Applicability of glass fiber as a support material for generation of gaseous standard mixtures using thermal decomposition of the surface compound. Calibration of analytical equipment

The use of glass fiber as a support material for a surface compound serving to generate gaseous standard mixtures of ethene is described. The technique is based on the process of thermal decomposition of the surface compound in a desorber connected on‐line via a multi‐port valve to the calibrated device. The surface compound undergoes thermal decomposition at 245°C, yielding known amounts of ethene. The method enables on‐line preparation of a standard mixture immediately before the calibration step. Consequently, it can be also applied for the generation of standard mixtures containing volatile, malodorous, unstable, and toxic compounds.     

Influence of process parameters on the composition of gaseous standard mixtures of n-propanethiol obtained by thermal decomposition of surface compounds

A method for preparation of gaseous standard mixtures of n-propanethiol based on thermal decomposition of surface immobilized compounds bonded to silica gel is described. On the basis of the experimentally established relationships between n-propanethiol concentration and generation time, the influence of the mixture composition on diluting gas flow rate, decomposition temperature, and surface compound bed mass was determined.     

Influence of process parameters on the composition of gaseous standard mixtures of n-propanethiol obtained by thermal decomposition of surface compounds

A method for preparation of gaseous standard mixtures of n-propanethiol based on thermal decomposition of surface immobilized compounds bonded to silica gel is described. On the basis of the experimentally established relationships between n-propanethiol concentration and generation time, the influence of the mixture composition on diluting gas flow rate, decomposition temperature, and surface compound bed mass was determined.     

Thermal decomposition of silica based surface compounds as a source of volatile standards. A new approach to the generation of gaseous calibration mixtures

A new approach to the preparation of standard gaseous mixtures based on thermal decomposition of surface compounds, has been reviewed. The method is used to prepare standard mixtures containing volatile analytes such as thiols, isothiocyanates, carbon monoxide, carbon dioxide, ammonia, amines, ethylene and methyl chloride. The method enables the preparation of the standard immediately before a calibration step. It is specially useful for the generation of standard mixtures containing volatile, malodorous, unstable and toxic compounds. Presented at Balaton Symposium on High Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

Relation between chemical structure of amino acids and their thermal decomposition

The influence of substituents on the thermal decomposition of monomeric organic compounds was studied. For this purpose the thermal destruction of dozen or so α-amino acids of the diversified chemical constitution, among others compounds containing the second amine group, additional carboxyl group, amino acids containing hydroxyl or sulfhydryl groups and amino acids connected with five-member heterocyclic ring or existing in the form of hydrates or hydrochlorides were investigated. The analyses were performed using a derivatograph in an air atmosphere, sample sizes were from 50 to 200 mg and heating rate from 3 to 15 K min^-1. It has been established that the thermal decomposition of studied compounds occurs in three stages. The temperature ranges, in which the analyzed compounds undergo thermal transformations were established. For evaluation of the thermoanalytical results an advanced multivariate data processing method, principal component analysis (PCA), was used. By this method the influence of the specific functional groups on the thermal decomposition of α-amino acids was determined. The stage of decomposition, in which the thermoanalytic data are the best correlated to the chemical constitution of the compound, is the second stage. It has also been recognized, that better discrimination among the analyzed compounds was obtained for the data set of the DTA.     

Thermal decomposition of immobilized compounds for the generation of gaseous standard mixtures containing ammonia and amines

The thermal decomposition of phthalamic acid derivatives chemically bonded to the surface of silica gel was examined and utilized for the generation of single-component gaseous standard mixtures of ammonia, methylamine, diethylamine and triethylamine. The conditions of the thermal decomposition (temperature, time, diluent gas flow rate) were optimized to ensure complete liberation and rapid elution of the compounds from the bed of modified silica gel. The total amounts of these four compounds that can be released from unit mass of the modified silica gel are in the range of several mg.     

Modification of Surface Reactivity by CO: Effects on Decomposition and Polymerization of Acetaldehyde on Ru(0001)

The adsorption and reaction of acetaldehyde on the clean and CO pre-covered Ru(0001) surfaces have been investigated using temperature programmed desorption method. On the clean Ru(0001) surface, the decomposition of acetaldehyde is the main reaction channel, with little polymerization occurring. However, on the CO pre-covered Ru(0001) surface, the decomposition of acetaldehyde is inhibited considerably with increasing CO coverage. Whereas, the polymerization occurs efficiently, especially at high CO coverage (θ_CO>0.5 ML), which is strongly CO coverage dependent. Combined with previous studies, the well-ordered hexagonal structure of CO layer formed on the Ru(0001) surface at high CO coverage that matches the configuration of paraldehyde is likely to be the origin of this remarkable phenomenon.     

