The first step of biodiesel autoxidation by differential scanning calorimetry and DFT calculations

The radical abstraction reaction of (bis)-allylic hydrogen in biodiesel fatty acid chains is the first step of autoxidation, being the rate-determining one. Some intrinsic features of the intermediary radicals can determine the oxidative stability (OS) under thermodynamic control. In the present study, some common fatty acid methyl esters present in biodiesel (stearate, oleate, ricinoleate, and linoleate) were oxidized in non-isothermal conditions, using differential scanning calorimetry, and the results were compared with quantum chemical calculations at the density functional theory level. The OS order (saturated > monounsaturated > polyunsaturated) was observed in both approaches (experimental and theoretical). A slight deviation observed between oleate and ricinoleate OS’s was explained based on their allylic radicals resonance and the influence of vicinal hydroxyl group. A linear relationship was found between oxidation temperature by pressurized differential scanning calorimetry and the calculated bond dissociation energy (C–H) for the first step of autoxidation.     

New Data on Sixteen Reference Materials Obtained by INAA

We report instrumental neutron activation analysis results for 15 elements (K, Fe, Sc, Cr, Co, Zn, As, Rb, Sb, Cs, Ba, Hf, Ta, Th, and U) in 16 geochemical reference samples, namely SDC-1, SCo-1, SGR-1, STM-1, RGM-1, BIR-1, MAG-1 and BHVO-1 from USGS (United States Geological Survey, Reston), Soil-5, Soil-7 and SL-1 from IAEA (International Atomic Energy Agency, Vienna), and GSS-1, GSS-4, GSS-7, GSR-2 and GSR-3 from IGGE Institute of Geophysical and Geochemical Prospecting, People's Republic of China), The results are compared with literature values. In general our results agree well with recommended or proposed values.     

Behaviour of methyl red under fast atom bombardment conditions

The influence of the pH, the nature of the matrix and the presence of a surfactant on the positive- and negative-ion abundances in the molecular mass region in the fast atom bombardment (FAB) mass spectra of methyl red was investigated. A small but significant pH effect was observed which was attributed to the non- or at the most low surface-active character of the intact methyl red molecule. As expected, the more basic the solution, the less protonated molecules with respect to M⁺˙ are observed and in the negative-ion mode less [M + H]^? and more [M – H]^? ions with respect to M^?˙ were found. In contrast to neutral solutions, both acidic and basic solutions give a long-lasting stable response of all methyl red ions. For dyes with a moderately negative redox potential such as methyl red, beam-induced redox reactions seem to play a role in the ionization process, the neutral medium offering the best conditions for reduction processes. The ion intensities in the molecular mass region depend on the nature of the matrix. Protonation of the molecule has been found to be more effective in glycerol than in 3-nitrobenzyl alcohol; the former also appears to offer the best conditions for reduction processes. Anionic and cationic surfactants effectively suppress the contribution of ions from glycerol in both positive- and negative-ion spectra and generally promote the formation of analyte ions at the surface. The most important effect of the surfactant in a neutral medium seems to be the promotion of a regular transport of ions and molecules to the surface, which permits the creation of stable ion currents, instead of an unstable ion beam if the surfactant is absent. Moreover, when the surfactant is present an increase of the sample ion abundances is observed. Redox reactions involving molecules and molecular ions and also molecules and preformed ions in the solution, brought to the surface by micelles, have been proposed to give some contribution for the small but significant enhanced abundance of [M + nH]⁺ (n > 1) ions with respect to [M + H]⁺ ions, in the presence of a surfactant. The results have been rationalized in terms of the surface phenomena while the important role of surfactants for obtaining better FAB mass spectra is emphasized.     

Characterization and kinetic compensation effect of corn biodiesel

This work presents the characterization and the kinetic compensation effect of corn biodiesel obtained by the methanol and ethanol routes. The biodiesel was characterized by physico-chemical analyses, gas chromatography, nuclear magnetic resonance and thermal analysis. The physico-chemical properties indicated that the biodiesel samples meet the specifications of the Brazilian National Agency of Petroleum, Natural Gas and Biofuels (ANP) standards. The analyses by IR and ¹H NMR spectroscopy indicated the ester formation. Gas chromatography indicated that biodiesel was obtained with an ester content above 97%. The kinetic parameters were determined with three different heating rates, and it was observed that both the methanol and ethanol biodiesel obeyed the kinetic compensation effect.     

Sur l'isotopie des formes fermées en dimension 3

Sans résumé     

Ion transport and structure in silver borate glasses

A study of ionic conductivity in silver borate glasses is presented, which includes (i) a full characterization of the ion transport properties in the AgI:Ag₂O:B₂O₃ glasses, (ii) results for glasses where iodine is totally or partially substituted by another halide, (iii) mixed-cation effects. A substantial portion of the paper discusses the relationship between local order and ion transport properties of borate glasses and vitreous electrolyte systems.     

Stable and metastable phases in Agl-containing silver borate glasses

This paper studies the thermal stability and the glass formation region the Agl-Ag₂O-B₂O₃ system. The miscibility range of the components has been investigated by direct visual observation at 1000°C, i.e. where the system is liquid over the entire compositional range. By combining X-ray and differential thermal analysis (DTA) measurements, the large region of glass formation of the system has been determined. The dependence of the glass transition temperature upon the glass composition is discussed. Series of glasses have been annealed over extended time intervals above the glass transition temperature in order to characterize the “equilibrium” phase diagram for the system. Analysis of the annealed materials shows that several crystalline compounds form upon annealing.     

Direct introduction of slurry samples in multi-syringe flow injection analysis: determination of potassium in plant samples.

The present work explores the slurry sampling approach for automatic, flow-based plant analysis. For this purpose, pinch valves were introduced into a multi-syringe flow injection analysis manifold to provide the repeatable aspiration of a few microliters of plant suspension before the material was further processed through the flow system. For validation of the proposed approach, the determination of potassium by flame emission spectrometry was implemented. Several parameters were studied: the concentration of plant particles in the sample suspension and the utilization of matrix modifiers. Microwave digestion was also implemented; no significant difference was found when certified reference material was analyzed with or without the in-line digestion step. The system was successfully applied to 13 samples within a concentration range of 2.5 to 100 mg g(-1). A determination frequency of 28 h(-1) was achieved and the precision was better than 4.0% (n = 12).     

The mass spectra of dimethoxytoluenes

The primary fragmentation reactions in the dimethoxytoluenes have been studied and compared with those in dimethoxybenzenes and monomethoxytoluenes. Labelled analogues and metastable ion characteristics were used to elucidate the reaction mechanisms. The formation of stable quinoid-type ions directs many fragmentation pathways. Analytically useful spectral characteristics are indicated.