The reduction of methylene blue by sulfide ion in the absence and presence of oxygen: Simulation of the methylene blue-O2?HS?-CSTR oscillations

A mechanism is discussed which reproduces in simulations the oscillations during the methylene-blue catalyzed reduction of O₂ by HS^– in a continuous-flow, strirred tank reactor (CSTR). It contains 14 reactions and is based on experiments and simulations of simpler reactions including the reduction of MB⁺ by HS^– in the absence and presence of O₂ and the reactions of H₂O₂ and O₂ with HS^–. All experiments on component reactions as well as the CSTR oscillations can be simulated by the same set of reactions and rate constants. The major dynamic feature of the mechanism is the competition for MB^. by the oxidizing agents O₂ and H₂O₂ and the reducing agents HS^– and HS^.. The species MB^. is the radical intermediate between the colored (MB⁺) and colorless (MBH) forms of methylene blue.     

111In(111Cd) quadrupole interaction in the chalcogenide spinel CdCr2Se4

The¹¹¹Cd quadrupole coupling in the spinel CdCr₂Se₄ is investigated by TDPAC after different sample treatments. The experiments are consistent with Se vacancies causing the non-cubic probe environments and with the assumption that the majority of the¹¹¹In atoms are positioned at A-sites.     

Flow sorption calorimetry, a powerful tool to investigate the acid-base character of organic polymer surfaces

Flow sorption calorimetry (FSC) can be a useful tool to estimate the Lewis acid-base behaviour of the surface of organic polymers with less than 1 m²/g surface area. Polyethylene powders grafted with different amounts of acrylic acid have been chosen as samples. The surface of the powders was investigated by means of X-ray photoelectron spectroscopy (XPS) at first and than brought into contact with the strong electron donor triethylamine in a special FSC measuring system. The heat of the displacement isotherm together with the surface excess isotherm of triethylamine adsorbed onto the powders was determined. The results obtained with FSC and XPS were well correlated and gave a comprehensive picture of the acidic surface character of modified polyethylene. Received: 19 September 1996 / Revised: 22 January 1997 / Accepted: 26 January 1997     

Alkyl-Diol Silica (ADS): restricted access precolumn packings for direct injection and coupled-column chromatography of biofluids

The direct and repetitive injection of untreated biological fluids (e.g., hemolyzed blood, plasma, serum, cell culture and tissue homogenates) onto an HPLC-system and the subsequent analysis of low-molecular weight compounds (e.g. drugs, xenobiotics, metabolites) is rendered possible by a coupled-column configuration and special precolumn packings. For this purpose a new family of chemically and enzymatically tailored reversed-phase packing materials have been prepared. The LC-integrated sample clean-up with these restricted access (bimodal) phases is based on the complete nonadsorptive size exclusion of macromolecules (e.g. proteins) and on the simultaneous dynamic partitioning of the target molecules. The bonded phase which exclusively covers the internal pore surface of a glyceryl-modified silica is a butyryl-(C-4), capryloyl-(C-8) or stearoyl-(C-18) moiety. These ligands allow a classical reversed-phase or ion-pair chromatography during the sample work-up step. The capacity of the n-alkyl phase is comparable with conventional silica based RP-materials. The broad hydrophobic retentive capability of these packings allows the extraction of a wide variety of compounds of biomedical interest. The electroneutral and hydrophilic particle exterior (glyceryl-residues) was generated using either soluble or immobilized enzymes (lipase, esterase) which cleave the fatty acid esters exclusively at the outer surface. Unwanted macromolecular components of a sample (e.g. proteins) are quantitatively eluted in the void volume due to the restricted access given by the pore size (6 nm) and the nonadsorptive external diol coverage. The lifetime of a precolumn (25 × 4 mm I.D.) packed with these novel bimodal, i.e. RP-SEC phases exceeds more than 200 injections of 500 l plasma. In addition to the synthesis, this paper describes an application of each of these Alkyl-Diol Silica (ADS) precolumn packings in fully automated coupled-column HPLC systems for the analysis of drugs and endogenous compounds in different biological matrices.Dedicated to Professor Dr. Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday     

Cyclic esters of aliphatic diacids with pyrocatechol and hydroquinone

Cyclic esters of adipic acid, suberic acid and sebacic acid were prepared by reaction of the acid dichlorides and pyrocatechol or hydroquinone in benzene under high dilution conditions. While only the cyclic dimers could be obtained from hydroquinone, pyrocatechol formed cyclic monomers as well as cyclic dimers (and also a cyclic dimer with succinic acid). The structure of all compounds was confirmed by¹H-NMR- and mass spectra. The crystal structures of the pyrocatechol esters were determined by single crystal X-ray analysis.

