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C Vidal-Madjar

Publications and source records attributed to C Vidal-Madjar.

At least 19 recordsLinked to original sources
PubMed2005

Complex organic matter in Titan's atmospheric aerosols from in situ pyrolysis and analysis.

Aerosols in Titan's atmosphere play an important role in determining its thermal structure. They also serve as sinks for organic vapours and can act as condensation nuclei for the formation of clouds, where the condensation efficiency will depend on the chemical composition of the aerosols. So far, however, no direct information has been available on the chemical composition of these particles. Here we report an in situ chemical analysis of Titan's aerosols by pyrolysis at 600 degrees C. Ammonia (NH3) and hydrogen cyanide (HCN) have been identified as the main pyrolysis products. This clearly shows that the aerosol particles include a solid organic refractory core. NH3 and HCN are gaseous chemical fingerprints of the complex organics that constitute this core, and their presence demonstrates that carbon and nitrogen are in the aerosols.

Aerosols↗
PubMed2003

Solvent extraction of organic molecules of exobiological interest for in situ analysis of the Martian soil.

A solid-liquid extraction method able to perform in situ extraction of organic compounds on Mars is proposed. The extraction efficiency of various organic solvents was tested and compared to that of water. The selected key compounds are molecules of exobiological interest: glycine, alanine, serine, glutamic acid, oxalic acid, benzoic acid, phthalic acid, isophthalic acid, terephthalic acid and 1,3,5-benzenetricarboxylic acid. Among the organic solvents, propanol gives the highest yield of extraction for all the targeted compounds except for benzoic acid. A mixture of propanol and ethyl acetate increases significantly the extraction yield of benzoic acid. The extraction time was considerably reduced (140 h to 15 min) by using sonication. The method is discussed for an easy automation with coupling to an in situ GC-MS space instrument.

Exobiology↗
PubMed2002

Gas chromatography for in situ analysis of a cometary nucleus. IV. Study of capillary column robustness for space application.

As part of the development of the European Space Agency Rosetta space mission to investigate a cometary nucleus, the selection of columns dedicated to the gas chromatographic subsystem of the Cometary Sampling and Composition (COSAC) experiment was achieved. Once the space probe launched, these columns will be exposed to the harsh environmental constraints of space missions: vibrations, radiation (by photons or energetic particles), space vacuum, and large temperature range. In order to test the resistance of the flight columns and their stationary phases, the columns were exposed to these rough conditions reproduced in the laboratory. The comparison of the analytical performances of the columns, evaluated prior and after the environmental tests, demonstrated that all the columns withstand space constraints, and that their analytical properties were preserved. Therefore, all the selected capillary columns, even having porous layer or chiral stationary phases, were qualified for space exploration.

Chromatography, Gas↗
PubMed2002

Gas chromatography for in situ analysis of a cometary nucleus III. Multi-capillary column system for the cometary sampling and composition experiment of the Rosetta lander probe.

The cometary sampling and composition (COSAC) experiment is one of the principal experiments of the surface lander probe of the European Space Agency Rosetta mission to be launched in January 2003. The instrument is designed for the in situ chemical analysis of a cometary nucleus as the details of the nucleus composition are of primary importance for understanding both the formation of the solar system, and the origin of life on Earth. The COSAC experiment consists of an evaporation/pyrolysis device and two analytical systems: a multi-column gas chromatograph and a high-resolution time-of-flight mass spectrometer which may either be operated alone or in a coupled mode. The gas chromatograph includes five general purpose chromatographic columns and three chiral ones, all mounted in parallel. Taking into account the chemical species potentially present in the cometary nucleus as well as the space constraints, a set of five complementary columns was selected to perform the separation and identification of the compounds present in the cometary nucleus. This set of columns includes a carbon molecular sieve porous-layer open tubular (PLOT) column used for the separation of both the noble and other permanent gases, and the C1-C2 hydrocarbons. A second PLOT column uses a divinylbenzene-ethylene glycol-dimethylacrylate porous polymer as stationary phase for the analysis of a wide range of C1-C2 organic molecules, Two complementary wall-coated open tubular (WCOT) columns with polydimethylsiloxane (PDMS) liquid stationary phases, one containing cyanopropyl-phenylsiloxane and the other diphenylsiloxane groups, are designed to target the same range of organic compounds (C3-C7) which could be representative of the widest range of cometary compounds. A third WCOT column with an apolar stationary phase made of non-substituted PDMS is used for the separation and identification of higher-molecular-mass compounds (up to C10) and aromatic species (monoaromatic and polyaromatic). This paper describes these five general-purpose capillary PLOT and WCOT columns, selected to be used in the COSAC GC system. The analytical capabilities are examined with a special emphasis on the exobiological and planetological implications.

