Polydimethylsiloxane as a packing material in a programmed temperature vaporizer to introduce large-volume samples in capillary gas chromatography

The viability of using polydimethylsiloxane (PDMS) as a retaining material inside a programmed temperature vaporizer injector for the introduction of large-volume samples in gas chromatography is assessed. To that end, materials made up of Volaspher A-2 and coated with different percentages of PDMS (5%, 15%, and 50%) are considered. In addition, adsorbent (Tenax TA) and absorbent (PDMS) materials are comparatively studied in terms of their retention capacity. A relative standard deviation lower than 5.0% is obtained from the injection of PDMS, whereas values up to 49% are provided by Tenax TA. Significantly higher amounts of different volatile compounds are retained by PDMS in comparison with Tenax TA. In conclusion, the use of PDMS as a packing material seems to be viable for large-volume sampling and particularly recommendable for minor compounds occurring in complex matrices.     

Determination of coumarin, vanillin, and ethyl vanillin in vanilla extract products: liquid chromatography mass spectrometry method development and validation studies

A LC-MS method was developed for the determination of coumarin, vanillin, and ethyl vanillin in vanilla products. Samples were analyzed using LC-electrospray ionization (ESI)-MS in the positive ionization mode. Limits of detection for the method ranged from 0.051 to 0.073 microg mL(-1). Using the optimized method, 24 vanilla products were analyzed. All samples tested negative for coumarin. Concentrations ranged from 0.38 to 8.59 mg mL(-1) (x =3.73) for vanillin and 0.33 to 2.27 mg mL(-1) (x =1.03) for ethyl vanillin. The measured concentrations are compared to values calculated using UV monitoring and to results reported in a similar survey in 1988. Analytical results, method precision, and accuracy data are presented.     

Characterization of the high-pressure structures and phase transformations in SnO2. A density functional theory study

Theoretical investigations concerning the high-pressure polymorphs, the equations of state, and the phase transitions of SnO2 have been performed using density functional theory at the B3LYP level. Total energy calculations and geometry optimizations have been carried out for all phases involved, and the following sequence of structural transitions from the rutile-type (P42/mnm) driven by pressure has been obtained (the transition pressure is in parentheses): --> CaCl2-type, Pnnm (12 GPa) --> alpha-PbO2-type, Pbcn (17 GPa) --> pyrite-type, Pa (17 GPa) --> ZrO2-type orthorhombic phase I, Pbca (18 GPa) --> fluorite-type, Fmm (24 GPa) --> cotunnite-type orthorhombic phase II, Pnam (33 GPa). The highest bulk modulus values, calculated by fitting pressure-volume data to the second-order Birch-Murnaghan equation of state, correspond to the cubic pyrite and the fluorite-type phases with values of 293 and 322 GPa, respectively.     

A theoretical study on the electronic structure of Au-XO(0,-1,+1) (X=C, N, and O) complexes: effect of an external electric field

A theoretical study on the nature of Au-XO(0,-1,+1) (X=C, N, O) interaction is carried out in order to provide a better understanding on the adsorption process of XO molecules on Au surfaces or Au-supported surfaces. The effect of the total charge as well as the presence of an external electric field on the formation processes of the Au-XO complex are analyzed and discussed using DFT (B3LYP) and high-level ab initio (CCSD(T)//MP2) methods employing a 6-311+G(3df) basis set for X and O atoms and Stuttgart pseudopotentials for Au atom. The presence of an electric field can increase the binding of O2 molecule to Au while weakening the formation of the Au-CO complex. These behaviors are discussed in the context of adsorption or deadsorption of these molecules on Au clusters. The formation of the Au-XO complex, the effect of addition/removal of one electron, and the role of the electric field are rationalized by studying the nature of the bonding interactions by means of the electron localization function (ELF) analysis. The net interaction between Au and XO fragments is governed by the interplay of three factors: (i) the amount of charge transfer from Au to XO, (ii) the sharing of the lone pair from X atom by the Au core (V(X, Au) basin), and (iii) the role of the lone pair of Au (V(Au) basin) mainly formed by 6s electrons. The total charge of the system and the applied electric field determine the population and orientation of the V(Au) basin and, subsequently, the degree of repulsion with the V(X, Au) basin.     

低温等离子体转化NO/O2/N2气氛中NO的实验研究

通过建立低温等离子体实验系统, 研究了介质阻挡放电型低温等离子体反应器作用于NO/O2/N2混合气体系时, NO, O2初始浓度对NO的转化效率的影响以及NOx, O3浓度随能量密度的变化关系. 低温等离子体作用于NO/O2/N2混合气体系时, NO同时发生氧化还原反应, 氧化反应占主导地位, 大部分NO转化为NO2; NO转化率随O2, NO初始浓度增大而降低, 能量密度在450～600 J/L时转化率较高; 产生的O3浓度随能量密度的增大呈先增后减的趋势.     

Uses and perspectives of Mössbauer spectroscopic studies of iron minerals in coal

The processes involved in the utilization of coal are affected by the minerals contained in it. Due to the presence of iron as a major constituent of coal mineral matter, and to the fact that the iron minerals, especially pyrite, and their transformation products play an important role in coal uses, ⁵⁷Fe-Mössbauer spectroscopy appears as an attractive tool in coal research. Mössbauer studies related to the characterization of iron phases, coal oxidation and quantitative determination of pyritic sulphur are discussed in this work.     

