Diastereoselective nucleophilic substitution reactions of oxasilacyclopentane acetals: application of the "inside attack" model for reactions of five-membered ring oxocarbenium ions

The additions of nucleophiles to oxocarbenium ions derived from oxasilacyclopentane acetates proceeded with high diastereoselectivity in most cases. Sterically demanding nucleophiles such as the silyl enol ether of diethyl ketone add to the face opposite the C-2 substituent. These reactions establish the syn stereochemistry about the newly formed carbon-carbon bond. Small nucleophiles such as allyltrimethylsilane do not show this same stereochemical preference: they add from the same face as the substituent in C-2-substituted oxocarbenium ions. The stereoselectivities exhibited by both small and large nucleophiles can be understood by application of the "inside attack" model for five-membered ring oxocarbenium ions developed previously for tetrahydrofuran-derived cations. This stereoelectronic model requires attack of the nucleophile from the face of the cation that provides the products in their lower energy staggered conformations. Small nucleophiles add to the "inside" of the lower energy ground-state conformer of the oxocarbenium ion. In contrast, sterically demanding nucleophiles add to the inside of the envelope conformer where approach is anti to the C-2 substituent of the oxocarbenium ion, regardless of the ground-state conformer population.     

Novel combination of non-aqueous capillary electrophoresis and multivariate curve resolution-alternating least squares to determine phenolic acids in virgin olive oil

This paper presents the development of a non-aqueous capillary electrophoresis method coupled to UV detection combined with multivariate curve resolution-alternating least-squares (MCR-ALS) to carry out the resolution and quantitation of a mixture of six phenolic acids in virgin olive oil samples. p-Coumaric, caffeic, ferulic, 3,4-dihydroxyphenylacetic, vanillic and 4-hydroxyphenilacetic acids have been the analytes under study. All of them present different absorption spectra and overlapped time profiles with the olive oil matrix interferences and between them. The modeling strategy involves the building of a single MCR-ALS model composed of matrices augmented in the temporal mode, namely spectra remain invariant while time profiles may change from sample to sample. So MCR-ALS was used to cope with the coeluting interferences, on accounting the second order advantage inherent to this algorithm which, in addition, is able to handle data sets deviating from trilinearity, like the data herein analyzed. The method was firstly applied to resolve standard mixtures of the analytes randomly prepared in 1-propanol and, secondly, in real virgin olive oil samples, getting recovery values near to 100% in all cases. The importance and novelty of this methodology relies on the combination of non-aqueous capillary electrophoresis second-order data and MCR-ALS algorithm which allows performing the resolution of these compounds simplifying the previous sample pretreatment stages.     

Direct quantitation of volatile organic compounds in packaging materials by headspace solid-phase microextraction-gas chromatography-mass spectrometry

The quantification of volatile organic compounds (VOCs) in flexible multilayer packaging materials using headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) was studied. The analytes imclude 22 compounds such as aldehydes. ketones, carboxylic acids and hydrocarbons formed by thermooxidative degradation of polyethylene during the extrusion coating process in the manufacture of the packaging, and many of them are involved in the unpleasant and undesirable odour of these materials. External standard calibration using a solution of the analytes in an appropriate solvent was the first approach studied. Aqueous solutions of the analytes provided low reproducibility and the reduction of aldehydes to alcohols under the HS-SPME conditions. Hexadecane was chosen as the solvent since its polarity is similar to that of polyethylene and its volatility is lower than that of the analytes. However, hexadecane should be added to the sample before the analysis as it modifies the absorption capacity of the fibre. A 75-microm Carboxen-poly(dimethylsiloxane) fibre was used to extract the VOCs from the headspace above the packaging in a 15-ml sealed vial at 100 degrees C after 5 min of preincubation. The influence of the extraction time on the amount extracted was studied for a standard solution of the analytes in hexadecane, together with the influence of the volume of the standard solution and the amount of the sample placed in the vial. Standard addition and multiple HS-SPME were also studied as calibration methods and the results obtained in the quantitative analysis of a packaging material were compared.     

Direct capillary electrophoretic determination of three chemotherapeutic drugs in human urine

Summary Capillary zone electrophoresis (CZE) has been used for direct determination of 6-thioguanine, methotrexate, and 5-fluorouracil in human urine, by use of a fused-silica capillary (60.2 cm×75 μm i.d.). Separation was performed after hydrodynamic injection for 7 s; the separation potential and capillary temperature were 25 kV and 35°C, respectively. A 45mm borate buffer solution (pH 9.2) was used as separation electrolyte. Under these conditions the analysis takes approximately 10 min and interday precision of migration times and corrected peak areas is satisfactory. A linear response over the concentration range 3.0–20.0 mg L¹ was observed for the three chemotherapeutic drugs in diluted human urine. Detection limits (s/n=3) for 6-thioguanine and methotrexate were approximately 1.60 mg L¹ in diluted human urine; that for fluorouracil was 2.60 mg L¹. A 2-ml volume of human urine was diluted with 2-mL of water and introduced directly into the electrophoresis system. CZE was shown to be a good method with regard to simplicity, satisfactory precision, and sensitivity. This method resulted in especially excellent recoveries for determination of methotrexate in all the different urine samples analysed (n=10).     

