Simultaneous microdetermination of carbon, hydrogen, chlorine, bromine, sulfur, and metals in organometallic compounds

A method for the simultaneous microdetermination of carbon, hydrogen, chlorine, bromine, sulfur, and metals which combines features of the known intermediate storage technique in organic microanalysis and formation of the metal-oxine complex, is described. Carbon and hydrogen are determined gravimetrically as usual. Sulfur and halogens are absorbed and stored on electrolytic silver. After completion of the combustion, the oxygen is replaced first by nitrogen and secondly by hydrogen which liberates the sulfur as sulfur dioxide and halogens as hydrogen halides and regenerates the silver layer. The liberated gases can be easily absorbed by dilute hydrogen peroxide and determined titrimetrically. The metal oxide, remaining in the platinum boat, is dissolved in a mixture of nitric-perchloric acids in a test tube and the metal content is determined gravimetrically and titrimetrically using 8-hydroxyquinoline.     

Polymerization of diethynyldiphenylmethane: DSC,FTIR, NMR,and dielectric characterization

4,4′-Diethynyldiphenylmethane thermally polymerizes by a free radical mechanism to a highly crosslinked structure of interest as a high temperature composite matrix resin. The polymerization reaction was characterized by differential scanning calorimetry, Fourier-transform infrared (FTIR) spectroscopy, ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, and microdi-electrometry. The predominant reaction mechanism appears to be linear polymerization through the acetylene end groups, which follows first-order kinetics. However, during the early stages of reaction a second, more rapid polymerization mechanism is evident; it is postulated that this is the formation of a cyclic trimer, which is kinetically favored but sterically prohibited as the crosslinked network grows. Formation of a liquid crystalline trimer is hyppthesized; such intermediate formation is supported by intensity increases in the aromatic region of the NMR spectra, by FTIR difference spectroscopy comparisons with model compounds, and by enhancement of conductivity as observed by microdielectrometry.     

Assimilation, dissimilation, and detoxification of formaldehyde, a central metabolic intermediate of methylotrophic metabolism

Methanol is a valuable raw material used in the manufacture of useful chemicals as well as a potential source of energy to replace coal and petroleum. Biotechnological interest in the microbial utilization of methanol has increased because it is an ideal carbon source and can be produced from renewable biomass. Formaldehyde, a cytotoxic compound, is a central metabolic intermediate in methanol metabolism. Therefore, microorganisms utilizing methanol have adopted several metabolic strategies to cope with the toxicity of formaldehyde. Formaldehyde is initially detoxified through trapping by some cofactors, such as glutathione, mycothiol, tetrahydrofolate, and tetrahydromethanopterin, before being oxidized to CO2. Alternatively, free formaldehyde can be trapped by sugar phosphates as the first reaction in the C1 assimilation pathways: the xylulose monophosphate pathway for yeasts and the ribulose monophosphate (RuMP) pathway for bacteria. In yeasts, although formaldehyde generation and consumption takes place in the peroxisome, the cytosolic formaldehyde oxidation pathway also plays a role in formaldehyde detoxification as well as energy formation. The key enzymes of the RuMP pathway are found in a variety of microorganisms including bacteria and archaea. Regulation of the genes encoding these enzymes and their catalytic mechanisms depend on the physiological traits of these organisms during evolution.     

A review of silver nanoparticles in food packaging technologies: Regulation,methods, properties,migration, and future challenges

The development of antimicrobial food packaging is needed for food preservation and quality maintenance. Silver nanoparticles (AgNPs) have been widely used as an antimicrobial agent in food packaging technologies. However, the risks associated with their potential migration into foods are a major concern. This paper comprehensively reviews the use of AgNPs in food packaging technologies. The application of AgNPs in food packaging technologies has been regulated by the United States Food and Drug Administration and the European Food Safety Authority. The addition of AgNPs into food packaging can improve their barrier, mechanical, and antibacterial properties, as well as maintain the quality of foods. Migration of AgNPs from food packaging into foods is still a concern as it has implications for human health associated with their toxicity properties. A study on the toxicological properties of AgNPs released from food packaging needs to be carried out intensively to ensure their safety before being widely implemented. Moreover, comprehensive economic evaluation to implement AgNPs in food packaging is needed as such a study is missing in the literature.     

