Kinetic studies of laser-induced synthesis of thin vanadium pentoxide films

The kinetics of Ar⁺ laser-induced oxidation of 100 nm thick vanadium films on glass substrates is followed by measuring time-dependent changes in reflected and transmitted intensity of a He-Ne probe beam. The growth rate of the vanadium pentoxide layer increases with increasing laser powerP as = ₀ exp(-a/P). At power densities above kW/cm² vanadium pentoxide crystallizes from the melt.     

Description of the shape of thermoanalytical curves

Empirical parameters were chosen for the characterization of peaks (or steps) of thermoanalytical curves. The parameters were applied in studies on the repeatability, the relationship between kinetic constants and peak shape, the effect of sample thermal resistance. Kinetic constants can be estimated on the basis of peak shape parameters. Besides, approximate criteria were formulated for experiments allowing kinetic evaluation with the neglect of the heat transport within the sample. The empirical parameters were also used in checking the suitability of DSC data for purity analysis and in detecting changes of the thermal decomposition of papers caused by ageing.Zur Beschreibung von Peaks bzw. Stufen thermoanalytischer Kurven wurden emperische Parameter ermittelt, die bei der Untersuchung der Reproduziertbarkeit, der Beziehung zwischen kinetischen Konstanten und Peakform so wie des Einflusses des thermischen Wiederstandes der Probe angewendet wurden. Kinetische Konstanten können auf der Basis von Peakformparametern geschÄtzt werden. Weiterhin wurden Kriterien für Experimente ermittelt, die eine kinetische NÄherungslösung unter VernachlÄssigung des WÄrmetransportes innerhalb der Probe ermöglichen. Die empirischen Parameter wurden au\erdem bei der Untersuchung der Eignung von DSC-daten zur Reinheitsanalyse angewendet, weiterhin zum Nachweis von alterungsbedingten Änderungen der thermischen Zerzetzung von Papier.

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The authors express their thanks to Mrs E. Tóth for her valuable technical help. The permission to reproduce Figs 2–6 and 8–10 from Analytica Chimica Acta and Thermochimica Acta (Elsevier, Amsterdam) is also acknowledged.     

Preparation of 3-hydroxyl-vinyl selenides and 1,3-bis(seleeno)-propenes

Z and E 3-hydroxy-vinyl selenides can be prepared with very high stereoselectivity respectively by nucleophilic addition of selenolates to propargyl alcohols and by selenylation followed by LiAlH₄ reduction of the same propargyl alcohols. 1,3-Bis(seleno)-propenes are available either from the reaction of 3-hydroxy-vinyl selenides with selenols in the presence of zinc chloride or by Wittig condensation of -selenoaldehydes with selenophosphorus ylids. Both reactions lead to mixtures of E and Z products which undergo sigmatropic rearrangements under acidic conditions.     

Modelling the gas chromatographic separation of multicomponent samples on a system of three open-tubular columns coupled in series

A procedure was developed for modelling the gas chromatographic separation on a system of columns coupled in series. This procedure can be used for computer based optimization of lengths and order of serially coupled columns at isothermal conditions. The sample component retention factors and column resistances needed for the model can easily be measured on each individual column. The proposed procedure was verified by analyzing a 52 component hydrocarbon mixture on three columns of different polarity coupled in series by various column orders. Good agreement between experimental and calculated retention data was found. The procedure is also well suited for optimization of the chromatographic selectivity by coupling columns with different selectivity in series.Dedicated to Professor Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday     

New synthetic routes to vinyl sulfides,ketene thioacetals and their seleno analogues from carbonyl compounds

Title compounds have been prepared from carbonyl compounds by formal removal of hydroxyl and sulfenyl or hydroxyl and selenyl moieties from the corresponding functionalized β-hydroxysulfides or β-hydroxyselenides.     

Preparation of a polyacrylonitrile/multi-walled carbon nanotubes composite by surface-initiated atom transfer radical polymerization on a stainless steel wire for solid-phase microextraction

We report on the fabrication and performances of a solid-phase microextraction (SPME) fiber based on a stainless steel wire coated with a covalently attached polyacrylonitrile (PAN)/multi-walled carbon nanotubes (MWCNTs) composite. This new coating is obtained by atom transfer radical polymerization (ATRP) of acrylonitrile mixed with MWCNTs. ATRP is initiated from 11-(2-bromo-2-methylpropionyloxy)-undecyl-phosphonic acid molecules grafted on the wire surface via the phosphonic acid group. The extraction performances of the fibers are assessed on different classes of compounds (polar, non-polar, aromatic, etc.) from water solutions by headspace extraction. The optimization of the parameters affecting the extraction efficiency of the target compounds was studied as well as the reproducibility and the repeatability of the fiber. The fibers sustain more than 200 extractions during which they remain chemically stable and maintain good performances (detection limits lower than 2 μg/l, repeatability, etc.). Considering their robustness together with their easy and inexpensive fabrication, these fibers could constitute promising alternatives to existing products.