Astrophysical S-factors from asymptotic normalization coefficients

S-factors for direct capture reactions can be found at astrophysical energies from asymptotic normalization coefficients which provide the normalization of the tail of the overlap function. For example the overlap for ⁸B → ⁷Be+p defines the S-factor for ⁷Be (p, γ)⁸B. Peripheral transfer reactions offer a technique to determine these asymptotic normalization coefficients. As a test of the technique, the ¹⁶O(³He, d)¹⁷F reaction has been used to determine asymptotic normalization coefficients for transitions to the ground and first excited states of ¹⁷F. The S-factors for ¹⁶O(p, γ)¹⁷F calculated from these ¹⁷F → ¹⁶O+p asymptotic normalization coefficients are found to be in very good agreement with recent measurements. Following the same technique, the ¹⁰B(⁷Be, ⁸B)⁹Be and ¹⁴N(⁷Be, ⁸B)¹³C reactions have been used to measure the asymptotic normalization coefficient for ⁷Be(p, γ)⁸B. This result provides an indirect determination of S ₁₇(0).     

Importance of MS selectivity and chromatographic separation in LC‐MS/MS‐based methods when investigating pharmaceutical metabolites in water. Dipyrone as a case of study

Pharmaceuticals are emerging contaminants of increasing concern because of their presence in the aquatic environment and potential to reach drinking‐water sources. After human and/or veterinary consumption, pharmaceuticals can be excreted in unchanged form, as the parent compound, and/or as free or conjugated metabolites. Determination of most pharmaceuticals and metabolites in the environment is commonly made by liquid chromatography (LC) coupled to mass spectrometry (MS). LC coupled to tandem MS is the technique of choice nowadays in this field. The acquisition of two selected reaction monitoring (SRM) transitions together with the retention time is the most widely accepted criterion for a safe quantification and confirmation assay. However, scarce attention is normally paid to the selectivity of the selected transitions as well as to the chromatographic separation. In this work, the importance of full spectrum acquisition high‐resolution MS data using a hybrid quadrupole time‐of‐flight analyser and/or a suitable chromatographic separation (to reduce the possibility of co‐eluting interferences) is highlighted when investigating pharmaceutical metabolites that share common fragment ions. For this purpose, the analytical challenge associated to the determination of metabolites of the widely used analgesic dipyrone (also known as metamizol) in urban wastewater is discussed. Examples are given on the possibilities of reporting false positives of dypirone metabolites by LC‐MS/MS under SRM mode due to a wrong assignment of identity of the compounds detected. Copyright © 2012 John Wiley & Sons, Ltd.     

Single-laboratory validation of a method for the determination of furan in foods by using static headspace sampling and gas chromatography/mass spectrometry

A headspace gas chromatography/mass spectrometry method was developed and validated in-house for the determination of furan in foods. The method of standard additions with d4-furan as the internal standard was used to quantitate furan. The limit of detection and limit of quantitation (LOQ) values ranged from 0.2 and 0.6 nglg, respectively, in apple juice to 0.9 and 2.9 ng/g, respectively, in peanut butter. Recoveries were obtained at 0.5, 1, 2, and 3 times the LOQ. At 1, 2, and 3 times the LOQ, the recoveries ranged from 89.4 to 108%, and the relative standard deviations ranged from 3.3 to 17.3% for all the matrixes. For apple juice, chicken broth, and infant formula, the averaged coefficients of determination from the linear regression analyses were >0.99 with each food fortified at 0.5, 1, 2, and 3 times the LOQ. The coefficients of determination were >0.99 for green beans and 0.96 for peanut butter with the foods fortified at 1, 2, and 3 times the LOQ. Within-laboratory precision was determined by comparing the amounts of furan found in 18 samples by 2 analysts on different days with different instruments. For most of the foods, the difference between the amounts found by each analyst was <18%. The method was used to conduct a survey of >300 foods. The furan levels found ranged from none detected to 174 ng/g.     

Relevance in the Fischer-Tropsch synthesis of the formation of Fe-O-Ce interactions on iron-cerium mixed oxide systems

A series of Fe-Ce mixed oxides (95 atom % Fe-5 atom % Ce) has been prepared by different methods: coprecipitation, impregnation, and physical mixture of Ce and Fe oxides. These solids have been tested in the Fischer-Tropsch synthesis. The characterization of the catalytic precursors was carried out by X-ray diffraction (XRD), Raman, M?ssbauer, and X-ray photoelectron (XPS) spectroscopic techniques. When the preparation method ensures a microscopic contact between Fe and Ce cations in the solid, several types of Fe-Ce interactions are present in the calcined solids. The interactions take the shape of Fe-O-Ce bridges that can exist either in the hematite-like solid solution or in the interphase between the Fe oxide covered by microcrystals of Ce oxide. In the case of the hematite-like solid solution, Ce(IV) cations are dissolved in the alpha-Fe2O3 network. The promotion by Ce of the catalytic properties observed in the final catalysts can be directly related with the detection of these Fe-O-Ce bridges in the calcined solids. The Ce promotion results in a larger yield to hydrocarbons, a higher production of olefins, and a higher selectivity to medium and large chain hydrocarbons (larger than six carbon atoms). It is proposed that the Ce promotion is due to the presence of Fe0-Ce(III) ensembles in the final catalysts arising from the initial Fe-O-Ce bridges developed in the parent calcined samples.