Simultaneous activation determinatio of aluminium,magnesium, and silicon in rocks,glasses, and pottery

A method for simultaneous neutron activation determination of Al, Mg and Si using two irradiations in a nuclear reactor—the first one with epithermal and the second one with pile neutrons is proposed. The applicability of the method is demonstrated by the analysis of standard reference materials—rocks, glass, as well as of archaeological finds from medieval glass and sgraffito ceramics.     

TXRF, PIXE, SYXRF; Principles, critical comparison and applications

Summary At first glance X-Ray fluorescence analysis seems to be a very sensitive and effectful method to detect element traces in the multielement mode. But in praxi the application range is restricted, if X-Ray tubes are used as excitation sources. To overcome this situation, it is necessary to improve the conditions of excitation and to reduce the background, produced by different scattering effects. TXRF, PIXE and SYXRF, which allow multielement analysis in the trace- and ultratrace region are using this strategy. In the case of TXRF a remarkable background reduction is achieved if the sample is prepared as a thin amorphous film on a planar sample holder and the excitation beam of a X-Ray tube is totally reflected on its surface. In the case of PIXE a particle beam of high intensity is used as excitation source, improving the conditions of excitation and giving the opportunity of spatial resolved analyses. In the case of SYXRF the X-Ray fraction of synchrotron radiation is used as excitation source, giving the opportunity, to improve the conditions of excitation as well as to reduce the background by using the high polarisation of the beam. In this case, too, spatial resolved analysis are possible. The principles of the three methods are described, their advantages and disadvantages are critically compared and advanced applications from different analytical fields are presented.     

Electrothermal vaporization-ICP-MS for the determination of metals in micro amounts of high-purity non-metals (P, As, Sb, S, Se, Te, I)

A thermal trace-matrix separation procedure has been developed for the determination of ultra-traces of metals in solid non-metals (P, As, Sb, S, Se, Te and I). These matrices are dissolved in high-purity hydrogen peroxide to form the corresponding acids. By applying a suitable temperature profile, coating the graphite tube with WC and using hydrogen as a gaseous modifier more than 90% of the matrix can be removed. Analyte recoveries are 90 to 105% with a standard deviation of 5 to 10%. Since the separation principle is not specific, non-metallic analytes show poor recoveries of 10 to 40% (20–100% RSD). Elements forming persistent carbides, such as refractory metals, require either HF or freon as an additional modifier. Separation of the matrix leads to higher sensitivities, fewer spectral and non-spectral interferences and to better precision. The graphite tube is only little contaminated and its lifetime is drastically increased. The improvements result in limits of detection in the lower pg/g-range in the solid non-metals. Furthermore, this method can be employed for amounts of sample around 1 mg at absolute detection limits below 1 pg. Best performance of the coupling of ETV to ICP-MS is only obtained in the single-ion monitoring mode which means only semi-sequential multi-element capabilities.     

Electrothermal vaporization-ICP-MS for the determination of metals in micro amounts of high-purity non-metals (P, As, Sb, S, Se, Te, I)

A thermal trace-matrix separation procedure has been developed for the determination of ultra-traces of metals in solid non-metals (P, As, Sb, S, Se, Te and I). These matrices are dissolved in high-purity hydrogen peroxide to form the corresponding acids. By applying a suitable temperature profile, coating the graphite tube with WC and using hydrogen as a gaseous modifier more than 90% of the matrix can be removed. Analyte recoveries are 90 to 105% with a standard deviation of 5 to 10%. Since the separation principle is not specific, non-metallic analytes show poor recoveries of 10 to 40% (20–100% RSD). Elements forming persistent carbides, such as refractory metals, require either HF or freon as an additional modifier. Separation of the matrix leads to higher sensitivities, fewer spectral and non-spectral interferences and to better precision. The graphite tube is only little contaminated and its lifetime is drastically increased. The improvements result in limits of detection in the lower pg/g-range in the solid non-metals. Furthermore, this method can be employed for amounts of sample around 1 mg at absolute detection limits below 1 pg. Best performance of the coupling of ETV to ICP-MS is only obtained in the single-ion monitoring mode which means only semi-sequential multi-element capabilities. Received: 28 April 1997 / Revised: 16 June 1997 / Accepted: 21 June 1997     

