Percolative conductivity in alkaline earth silicate melts and glasses

Ion conducting $(CaO)_x(SiO_2)_{1-x}$ glasses and melts show a threshold behaviour in dc conductivity near $x=x_t=0.50$, with conductivities increasing linearly at $x>x_t$. We show that the behaviour can be traced to a rigid ($x0.50$) elastic phase transition near $x=x_t$. In the floppy phase, conductivity enhancement is traced to increased mobility or diffusion of $Ca^{2+}$ carriers as the modified network elastically softens.Comment: 15 pages, 5 figures. Europhysics Letters (2003), in pres

Oligomer formation during gas-phase ozonolysis of small alkenes and enol ethers: new evidence for the central role of the Criegee Intermediate as oligomer chain unit

An important fraction of secondary organic aerosol (SOA) formed by atmospheric oxidation of diverse volatile organic compounds (VOC) has recently been shown to consist of high-molecular weight oligomeric species. In our previous study (Sadezky et al., 2006), we reported the identification and characterization of oligomers as main constituents of SOA from gas-phase ozonolysis of small enol ethers. These oligomers contained repeated chain units of the same chemical composition as the main Criegee Intermediates (CI) formed during the ozonolysis reaction, which were CH<sub>2</sub>O<sub>2</sub> (mass 46) for alkyl vinyl ethers (AVE) and C<sub>2</sub>H<sub>4</sub>O<sub>2</sub> (mass 60) for ethyl propenyl ether (EPE). In the present work, we extend our previous study to another enol ether (ethyl butenyl ether EBE) and a variety of structurally related small alkenes (<i>trans</i>-3-hexene, <i>trans</i>-4-octene and 2,3-dimethyl-2-butene). <br><br> Experiments have been carried out in a 570 l spherical glass reactor at atmospheric conditions in the absence of seed aerosol. SOA formation was measured by a scanning mobility particle sizer (SMPS). SOA filter samples were collected and chemically characterized off-line by ESI(+)/TOF MS and ESI(+)/TOF MS/MS, and elemental compositions were determined by ESI(+)/FTICR MS and ESI(+)/FTICR MS/MS. The results for all investigated unsaturated compounds are in excellent agreement with the observations of our previous study. Analysis of the collected SOA filter samples reveal the presence of oligomeric compounds in the mass range 200 to 800 u as major constituents. The repeated chain units of these oligomers are shown to systematically have the same chemical composition as the respective main Criegee Intermediate (CI) formed during ozonolysis of the unsaturated compounds, which is C<sub>3</sub>H<sub>6</sub>O<sub>2</sub> (mass 74) for ethyl butenyl ether (EBE), <i>trans</i>-3-hexene, and 2,3-dimethyl-2-butene, and C<sub>4</sub>H<sub>8</sub>O<sub>2</sub> (mass 88) for extit{trans}-4-octene. Analogous fragmentation pathways among the oligomers formed by gas-phase ozonolysis of the different alkenes and enol ethers in our present and previous study, characterized by successive losses of the respective CI-like chain unit as a neutral fragment, indicate a similar principal structure. In this work, we confirm the basic structure of a linear oligoperoxide – [CH(R)-O-O]<sub>n</sub> – for all detected oligomers, with the repeated chain unit CH(R)OO corresponding to the respective major CI. The elemental compositions of parent ions, fragment ions and fragmented neutrals determined by accurate mass measurements with the FTICR technique allow us to assign a complete structure to the oligomer molecules. We suggest that the formation of the oligoperoxidic chain units occurs through a new gas-phase reaction mechanism observed for the first time in our present work, which involves the addition of stabilized CI to organic peroxy radicals. Furthermore, copolymerization of CI simultaneously formed in the gas phase from two different unsaturated compounds is shown to occur during the ozonolysis of a mixture of extit{trans}-3-hexene and ethyl vinyl ether (EVE), leading to formation of oligomers with mixed chain units C<sub>3</sub>H<sub>6</sub>O<sub>2</sub> (mass 74) and CH<sub>2</sub>O<sub>2</sub> (mass 46). We therefore suggest oligoperoxide formation by repeated peroxy radical-stabilized CI addition to be a general reaction pathway of small stabilized CI in the gas phase, which represents an alternative way to high-molecular products and thus contributes to SOA formation

Development of an LC-MS-MS method for the quantification of taurine derivatives in marine invertebrates.