Detection of organic products of polymer pyrolysis by thermogravimetry-supersonic jet-skimmer time-of-flight mass spectrometry (TG-Skimmer-SPI-TOFMS) using an electron beam pumped rare gas excimer VUV-light source (EBEL) for soft photo ionisation

A commercial thermogravimetry—supersonic jet-skimmer quadrupole mass spectrometer system (TG-Skimmer-QMS, Netzsch GmbH, Germany) was successfully converted for soft single photon ionisation time-of-flight mass spectrometric (SPI-TOFMS) detection of organic compounds. VUV light for SPI was generated by an electron beam pumped argon excimer light source (EBEL; E _photon = 9.8 eV). Furthermore, the versatility of the system was conserved, as high temperature TG and DSC measurements as well as electron ionisation mass spectrometry for the detection of inorganic compounds are still possible. The new system was tested with two polymers and a hydrocarbon mixture (diesel). It was demonstrated that aliphatic and aromatic organic compounds can be detected without fragmentation. Thus the system allows the recording of a readily interpretable organic signature of, e.g. thermal polymer decomposition. The thermal degradation of polystyrene shows a rich signature of the monomer, some oligomers and minor products of irregular cleavings of carbon chains. Polycarbonate exhibits a thermal decomposition fingerprint which is dominated by products of bisphenol A. The bisphenol A monomer, however, is also detectable.     

Formation of low molecular weight carbonyl compounds by sensitized photochemical decomposition of aliphatic hydrocarbons in seawater

Summary Anthraquinone has been found to be a compound occuring frequently in seawater. Excited by solar radiation it acts as one of many natural and man-made photosensitizers and can thus be used as a model substance for the study of sensitized photochemical reactions of environmental chemicals. Experimentally it has been shown to mediate the oxidative photochemical decomposition of aliphatic hydrocarbons which, lacking absorption bands in the solar UV range at sea level, are by themselves photochemically inert. Formaldehyde, smaller amounts of acetaldehyde and acetone as well as a still unidentified carbonyl compound are the principal low molecular weight products generated in the anthraquinone-sensitized photooxidation, with natural as well as artificial sunlight, of straight chain saturated hydrocarbons accommodated in high purity water. Qualitatively the same results were obtained in natural seawater as reaction medium from which particles were removed by glass fiber filtration and organic compounds by adsorption on activated charcoal. The concomittant generation of homologous series of methylketones and terminal alkenes suggest a decomposition mechanism involving cyclic electron rearrangement in a 6-membered transition state. Based on HPLC analysis of their 2,4-dinitrophenylhydrazones, the rates of volatile carbonyl generation in the sensitized photo-oxidation of n-tetradecane was determined in the liquid phase. Also determined was the rate of formaldehyde formation in the gas phase from n-tetradecane and from two Brazilian crude oils. The rates of generation of acetaldehyde and acetone could not be determined in the experiments with crude oils because of irregular changes of concentrations with time. The rate of concentration increase of formaldehyde in the vapour phase over the hydrocarbon surface film was similar to that in the water underneath. The artifical light source was a high pressure xenon lamp whose emission spectrum closely resembles that of natural sunlight at sea level. It was calibrated against the intensity of natural sunlight using pnitroacetophenone/pyridine as binary chemical actinometer.     

Thermal decomposition behaviors of β-cyclodextrin,its inclusion complexes of alkyl amines,and complexed β-cyclodextrin at different heating rates

The present work revealed there was a conceptual difference in the thermal decomposition behaviors between the complexed β-cyclodextrin (CD) in an inclusion system and the β-CD complex of guest. The thermal decomposition behaviors of the solid inclusion complexes of β-CD with ethylenediamine (Eda), diethylenetriamine (Dta) and triethylamine (Tea) were investigated using nonisothermal thermogravimetry (TG) analysis based on weight loss as a function of temperature. In view of TG profiles, a consecutive mechanism describing the formation and thermal decomposition of the three solid supermolecules of β-CD was presented. Heating rate has very different effects on the thermal decomposition behaviors of these complexes. The faster the heating rate is, the higher the melting-decomposition point of the complexed β-CD in an inclusion system is, and on the whole the bigger the rate constant (k) of the thermal decomposition reaction of the complexed β-CD is. The thermal decomposition process of the complexed β-CD for each inclusion system is determined to be simple first-order reaction using Ozawa method. The apparent activation energies (E _a) and frequency factors (A) of the thermal decomposition reactions of the complexed β-CD molecules have been also calculated. It is found that when the decomposition reaction of the complexed β-CD encountered a large value of E _a, such as that in Dta–β-CD system, an apparent compensation effect of A on E _a can provide enough energy to conquer the reaction barrier in prompting the k value of thermal decomposition reaction of the complexed β-CD according to Arrhenius equation.     