Electroreduction of aromatic oximes: diprotonation, adsorption, imine formation, and substituent effects

Aromatic oximes are reduced in aqueous solution in a four-electron process. The reducible species in the pH range 5-8 is a diprotonated form of the oxime. This species is generated in the course of electrolysis in the vicinity of the electrode surface from the adsorbed neutral form of the oxime. The reduction is initiated by a cleavage of the N-O bond. The diprotonation facilitates the reduction process by the preformation of OH2+ as a good leaving group and by a positive charge on the azomethine nitrogen. Diprotonation has been proven based on shapes of i = f(pH) plots, by observed shifts of half-wave potentials with pH and by comparison with the reduction of nitrones. Some observed deviations from theoretical i = f(pH) plots were attributed to the role of adsorption on the rate of protonation. Adsorption is also responsible for dips on some of the i-E curves. Adsorption plays a role at concentrations as low as 1 x 10(-5) M, when the electrode surface is still not fully covered. This indicates that catalyzed protonation occurs on islets of adsorbed materials. At pH 2-5 the studied oximes in the vicinity of the electrode are predominately present in a protonated form, which is less strongly adsorbed. In this pH range the protonation takes place in a homogeneous reaction layer of the electrode. It yields a monoprotonated form, which is reduced. The separation of two two-electron waves observed for some oximes in acidic media serves as an experimental proof of the formation of imines as reduction intermediates. This separation is caused by the differences in pKa values of protonated forms of oximes and imines. The effects of substituents in the para position on the benzene ring are characterized by correlation with the Hammett substituent constant sigmax. This has been proven at pH 1.5 for substituted benzaldehyde oximes and at pH 5.0 for substituted acetophenone oximes.     

A protecting-group-free synthesis of arbusculone,andirolactone, pinnatolide,ipomolactone, cyclocapitelline and isocyclocapitelline

A general approach for a collective synthesis of natural products containing substituted THF ring is described. In this paper, Arbusculone, a small molecule natural product accomplished using a short route, is used as the key intermediate to achieve the total synthesis of Andirolactone, Pinnatolide, Ipomolactone, Cyclocapitelline, Isocyclocapitelline and their two isomers in less than ten steps. The present effort highlights protecting-group-free total syntheses and the shortest route to access these natural products from commercially available cheap starting materials.     

Liquid Chromatography of Adenine,Caffeine, Theobromine,Theophylline, Tannic Acid,Xanthine and its Application to Tea Analysis

A polystyrenesulfonic acid resin in the hydrogen form is a useful stationary phase for chromatographic separation of adenine, caffeine, theobromine, theophylline, tannic acid, xanthine and tea components with aqueous ethanol as eluent. The results indicated that caffeine, theophylline, theobromine, adenine, xanthine and tannic acid could be separated on a column of Dowex-50W-X8(H)⁺ by elution with 25% ethanol as eluent.     

Effects of Acetone,Ethanol, Isopropanol,and Dimethyl Sulfoxide on Amylose-Iodine Complex

ABSTRACT

The amylose-iodine (AI) complex formation was studied by absorption spectra in water and water containing varying proportions of ethanol, acetone, isopropanol and dimethyl sulfoxide (DMSO). Complex formation is most favored in pure water and decreases as the proportion of nonaqueous solvent is increased. A decrease in the absorbance intensity at around 615 nm (for AI complex) is accompanied by a peak shift towards 550 nm and an increased absorbance at around 350 nm (for unbound iodine). The amount of the nonaqueous solvent added, as well as the order in which it is added relative to amylose and iodine solution, change remarkably the extent of the AI complex formation. A mechanism of the complex formation is proposed.     

Hydration, charge, size, and shape characteristics of peptides from their CZE analyses

A CZE model is presented for peptide characterization on the basis of well-established physicochemical equations. The effective mobility is used as basic data in the model to estimate relevant peptide properties such as, for instance, hydration, net and total electrical charge numbers, hydrodynamic size and shape, particle average orientation, and pH-microenvironment from the charge regulation phenomenon. Therefore 102 experimental effective mobilities of different peptides are studied and discussed in relation to previous work. An equation for the estimation of peptide hydration as a function of ionizing, polar, and non-polar amino acid residues is included in the model. It is also shown that the shape-orientation factor of peptides may be either lower or higher than one, and its value depends on a complex interplay among total charge number, molar mass, hydration, and amino acid sequence.     