Chromatography, Gas↗
PubMed2001

Analysis of complex mixtures recovered from space missions statistical approach to the study of Titan atmosphere analogues (tholins).

To study Titan, the largest moon of Saturn, laboratory simulation experiments have been performed to obtain analogues of Titan's aerosols (named tholins) using different energy sources. Tholins, which have been demonstrated to represent aerosols in Titan's haze layers, are a complex mixture, resulting from the chemical evolution of several hydrocarbons and nitriles. Their chromatographic analysis yields complex chromatograms, which require the use of mathematical procedures to extract from them all the information they contain. Two different chemometric approaches (the Fourier analysis approach and the statistical model of peak overlapping) have been successfully applied to pyrolysis-GC-MS chromatogram of a tholin sample. Fundamental information on the mixture's chemical composition (number of components, m) and on the separation system performance (separation efficiency, sigma) can be easily estimated: the excellent correspondence between the data calculated by the two independent procedures proves the reliability of the statistical approaches in characterizing a tholin chromatogram. Moreover, the plot of autocorrelation function contains, in a simplified form, all the information on the retention pattern: retention recursivities can be easily singled out and related to specific molecular structure variations. Therefore, the autocorrelation function (ACF) plot constitutes a simplified fingerprint of the pyrolysis products of tholins, which can be used as a powerful tool to characterize a tholin sample.

Aerosols↗
PubMed2001

Chemical derivatization of amino acids for in situ analysis of Martian samples by gas chromatography.

Three different methods of derivatization are tested in order to select and optimize one for the in situ analysis of amino acids in Martian samples. The silylation procedure can easily be automated with a high yield and a linear response in a large range of concentrations. The alkylation method is simple and easily automated, but irreproducible data are obtained for the reaction in the GC liner at quite a high temperature (300 degrees C). Moreover by-products of the reaction interfere in the GC chromatograms and mass spectrometry detection is needed for product identification. The chloroformate derivatization has several advantages such as one-step reaction and short time analysis. The main problem of this procedure is the shaking step which difficult to develop in space application.

Acylation↗
PubMed2001

Study of the retention properties of warfarin enantiomers on a beta-cyclodextrin polymeric support.

High-performance liquid chromatography was used to study the retention properties of (R)- and (S)-warfarins on a silica support coated with a beta-cyclodextrin polymer. The influence of the methanol content of the acetate buffer eluent was investigated at pH 4. The measure of the variations of retention time with temperature enables one to determine the enthalpy and the entropy of adsorption. The plot of the two thermodynamic functions shows a minimum around 30% (v/v) methanol. At low methanol contents, the decrease of the hydrophobic interactions with increasing methanol content explains the decrease of the enthalpic and entropic terms. Above 40% (v/v) methanol, the decrease of the adsorption enthalpy absolute value is due to the solvation by the organic component. From the analysis of peak shape in mass-overload conditions, the column capacity toward each enantiomer was determined. A lower capacity was found toward (S)-warfarin, the more retained enantiomer. Peak shape analysis in mass-overload conditions was used to determine the adsorption isotherm. A Langmuir-type adsorption isotherm accounts well for the experimental data.

Adsorption↗
PubMed2001

Detection of martian amino acids by chemical derivatization coupled to gas chromatography: in situ and laboratory analysis.

If there is, or ever was, life in our solar system beyond the Earth, Mars is the most likely place to search for. Future space missions will have then to take into account the detection of prebiotic molecules or molecules of biological significance such as amino acids. Techniques of analysis used for returned samples have to be very sensitive and avoid any chemical or biological contamination whereas in situ techniques have to be automated, fast and low energy consuming. Several possible methods could be used for in situ amino acid analyses on Mars, but gas chromatography would likely be the most suitable. Returned samples could be analyzed by any method in routine laboratory use such as gas chromatography, already successfully performed for analyses of organic matter including amino acids from martian meteorites. The derivatization step, which volatilizes amino acids to perform both in situ and laboratory analysis by gas chromatography, is discussed here.

Amino Acids↗
PubMed2000

Gas chromatography for in situ analysis of a cometary nucleus. II. Analysis of permanent gases and light hydrocarbons with a carbon molecular sieve porous layer open tubular column.