Sulfur amino acids, such as taurine, hypotaurine, and thiotaurine, were found in high quantities in tissues of marine symbiotic organisms (e.g., bivalves, tubeworms) living close to hydrothermal vent sites. Therefore, they are assumed to play a key role in the S-oxidizing base metabolism or sulfide detoxification. We propose here a specific, rapid, and original analytical procedure for the direct determination of sulfur amino acids at the level of a few parts per billion in biological samples, avoiding the classical low specific post-column ortho-phthaldialdehyde derivatization step required by non-ultraviolet-absorbing molecules. Indeed, by coupling liquid chromatography on a porous graphitic stationary phase under isocratic conditions (10 mM ammonium acetate buffer adjusted to pH 9.3) to tandem mass spectrometry (ionization process by pneumatically assisted electrospray in negative ion mode), it is possible to perform specific quantification of these metabolites in less than 10 min directly in biological matrices without any derivatization step or other tedious sample treatments. Thus, taurine, hypotaurine, and thiotaurine have been identified and assayed in several deep sea organisms, showing that the developed method is well suited for this kind of application

Parameter optimization for the analysis of underivatized protein amino acids by liquid chromatography and ionspray tandem mass spectrometry

International audienc

Formation of secondary organic aerosol and oligomers from the ozonolysis of enol ethers

International audienceFormation of secondary organic aerosol has been observed in the gas phase ozonolysis of a series of enol ethers, among them several alkyl vinyl ethers (AVE, ROCH=CH2), such as ethyl, propyl, n-butyl, iso-butyl, t-butyl vinyl ether, and ethyl propenyl ether (EPE, C2H5OCH=CHCH3). The ozonolysis has been studied in a 570 l spherical glass reactor at atmospheric pressure (730 Torr) and temperature (296 K). Gas phase reaction products were investigated by in-situ FTIR spectroscopy, and secondary organic aerosol (SOA) formation was monitored by a scanning mobility particle sizer (SMPS). The chemical composition of the formed SOA was analysed by a hybrid mass spectrometer using electrospray ionization (ESI). The main stable gas phase reaction product is the respective alkyl formate ROC(O)H, formed with yields of 60 to 80%, implying that similar yields of the corresponding Criegee Intermediates (CI) CH2O2 for the AVE and CH3CHO2 for EPE are generated. Measured SOA yields are between 2 to 4% for all enol ethers. Furthermore, SOA formation is strongly reduced or suppressed by the presence of an excess of formic acid, which acts as an efficient CI scavenger. Chemical analysis of the formed SOA by ESI(+)/MS-TOF allows to identify oligomeric compounds in the mass range 200 to 800 u as its major constituents. Repetitive chain units are identified as CH2O2 (mass 46) for the AVE and C2H4O2 (mass 60) for EPE and thus have the same chemical compositions as the respective major Criegee Intermediates formed during ozonolysis of these ethers. The oligomeric structure and chain unit identity are confirmed by HPLC/ESI(+)/MS-TOF and ESI(+)/MS/MS-TOF experiments, whereby successive and systematic loss of a fragment with mass 46 for the AVE (and mass 60 for EPE) is observed. It is proposed that the oligomer has the following basic structure of an oligoperoxide, -[CH(R)-O-O]n-, where R=H for the AVE and R=CH3 for the EPE. Oligoperoxide formation is thus suggested to be another, so far unknown reaction of stabilized Criegee Intermediates in the gas phase ozonolysis of oxygen-containing alkenes leading to SOA formation