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Cyclische Ester aliphatischer Dicarbonsäuren mit Brenzcatechin und Hydrochinon

Zusammenfassung Es wurden cyclische Ester von Adipinsäure, Korksäure und Sebacinsäure durch Reaktion der entsprechenden Säure-Dichloride mit Brenzcatechin oder Hydrochinon bei hoher Verdünnung hergestellt. Während mit Hydrochinon lediglich die cyclischen Dimeren erhalten werden konnten, wurden mit Brenzcatechin sowohl cyclische Monomere als auch cyclische Dimere erhalten (ebenfalls das cyclische Dimer mit Bernsteinsäure). Die Strukturen wurden mittels¹H-NMR und Massenspektren gesichert. Die Kristallstrukturen der Brenzcatechin-Ester wurden mittels Röntgenstrukturanalyse bestimmt.

     

Ion-pair reversed phase HPLC determination of nucleotides,nucleosides and nucleobases — Application to nucleotide metabolism in hepatocytes

Chromatographia - Three groups of metabolites were analyzed in extracts of rat hepatocytes by an HPLC method: (i) nucleotides (ATP, ADP, AMP, GTP, GDP, UTP, UDP, IMP, UMP), (ii) nucleosides and...     

Versuche zur Hydrolyse der Siloxan-Bindungen an Siliciumdioxid-Oberflächen

Zusammenfassung Am Beispiel des Aerosils wurde die Ver?nderung der Oberfl?che des Siliciumdioxids bei der Entwicklung verschiedener hydrolytischer Agentien untersucht. Die Oberfl?che des Aerosils enthielt ursprünglich neben Silanolgruppen in erheblichem Umfang Siloxangruppen. Mit kaltem Wasser, mit ammoniakalischer Brenzkatechin-L?sung und bei kürzerer Einwirkung von siedendem Wasser wurde eine teilweise Hydrolyse der Siloxan-Bindungen erreicht, doch, wurden nie mehr als 3.3 Silanolgruppen pro 100 ?² erhalten. Nach l?ngerer Einwirkung von siedendem Wasser wurde eine Abnahme der spez. Oberfl?che und der Zahl der Silanolgruppen pro Fl?cheneinheit beobachtet. Im Elektronenmikroskop zeigten die Teilchen dieser Aerosile Hüllen geringerer Dichte aus feinporigem Siliciumdioxid. Infolge der extrem engen Poren wurde ein erheblicher Teil der Oberfl?che und der Silanolgruppen bei den Messungen nicht erfa?t. Konz. Ammoniak und in geringerem Ma?e auch lange Lagerung an der Atmosph?re hatten ?hnliche Wirkung. überhitzter Wasserdampf katalysierte bei 200° die Abspaltung von Wasser aus Silanolgruppen. Die Pr?parate verloren dabei mehr Wasser als bei gleicher Temperatur im Hochvakuum. Die Ergebnisse und die Mechanismen der Ver?nderungen werden diskutiert.

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Summary The influence of various agents on the surface of Aerosil, a fine particle size silicia was studied. Originally, siloxane as well as silanol groups were present in the Aerosil surface. Partial hydrolysis of the siloxane bonds was achieved with cold water, ammoniacal pyrocatechol solution and after shorter contact with boiling water. The number of silanol groups per 100 ?² never exceeded 3.3. On prolonged action of boiling water a significant decrease of the surface area and of the number of silanol groups was observed. Electron microphotographs show that the particles became coated by a layer of less dense porous silicia. As a result of the extremely narrow pores part of the surface and of the silanol groups escaped detection. Ammonia and, to a lesser extent, prolonged storing at atmospheric conditions had similar effects. At 200 °C the condensation of silanol groups to siloxane bonds was catalyzed by superheated steam as evidenced by a greater loss of silanol groups than in a high vacuum at the same temperature. The results and the mechanisms of the changes in the silica surface are discussed.