Considering the severe constraints of space instrumentation, a great improvement for the in situ gas chromatographic (GC) determination of permanent and noble gases in a cometary nucleus is the use of a new carbon molecular sieve porous layer open tubular (PLOT) column called Carbobond. No exhaustive data dealing with this column being available, studies were carried out to entirely characterize its analytical performances, especially when used under the operating conditions of the cometary sampling and composition (COSAC) experiment of the European Space Agency (ESA) Rosetta space mission to be launched in 2003 for a rendezvous with comet 46 P/Wirtanen in 2011. The high efficiency and speed of analysis of this column at both atmospheric and vacuum outlet column pressure is demonstrated, and the kinetic mass transfer contribution of this carbon molecular sieve adsorbent is calculated. Besides, differential adsorption enthalpies of several gases and light hydrocarbons were determined from the variation of retention volume with temperature. The data indicate close adsorption behaviors on the Carbobond porous layer adsorbent and on the carbon molecular sieve Carboxen support used to prepare the packed columns. Moreover, taking into account the in situ operating conditions of the experiment, a study of two columns with different porous layer thicknesses allowed one to optimize the separation of the target components and to select the column parameters compatible with the instrument constraints. Comparison with columns of similar selectivity shows that these capillary columns are the first ones able to perform the same work as the packed and micro-packed columns dedicated to the separation of this range of compounds in GC space exploration.

Adsorption↗
PubMed1998

Characterization of Human Serum Albumin Adsorbed on a Porous Anion-Exchange Support.

Fourier transform infrared (FTIR) spectroscopy and high-performance liquid chromatography (HPLC) were used to study the adsorption behavior of human serum albumin (HSA) onto a polymer-layer type anion exchanger, poly(vinylimidazole) (PVI) adsorbed and cross-linked on a porous silica support. The data are compared with previous results for HSA adsorbed on C6 alkyl chains grafted on the same silica matrix. For the adsorption onto the PVI support from a phosphate buffer solution of low ionic strength (pD 7.4), the FTIR experiments reveal only very weak structural and solvation changes. A large fraction of the protein remains irreversibly adsorbed and the amount retained at equilibrium is close to that observed for the adsorption on the reversed-phase support, although the structural effect of the stationary phase was much larger with the grafted C6 alkyl chains. Comparing to the solution state, only 2% of the HSA backbone is modified by adsorption on PVI, whereas 12% alterations are involved for the protein adsorbed on the reversed-phase support. When adsorbed from an eluent containing 20% acetonitrile, the amount of HSA retained by the PVI ion exchanger is about twice that measured with the buffer alone. This result is explained by a more compact structure of the protein when dissolved in the organoaqueous solvent. The presence of acetonitrile does not markedly affect the elution front of HSA adsorbed on the charged hydrophilic adsorbent. This result contrasts with the much lower apparent adsorption rate observed when HSA is adsorbed on the grafted alkyl chain support in the presence of acetonitrile in the buffer. Copyright 1998 Academic Press.

Journal Article↗
PubMed1998

Capillary electrophoretic behavior of milk proteins in the presence of non-ionic surfactants.

The electrophoretic behavior of alpha-lactalbumin and beta-lactoglobulins (A and B) in the presence of non-ionic surfactants was studied by capillary electrophoresis (CE), using a poly(ethylene glycol) coated capillary column. The surfactants (Tween 20, Brij 35 and 78) were used as buffer additives. The separation is based on the difference in the strength of protein-surfactant association complexes, which results in a change of the effective electrophoretic mobility. The modification of the electrophoretic mobilities of proteins was observed and this variation permitted the estimation of the interaction between protein and surfactant. The effect of surfactant type and concentration on the migration behavior of protein in CE is discussed. It is found that the retention behavior of the milk proteins (the alpha-lactalbumin and the beta-lactoglobulins) in CE is very different. The pH of the buffer and the surfactant type influence significantly the protein-surfactant interactions.

Animals↗
PubMed1995

Study of conformational effects of recombinant interferon gamma adsorbed on a non-porous reversed-phase silica support.

Reversed-phase chromatography is a powerful method for separating recombinant interferon gamma and one of its analogues differing only by a single amino acid residue. Structural differences of the proteins explain this separation ability as demonstrated from adsorption studies on a non-porous reversed-phase support. To reveal the structural differences occurring in the adsorbed state, two different and independent methods were employed. The variation of the retention with the slope of the linear gradient gave information about the molecular contact area of the protein with the support. For different experimental conditions, these data were correlated with the adsorbent capacities measured on an n-octadecyl-modified non-porous silica support. These supports are useful for these types of experiments because the protein is adsorbed exclusively at the external surface of the beads. Moreover, a small amount of protein is necessary to saturate the column, owing to its low capacity.

Adsorption↗
PubMed1995

Molecular approach to protein-polymer interactions in ion-exchange chromatography.