Light-scattering, X-ray diffraction, elemental analysis and infrared spectro- photometry characterization of chitosan, a chelating polymer

Measurements on chitosan, which is used as a selective chromatographic support in inorganic chromatography, show that its molecular weight is 1.2 x 10(5), the nitrogen content is 7.4%, and its crystalline structure is not lost after adsorption of metal ions, which are co-ordinated with the nitrogen atoms of the -NH(2) groups of the polymer. Chitosan treated with copper sulphate exhibits a new crystal structure and a distinctive infrared spectrum, is quite stable and does not shrink, and is proposed as a chromatographic support for ligand-exchange chromatography.     

High frequency t1tration and estimation of ions: Determination of barium,lead, thallium,calcium, cerium,copper and silver

Solutions of potassium chromate, ammonium oxalate and ferrocyanide are used as titrants for the high frequency titration of metal ions. Lead and barium, up to 20 mg, and thallium, up to 50 mg, are estimated with chromate und yield good results. Similarly, ammonium oxalate behaves satisfactorily when the concentration of calcium is not more than 10 mg and that of cerium is between 10 and 40 mg. Copper and silver can be titrated by ferrocyanide only up to a concentration of 10 and 20 mg, respectively. In all cases, the pH of the solution should preferably be between 6.0 and 6.5, though it can be extended in some cases to as low as 5.0.     

Biogeochemical migration of heavy metals, Ti, V, Mo, Ta, W, and U, in the profile of a low-laid peatbog in the country between the Ob and Tom rivers

Biogeochemical migration of heavy metals in the course of marsh formation is mainly determined by physical-chemical processes, such as the oxidation-reduction zonality of the peatbog thickness and the state of humous substances (the quality of humus adsorptive barriers). In the process of peatbog formation V, U, Ta are the weakly captured elements while Mo is captured to a relatively higher extent. Elements as Ti and W are found in separate layers. No technogenic degradation of the marsh ecosystem in the Ob and Tom has been observed.     

Theory of incomplete crystals,surfaces, defects,interfaces and layered structures

This paper presents systematically the surface Green function matching analysis for systems with inherently discrete structure - e.g. phonons or tight-binding electrons. The general interface problem, the free surface, the vacancy - as a particular case of an inner surface - and point defects and adsorbates - as examples of interfaces - can all be treated on the same footing. By combining ideas of scattering and matching a unified approach encompasses a variety of different problems while yielding a method which can be used in practice for calculating e.g. surface, interface or defect state eigenvalues or spectral functions of interest. This holds for any type of physical states - electrons, phonons, waves - and for any chosen model. The practical combination with transfer matrix algorithms for planar geometry is discussed and an algorithm for growing shells is presented for point defects. Phonons allow for a discussion of the long-wave limit and contract is thus established with the surface Green function matching analysis for continuous media. The analysis is extended to layered structures - quantum wells and superlaticces - both for discrete and continuous systems.     

Comparison of restricted, unrestricted, inverse, and dual kinetic balances for four-component relativistic calculations

Various kinetic balances for constructing appropriate basis sets in four-component relativistic calculations are examined in great detail. These include the well-known restricted (RKB) and unrestricted (UKB) kinetic balances, the less-known dual kinetic balance (DKB) as well as the unknown inverse kinetic balance (IKB). The RKB and IKB are complementary to each other: The former is good for positive-energy states, whereas the latter good for negative-energy states. The DKB combines the good of both RKB and IKB and even provides full variational safety. However, such an advantage is largely offset by its complicated nature. The UKB does not offer any particular advantages as well. Overall, the RKB is the simplest ansatz. Although the negative-energy states by a finite RKB basis are in error of O(c ⁰), there is no objection to using them as intermediates for a sum-over-states formulation of perturbation theory, provided that the magnetic balance is also incorporated in the case of magnetic properties. In particular, the RKB is also an essential ingredient for formulating two-component relativistic theories, while all the others are simply incompatible. As such, the RKB should be regarded as the cornerstone of relativistic electronic structure calculations.