Enzymatic determination of sorbic acid

 An enzymatic method for the determination of sorbic acid based on the spectrophotometric measurement of sorbyl coenzyme A (sorbyl CoA) at 300 nm was developed. Sorbic acid is converted to sorbyl CoA with acyl CoA synthetase (EC 6.2.1.3) in the presence of coenzyme A and adenosine-5′-triphosphate. The reaction is made quantitative by irreversibly hydrolyzing pyrophosphate formed during the acylation reaction to phosphate in the presence of inorganic pyrophosphatase (EC 3.6.1.1). The absorbance measured at 300 nm is specific for sorbyl CoA and its value is proportional to the amount of sorbic acid in the sample solution. Accuracy, repeatability and recovery rates after standard addition to sample solutions were evaluated. All steps of the enzymatic analysis were carried out on an automatic centrifugal analyzer. The method is fast, precise and reliable, and therefore well suited for routine determinations, especially for high sample throughputs. Received: 7 June 1999 / Revised version: 12 August 1999     

Growth and stability of Ga2O3 nanospheres

Gallium oxide thin films were prepared by thermal evaporation and deposition of Ga₂O₃ on NaCl(001) cleavage planes at varying substrate temperatures, oxygen pressures and deposition rates. The structure of the so-prepared thin films was checked by Transmission Electron Microscopy and Selected Area Diffraction and also characterized by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy, both in the as-deposited state and after different oxidative and reductive treatments. The substrate temperature proved to be most crucial for the structure of the gallium oxide films, ranging from low-contrast amorphous structures at low substrate temperatures (298 K) to nanosphere structures at higher temperatures (580 K). The stability of the films was found to be mainly determined by the interaction of substrate temperature and deposition rate. Crystalline β-Ga₂O₃ structures were obtained after oxidative, reductive and annealing treatments at and beyond 773 K suggesting that the crystallization is mainly a thermal annealing effect.     

Pd–Al interaction at elevated temperatures: a TEM and SAED study

Epitaxially grown Pd particles partly embedded in amorphous Al₂O₃ were subjected to annealing and reductive treatments in the temperature range 523–873 K to induce a possible Pd–Al interaction. The structural, morphological and compositional changes were monitored by transmission electron microscopy and selected area electron diffraction. Formation of Pd₄Al₃ and PdAl alloys has been observed upon annealing in 1 bar He for 1 h at T > 523 K and upon reduction in 1 bar H₂ for 1 h at T ≥ 523 K, respectively. Both alloys appear to be stable up to 873 K, although Pd₄Al₃ shows beginning decomposition at and above 873 K. The stability under oxidative conditions was found to be very similar, a transformation back into metallic Pd sets in for both compounds at around 573–623 K. In agreement with previous studies on Pd/SiO₂, the formation of an amorphous hydride phase and/or a heavily distorted Pd lattice has been detected after reduction in hydrogen at 523 K.     

The structure and composition of oxidized and reduced tungsten oxide thin films

The structure, morphology and composition of pure WO₃ thin films deposited onto vacuum-cleaved NaCl(001) single crystals have been studied at different substrate temperatures up to 580 K and under different oxidative and reductive treatments in the temperature range 373-873 K by Transmission Electron Microscopy, Selected-Area Electron Diffraction and X-ray Photoelectron Spectroscopy (XPS). A transition from an amorphous structure obtained after deposition at 298 K to a more porous structure with small crystallites at the highest substrate temperatures has been observed. XPS spectra reveal the presence of W⁶⁺ irrespective of the preparation procedure. Significant changes in the film structure were only observed after an oxidative treatment in 1 bar O₂ at 673 K, which induces crystallization of a monoclinic WO₃ structure. After raising the oxidation temperature to 773 K, the film shows additional reconstruction and a hexagonal WO₃ structure becomes predominant. This hexagonal structure persists at least up to 873 K oxidation temperature. However, these structural transformations observed upon oxidation were almost completely suppressed by mixing the WO₃ thin film with a second oxide, e.g. Ga₂O₃. Reduction of the WO₃ films in 1 bar H₂ at 723-773 K eventually induced the formation of the β-W metal structure, as evidenced by electron diffraction and XPS.     

Enzymatic determination of inulin in food and dietary supplements

 An enzymatic method for the determination of the oligofructoside inulin based on the enzymatic hydrolysis of inulin by inulinase was developed. Fructose formed in this reaction was phosphorylated by adenosine-5′-triphosphate and hexokinase, and isomerized to glucose-6-phosphate by phosphoglucose isomerase. Glucose-6-phosphate was finally oxidized to gluconate-6-phosphate by glucose-6-phosphate dehydrogenase. Reduced nicotinamide adenine dinucleotide phosphate formed in this sequence of reactions is determined spectrophotometrically by its absorbance at 340 nm. The absorbance is directly proportional to the initial amount of inulin. Precision, accuracy, repeatability and recovery rates of inulin following standard addition to sample solutions were evaluated. All steps of the enzymatic analysis were carried out on an automatic centrifugal analyzer. This, together with rapid and simple sample preparation, makes the method suitable for the routine determination of inulin in food surveilance and for quality control purposes. Received: 14 December 1998 / Revised version: 15 February 1999     

Surface reactions on inverse model catalysts: CO adsorption and CO hydrogenation on vanadia- and ceria-modified surfaces of rhodium and palladium

Reducible transition metal oxides are well-known promoters of the hydrogenation of CO on noble metal surfaces. In this study the promotional effect of vanadia and ceria adlayers on Rh and Pd surfaces was investigated with emphasis on the effect of the oxidation state on CO adsorption and catalytic activity. Inverse supported catalysts were prepared by UHV deposition of V and Ce on the noble metal surface (Rh(111), Pd(111) or Rh foil). After oxidation and specified reduction, the reaction kinetics on polycrystalline Rh was measured at atmospheric pressure, and the molecular and dissociative chemisorption of CO on Rh(111) and Pd(111) and the methanation kinetics on Rh(111) were investigated by molecular beam techniques. On Rh(111), the probability of CO dissociation and the reaction rate are enhanced by submonolayer VO _x deposits. Local pressures between 10^-2 and 1 mbar are sufficient to drive the methanation at 573 K with measurable amounts of products, accompanied by significant restructuring of the catalyst surface. Although the reaction on Rh is generally promoted by small quantities of vanadia and ceria, the reaction rates depend strongly on the extent and temperature of hydrogen reduction. The observed increase of the reaction rate by reduction up to 673 K can be correlated to concomitant changes of the structure and composition of the VO _x deposits. If the reduction temperature is raised above 673 K, metallic V is partially dissolved in the bulk, and the resulting V/Rh subsurface alloy exhibits a particularly high activity. Contrary to vanadia, ceria islands on Rh promote the initial reaction only after a low-temperature reduction, but the activity decreases after reduction above 573 K.