A model was developed and implemented to aid in understanding and predicting the retention behaviour of proteins in ion-exchange chromatography. The model structures chosen were calcium-loaded and -depleted alpha-lactalbumin (ALC) and hen egg white lysozyme (HEWL) and a comparison was made with chromatographic measurements. A characteristic charge of -3.4 was found under the experimental conditions applied for both forms of ALC, and HEWL was not retained. The model explicitly considers all of the atoms, each being assigned a set of force field parameters. Because of the computational time necessary to include them, water molecules were not taken into account, but a sigmoidal function of the dielectric permittivity was introduced in the calculations. Interaction potential energies from bulk down to the contact were evaluated for each protein. The results were in qualitative agreement with those of the chromatographic experiments. It was possible to reproduce the difference in retention between both forms of ALC and also the behaviour of HEWL.

Animals↗
PubMed1994

Kinetic study of the adsorption of human serum albumin on immobilized antibody using the split-peak effect in immunochromatography.

The split-peak effect was used to determine the association rate constant of the antigen-immobilized antibody reaction. The amount of immobilized human serum albumin antibody on the chromatographic support was varied in order to find the optimal conditions to reduce the mass transfer contribution in the stagnant mobile phase fluid and measure the effective association rate constant of human serum albumin with the immobilized antibody. Kinetic studies as a function of flow-rate demonstrate the validity of the method consisting in determining the association rate constant from measurements performed on columns of various capacities. These experiments show that limitations due to mass transfer to the surface of the adsorbent are minimized at high flow-rates and for a low density of immobilized ligand.

Adsorption↗
PubMed1992

Study of affinity supports based on reactive polymers immobilized on silica: affinity constant determination from isocratic zonal elution.

Polymers bearing benzamidine moieties have been prepared from reactive copolymer containing chloroformate functions and deposited on porous silica matrices. These high-performance affinity chromatography supports were characterized by quantitative methods, which analyse the zonal elution behaviour of trypsin in the presence of a soluble competitor (L-arginine). The column loading capacity for trypsin was measured by the zonal elution method in mass overload conditions. On the basis of a Langmuir isotherm, the influence of the protein capacity and the concentration of the soluble ligand on the elution volume was studied for the determination of the binding constants. The plate heights determined for silica supports of various porosities and particle diameters show that the strong affinity interactions between trypsin and p-aminobenzamidine are mainly responsible for the low efficiencies observed.

Amino Acid Sequence↗
PubMed1992

Study of protein-ligand binding effects by direct chromatographic on-column injection.

The direct zonal on-column injection method was applied to a high-performance liquid chromatographic study of pollutant-protein binding interactions in solution. The protein and the protein-ligand complex are excluded on the basis of the size from the diol support, and the free ligand penetrates into the pores and is more retained. The pattern of the ligand elution profile depends on the protein-ligand dissociation constant. This effect was quantitatively analysed by developing a numerical simulation algorithm in which the column is divided into slices of given thickness. The column length, flow-rate and shape of the injection signal are given as input parameters. A global dispersion coefficient accounts for peak broadening. A rapid equilibrium is assumed with the hypothesis that a monovalent ligand interacts with a single binding site on the protein. The interaction of bovine serum albumin with pentachlorophenol was studied, and an apparent dissociation constant for the protein-ligand complex was determined by fitting the theoretical profile to the experimental one. The effect of the acetonitrile content in the solvent was studied. An important decrease of the dissociation constant is observed that affects the chromatographic elution pattern.

Acetonitriles↗
PubMed1991

Anion-exchange chromatographic properties of alpha-lactalbumin eluted from quaternized polyvinylimidazole. Study of the role of the polymer coating.

The anion-exchange elution behaviour of alpha-lactalbumin was studied on cross-linked and quaternized polyvinylimidazole, deposited on various high-performance liquid chromatographic supports (porous silica and diol silica). The influence of the nature and thickness of the coating layer on the retention and band-width properties of the protein elution peak was examined by isocratic elution. The retention properties of alpha-lactalbumin were studied from the plot of log k' vs. log([NaCl]), where k' is the capacity factor and [NaCl] the displacer salt concentration in the aqueous phase. The retention depends on the amount of stationary phase deposited on the support, but an increased hydrophobic effect is found when the polymer films do not coat the chromatographic support uniformly. Band broadening of the elution peaks was studied in terms of plots of plate height vs. mobile phase velocity. An important mass-transfer contribution is found, which decreases with increasing k' and increases with the thickness of the coating layer. These effects reveal that the diffusion into the polymer layer is the controlling step of the ion-exchange process with non-uniform polymer layers of large mean thickness.

Anions↗