In-house quality control material of nicarbazin and narasin in eggs: preparation and inter-laboratory evaluation

Coccidiostats are a group of pharmacologically active substances widely used in veterinary practice. Their residues are detected relatively often in poultry tissues and egg samples analyzed as part of official residue control programs in the European Union. Therefore, accuracy of quantitative results needs to be monitored through internal and external quality control studies. In addition, the use of materials containing incurred residues would be welcome to for ongoing monitoring of the method accuracy. Unfortunately, in the field of veterinary drug residues, certified reference materials are often unavailable. Therefore, in-house quality control material of incurred lyophilized eggs containing narasin and nicarbazin has been produced and characterized. The eggs originated from hens receiving feed with coccidiostat premix Maxiban were mixed to obtain presumed concentrations of residues and freeze-dried. Homogeneity of the material was verified by the duplicate analysis of ten random samples, and the results proved that the between samples variation was negligible in comparison with the method repeatability. No measurable loss of analytes was observed within 1 year; the slope of the regression line of the results of stability measurements was not significantly different from zero. The assigned values were expressed as medians of the results of inter-laboratory comparison performed in four different European laboratories; the uncertainty of the material was estimated, taken into consideration all above tests, resulting in (14.4 ± 2.53) µg/kg for nicarbazin and (7.91 ± 1.52) µg/kg for narasin.     

Conduction calorimetric studies of ternary binders based on Portland cement, calcium aluminate cement and calcium sulphate

Different binders of Portland cement, calcium aluminate cement and calcium sulphate (PC/CAC/C $ {\bar{\text{S}}} $ ) have been investigated to determinate the influence the CAC and C $ {\bar{\text{S}}} $ amount in the reactions mechanism. Several mixtures were studied, ratios of 100, 85/15 and 75/25 of PC/CAC with 0, 3 and 5 % of C $ {\bar{\text{S}}} $ . Conduction calorimetric technique was used to follow the hydration during 100 h. The XRD and FTIR techniques were used as support in the analysis of the hydration products. The results have shown that the studied ternary systems form an extra amount of ettringite, and changes in the reactions mechanism with respect to a PC. The reactions mechanism depends on the CAC and C $ {\bar{\text{S}}} $ amount present in the different binders.     

Monitoring and Evolution of the Pollution by Volatile Organic Compounds (Vocs) in the Groundwaters of The Najerilla River Basin (Spain)

VOCs were detected and identified in the water of several wells in the area of Nájera (La Rioja). The past uncontrolled spills from an important painting and varnishing industry and from other smaller ones devoted to the manufacture of furniture and varnishing activities were the sources of the VOCs plume detected in this aquifer. This article shows the results obtained in the monitoring and assessment of the pollution at 18 sampling points for more than a year. It has allowed us to study the evolution of the different pollutants and the influence of a remediation process applied by the company in the main pollution source. The analytical method is based on headspace-solid-phase microextraction (SPME) using a 75-μm carboxen-polydimethylsiloxane fibre. Quantification was carried out by gas chromatography with flame ionization detection. This method has allowed us to determine the 13 VOCs identified in the polluted underground samples providing good sensitivity (LOD between 0.1 and 6.0 ng/mL) and reproducibility (r.s.d. less than 10%).     

ITQ-12: a new microporous silica polymorph potentially useful for light hydrocarbon separations

The new synthetic form of microporous crystalline silica, denoted as ITQ-12, shows a high potential for the separation of propane and propene from its mixtures.     

Mathematical models of solute retention in gas chromatography as sources of thermodynamic data. Part III. Alkylbenzenes as the test analytes

Direct determination of thermodynamic data has always been a complicated and troublesome experimental task. Gas chromatography is among long-established working tools well suited to performing this particular task indirectly. Our own results were first presented in the papers by Ciazynska-Halarewicz and Kowalska (1,2), and this article is Part III in the series. Present experiments are carried out on low- and medium-polarity stationary phases at five different temperatures ranging from 323K to 423K. They enable determination of two thermodynamic properties of the alkylbenzenes, molar enthalpy of vaporization (DeltaH(vap)), and the chemical potential of partitioning of the methylene group between the two phases of the chromatographic system (Delta micro (p(-CH2-))). These properties are obtained from eight nonempirical models and, as is apparent from their derivation (3-7), the terms of the models have clearly defined physical meaning, which enables calculation of thermodynamic properties.