Synthesis, characterization, and antioxidant evaluation of solid-state mefenamates of some bivalent metals

Solid-state M-mef compounds, where M stands for bivalent Mn, Fe, Co, Ni, Cu, or Zn and mef is mefenamate, have been synthesized and characterized by thermoanalytical and spectroscopic techniques, molecular modeling, and antioxidant capacity evaluations. The TG–DTA and DSC curves show that the cobalt, nickel, and zinc compounds were hydrated while the manganese, iron, and copper compounds were anhydrous. The results provided information on the composition, dehydration, thermal stability, ligand denticity, crystallinity, thermal decomposition, as well as the gaseous products that evolved during the thermal decomposition of these compounds. The thermal stability of the anhydrous compounds depended on the nature of the metal ion, and followed the order: Ni > Mn > Zn > Co > Cu–Fe. The antioxidant activity of M-mef was higher than that of mefenamic acid, indicating that they are good models in treating a variety of inflammatory diseases and may lead to the development of new drugs.     

Initial steps in the production of H2 from ethanol: A FT-IR study of adsorbed species on Ni/MgO catalyst surface

Reactivity of pure ethanol and acetaldehyde on Ni/MgO catalyst at different reaction temperatures was investigated confirming that hydrogen production is ascribable to a series of consecutive and/or parallel reactions. FT-IR studies on adsorbed dry molecules (ethanol or acetaldehyde) showed that ethanol decomposition occurs on catalyst surface through formation of acetaldehyde, which is converted to acetates and then to carbonates, or decomposes to CO, C and H₂.     

Studies on the Thermal Decomposition of Benzoic Acid and its Derivatives

The thermal decomposition of benzoic acid and its derivatives containing —OH, —NH₂, —COOH and —SO₃H functional groups as substituents in ortho, meta and (or) para position together with sulphanilic acid was investigated. The analyses were performed using derivatograph, sample mass ranged from 50 to 200 mg, heating rates from 3 to 15 K min⁻¹ and static air atmosphere. It has been established that thermal decomposition of these aromatic acids proceeds through three common stages. In the first stage the phase transformations occur. The following two stages are due to the formation of intermediate products of the thermal decomposition and their combustion. Principal component analysis (PCA) was applied for evaluation of the results. Thanks to this method the influence of specific functional groups and their positions on the benzene ring on the thermal decomposition of the compounds under investigation was determined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal study of zinc(II) 4-chlorosalicylate complex compounds with bioactive ligands

Three new complex compounds of general formula Zn{4-ClC₆H₃-2-(OH)COO}₂⋅L₂⋅nH₂O (where L=thiourea (tu), nicotinamide (nam), caffeine (caf), n=2,3), were prepared and characterized by chemical analysis, IR spectroscopy and their thermal properties were studied by TG/DTG, DTA methods. It was found that the thermal decomposition of hydrated compounds starts with the release of water molecules. During the thermal decomposition of anhydrous compounds the release of organic ligands take place followed by the decomposition of salicylate anion. Zinc oxide was found as the final product of the thermal decomposition performed up to 650°C. RTG powder diffraction method, IR spectra and chemical analysis were used for the determination of products of the thermal decomposition.     

Thermal behavior of residues (sludge) originating from the sugarcane industry

The Brazilian sugarcane industry shows a great amount of generated sludge which should be utilized adequately. Two sludge samples, aerobic and anaerobic, were collected. Both were evaluated by thermogravimetry and differential thermal analysis (DTA) as well as X-ray power diffraction. These compounds show variations of mass between 30 and 140 °C due to the dehydration stage. The DTA curves show that the compounds have an exothermic reaction between 450 and 550 °C, which indicates that this can be used as an energy source. Details concerning the kinetic parameters of the dehydration and thermal decomposition have also been described here. The kinetic study of these stages was evaluated in open crucibles under nitrogen atmosphere. The obtained data were evaluated with the isoconversional kinetic method. The results show that different activation energies were obtained for thermal decomposition.     