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Herrn Prof.U. Hofmann zum 60. Geburtstag gewidmet.

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Herrn Prof. Dr.U. Hofmann sind wir für stete F?rderung und gro?zügige Unterstützung unserer Arbeiten zu gro?em Dank verpflichtet. Dem Fonds der Chemie und dem Landesgewerbeamt des Landes Baden-Württemberg danken wir für finanzielle Unterstützung.     

Heating in a stream of oxidizing or reducing gas for the qualitative analysis of inorganic substances

Summary Observation of the phenomena resulting from heating a substance first in air and then in hydrogen is recommended for the preliminary examination of small specimens of inorganic matter. It may permit immediate identification of simple compounds of the heavy metals. Reduction by heating in hydrogen is also recommended for the investigation of acid insoluble residues.

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Zusammenfassung Das Verhalten beim Erhitzen in Luft und dann in einer Wasserstoff-atmosphäre eignet sich für die Vorprüfung kleiner Mengen anorganischer Substanz. In einzelnen Fällen können einfache Verbindungen der Schwermetalle daran erkannt werden. Die Reduktion durch Erhitzen in einer Wasserstoffatmosphäre empfiehlt sich ferner für die Untersuchung säureunlöslicher Rückstände.

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Résumé L'observation du phénomène résultant du chauffage d'une substance dans l'air puis dans l'hydrogène est recommandée comme examen préliminaire de petits échantillons minéraux. Elle peut permettre l'identification immédiate de composés simples des métaux lourds. La réduction par chauffage dans l'hydrogène est également recommandée pour l'examen de résidus insolubles dans les acides.

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Dedicated to Prof.Hans Lieb on occasion of his 70th birthday.     

Boride-based nano-laminates with MAX-phase-like behaviour

MAX-phases being usually composed of transition metals, group A elements and carbon/nitrogen are considered interesting materials for many applications because of their tremendous bulk modulus, “reversible” plasticity, and machinability. This is mainly due to their unique kind of bonding comprising covalent, ionic as well as metallic bonds providing “easy” planes of rupture and deformability due to the layered crystal structures.In transition metal boride systems, similar types of bonding are available. In particular the W₂B₅-structure type and its stacking variations allow the synthesis of strongly layered crystal structures exhibiting unique delamination phenomena.The paper presents ab initio calculations showing the similarities of bonding between the ternary carbides and the corresponding ternary or quaternary borides. Formation of boride-based nano-laminates from auxiliary liquid phases, from the melt as well as during sintering and precipitation from supersaturated solid solutions will be discussed by means of SEM and TEM studies. The role of impurities weakening the interlayer bonding will be addressed in particular. The pronounced cleavage parallel to the basal plane gives rise for crack deflection and pull-out mechanisms if the laminates are dispersed in brittle matrices such as boron carbide, silicon carbide or other transition metal borides.     

Europium-sensitized luminescence determination of oxytetracycline in catfish muscle

An europium-sensitized time-resolved luminescence (TRL) method was developed to determine oxytetracycline (OTC) in cultivated catfish muscle. Extraction of OTC from fish muscle was performed with pH 4.0 ethylenediaminetetraacetic acid (EDTA)-McIlvaine buffer and clean up with hydrophilic-lipophilic balanced copolymer solid phase extraction (SPE) cartridges. The eluate was used without further concentration for TRL measurement in pH 9.0 micellar tris(hydroxylmethyl)aminomethane (TRIS) buffer. Cetyltrimethylammonium chloride (CTACl) was used as surfactant and EDTA as a co-ligand. The excitation and emission wavelengths were set at 388 and 615 nm, respectively. The linear dynamic range was 0-1000 ng g⁻¹ (R²=0.9995). The recovery was 92-112% in the fortification range of 50-200 ng g⁻¹ and the limits of detection (LOD) ranged from 3 to 7 ng g⁻¹. Incurred catfish samples were used to demonstrate the performance of the method around 100 ng g⁻¹, the European Union maximum residue level.