within the Ginzburg-Landau theory of superconductivity. Mathematics Subject Classification (2000) 35Q60, 78M30, 78M35