Thermal and pyrolysis studies of copolyesters

Random copolyesters of dimethyl terephthalate (DMT), ethylene glycol (EG), and butane-1,4-diol (BD) and the homopolyesters poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) have been subjected to degradation and pyrolysis studies. Differential thermal analysis (DTA) showed that the decomposition temperature is dependent on the percentage of EG and BD present in the copolyesters. Thermal volatilization analysis (TVA) also showed that the decomposition temperature is dependent on the percentage of EG and BD present in the copolyesters. The trend for the decomposition temperatures obtained from TVA studies for these copolyesters is similar to such other thermal properties as melting temperature T_m, ΔH_f, ΔH_c, etc. The subambient thermal volatilization analysis (SATVA) curves obtained for these polymers are also presented. The SATVA curve is the fingerprint of the total volatile products formed during the degradation in high vacuum. The isothermal pyrolysis of these materials was carried out in high vacuum at 450°C. The products formed were separated in a gas chromatograph and were subsequently identified in a mass spectrometer. The major pyrolysis products from PBT were butadiene and tetrahydrofuran, whereas those from PET were ethylene and acetaldehyde. The ratio of acetaldehyde to ethylene increases with the EG content in the copolyester, suggesting a different decomposition mechanism compared to the decomposition mechanism of PBT and PET.     

Modifying the functional cover of the γ-Al2O3 surface using organic salts of aluminum

A method is suggested for modifying the surface properties of alumina without changing its chemical composition. The sorption of aluminum complexes with organic acid ligands on the γ-Al₂O₃ surface is reported. The thermal decomposition of the adsorbed oxalate complexes yields supported aluminum oxide compounds on the surface of the initial support. This modifies the functional cover of the γ-Al₂O₃ surface, altering the proportions of different types of surface hydroxyl groups, reducing their total number, and lowering the concentration of weak Lewis acid sites.     

Solubility Isotherm and Thermal Analysis of the Selenites of the Three-Component System La2O3—SeO2—H2O at 100°C

The solubility isotherm of the system La₂O₃—SeO₂—H₂O at 100°C was studied. The compounds of the three-component system were identified by the Schreinemakers method as well as by chemical and X-ray phase analyses. Simultaneous TG and DTA analyses of all compounds of the system were made by using a derivatograph. The mechanism of thermal decomposition was described.This revised version was published online in November 2005 with corrections to the Cover Date.     

Bio-sniffers for ethanol and acetaldehyde using carbon and Ag/AgCl coated electrodes

Monitoring of ethanol and acetaldehyde in expired gas after drinking is an effective method for assessment of alcohol metabolic function. We have developed bioelectronic gas sensors (bio-sniffers) for convenient measurement of ethanol and acetaldehyde. The bio-sniffers were fabricated using a simple process and inexpensive method. The process consisted of coating carbon and Ag/AgCl on a filter paper using screen-printing, and immobilizing alcohol oxidase and aldehyde dehydrogenase on the carbon electrode. These bio-sniffers showed good response to ethanol and acetaldehyde vapor and were used to sense these vapors in the concentration range 1.0 to 100 ppm and 0.2 to 4.0 ppm, respectively. The calibration ranges cover the alcohol and acetaldehyde concentration in breath air after drinking. The bio-sniffer for acetaldehyde is applicable also to evaluate other aldehydes (i.e. formaldehyde as volatile organic compounds).

     

Non-isothermal kinetic study of the thermal decomposition of diaminoglyoxime and diaminofurazan

Data on the thermal stability of organic materials such as diaminofurazan (DAF) and diaminoglyoxime (DAG) was required in order to obtain safety information for handling, storage and use. These compounds have been shown to be a useful intermediate for the preparation of energetic compounds. In the present study, the thermal stability of the DAF and DAG was determined by differential scanning calorimetery (DSC) and simultaneous thermogravimetery-differential thermal analysis (TG-DTA) techniques. The results of TG analysis revealed that the main thermal degradation for the DAF and DAG occurs in the temperature ranges of 230–275°C and 180–230°C, respectively. On the other hand, the TG-DTA analysis of compounds indicates that DAF melts (at about 182°C) before it decomposes. However, the thermal decomposition of the DAG started simultaneously with its melting. The influence of the heating rate (5, 10, 15 and 20°C min⁻¹) on the DSC behaviour of the compounds was verified. The results showed that, as the heating rate was increased, decomposition temperatures of the compounds were increased. Also, the kinetic parameters such as activation energy and frequency factor for the compounds were obtained from the DSC data by non-isothermal methods proposed by ASTM E698 and Ozawa. Based on the values of activation energy obtained by ASTM and Ozawa methods, the following order in the thermal stability was noticed: DAF>DAG.