Simultaneous Determination of Ascorbic Acid and Uric Acid by Using a PVC/TTF‐TCNQ Composite Electrode as Detector in a FIA System

A PVC/TTF‐TCNQ composite electrode has been employed as detector in a flow injection system. The proposed method allows the simultaneous detection of ascorbic acid (AA) and uric acid (UA) in mixtures by using a FIA system in a simple manner, without pre‐treatment or modified electrode. This method is based on the amperometric determination of (a) ascorbic acid at 0.15 V and (b) both analytes at 0.35 V, being the response linear in the range 1×10^?2–4×10^?4 M for both analytes with detection limits (S/N=3) of 1.2×10^?4 M and 8.1×10^?5 M for AA and UA, respectively.     

Temperature dependence of second critical micelle concentration of dodecyldimethylbenzylammonium bromide in aqueous solution

The specific conductivity of dodecyldimethylbenzylammonium bromide (C12BBr) in aqueous solutions, in the temperature range of 15 to 40 °C, has been measured as a function of molality. The two breaks which were found on the conductivity against molality plots were attributed to the critical micelle concentration, cmc, and second critical micelle concentration, 2^nd cmc, respectively. The ratio of the slopes, S, of the three linear fragments on the plots, S2/S1 and S3/S1, was attributed to the degree of ionization of the micelles at cmc and 2^nd cmc respectively. It was shown that the values of the 2^nd cmc estimated above 27 °C are only apparent due to thermal disintegration of the micelles. In the temperature range of 15 to 27 °C, the values of the 2^nd cmc increase gradually and the plot of the 2^nd cmc against temperature is concave. The ratio of 2^nd cmc/cmc for C12BBr at 25 °C amounts to 15 and appears to be high compared to the literature values for other surfactants. For comparative purposes the cmc and 2^nd cmc values were also estimated conductometrically for decyldimethylbenzylammonium bromide (C10BBr) at 25 °C. The 2^nd cmc value for this surfactant is higher compared to the value for the C12 homologue by a factor of 2.6.The standard Gibbs free energies of micellization at cmc and at the 2^nd cmc were estimated from the experimental data for both surfactants at 25 °C.     

Prediction of the behaviour of organic pollutants using cloud point extraction

A preconcentration study based on the cloud point phenomenon was carried out for a set of triazine herbicides, three of them chloro-substituted and three of them methylthio-substituted. Concentration factors and recoveries were calculated as function of the percentage of the non-ionic surfactant Triton X-114 employed. From these values, obtained from a cloud point extraction (CPE) procedure, the distribution coefficient between the Triton X-114 micelles and water, Kc, prior to CPE was calculated for each triazine and related to the corresponding octanol-water partition coefficient, Kow. In order to confirm the results obtained with the triazine herbicides, two sets of data from chemically different organic pollutants--organophosporous and chlorophenols--obtained from the literature were assessed, concluding that they display a similar behaviour to that of the triazine herbicides. This can be used to predict the CPE behaviour of other organic pollutants from their octanol-water partition coefficients. The Kc values were compared with the analyte concentration ratio in the surfactant-rich phase and aqueous phase (Ksa) with a view to obtaining a link between the analyte behaviour prior to and after cloud point extraction procedures.     

Comparative study of separation and determination of triazines by micellar electrokinetic capillary chromatography and nonaqueous capillary electrophoresis: application to residue analysis in natural waters

The separation and determination of a mixture of chloro- and methylthiotriazines in water samples by both micellar electrokinetic capillary chromatography (MEKC) and nonaqueous capillary zone electrophoresis (NA-CZE) were compared. The characteristics of both methods proved to be very similar in terms of separation efficiency and analysis times, but application of these methods for the analysis of triazines in natural waters, with a prior preconcentration step, revealed significant differences. A preconcentration step by solid-phase extraction (SPE) with Oasis HLB cartridges was accomplished for the determination of triazines at sub-ppb levels in drinking and river waters; when NA-CZE was used after this SPE step, electropherograms with fewer interferences and more stable baselines were obtained than when separation was carried out using MEKC. Another aspect related to the application to real samples was the lack of precision encountered upon evaluating the electrophoretic signals generated when using SPE coupled with NA-CZE. Here, we demonstrate the importance of choosing an appropriate internal standard for analyte quantification. It is recommended that a triazine belonging to the same family as that of the triazine to be determined should be used as internal standard.     

Gas phase intramolecular proton transfer in cationized glycine and chlorine substituted derivatives (M-Gly, M = Na+, Mg2+, Cu+, Ni+, and Cu2+): existence of Zwitterionic structures?

The intramolecular proton transfer in cationized glycine and chlorine substituted derivatives with M = Na+, Mg2+, Ni+, Cu+, and Cu2+ has been studied with the three parameter B3LYP density functional method. The coordination of metal cations to the oxygens of the carboxylic group of glycine stabilizes the zwitterionic structure. For all monocations the intramolecular proton transfer occurs readily with small energy barriers (1-2 kcalmol(-1)). For the dication Mg2+ and Cu2+ systems, the zwitterionic structure becomes very stable. However, whereas for Mg2+, the proton transfer process takes place spontaneously, for Cu2+ the reaction occurs with an important energy barrier. The substitution of the hydrogens of the amino group by chlorine atoms decreases the basicity of nitrogen, which destabilizes the zwitterionic structure. For monosubstituted glycine complexed with Na+, the zwitterionic structure still exists as a minimum, but for disubstituted glycine no minimum appears for this structure. In contrast, for Mg2+ complexed to mono- and disubstituted glycine, the zwitterionic structure remains the only minimum, since the enhanced electrostatic interaction with the dication overcomes the destabilizing effect of the chlorine atoms.     

A study of the electrospray ionisation and ion-trap fragmentation of [M - H](-) ions of new 3,5-disubstituted tetrahydro-2H-1,3,5-thiadiazin-2-thiones

The electrospray ionisation (ESI) in negative mode of the pharmacologically significant 3,5-disubstituted tetrahydro-2H-1,3,5-thiadiazin-2-thiones, and their subsequent fragmentations using an ion-trap mass spectrometer, have been investigated. Experiments on sequential product ion fragmentations (MS(n)) were performed in order to elucidate the degradation pathways for these compounds. The data presented show that the fragmentation of the even-electron [M - H](-) ions could proceed through an internal nucleophilic substitution displacement. Decarboxylation and extrusion of carbon disulfide are other fragmentations observed.     

Stereoselective syntheses of 1,4-dideoxy-1,4-imino-octitols and novel tetrahydroxyindolizidines

A new route for the preparation of four new indolizidines, (1R,2S,6S,7S,8aS)- and (1R,2S,6R,7R,8aS)-1,2,6,7-tetrahydroxyindolizidine (30 and 32) and (1S,2R,7S,8S,8aR)- and (1S,2R,7R,8R,8aR)-1,2,7,8-tetrahydroxyindolizidine (44 and 46), is reported. The synthesis is based on Knoevenagel homologation of the readily available enantiomerically pure pyrrolidin-carbaldehydes 13 and 37followed by asymmetric dihydroxylation of the subsequent alkenyl pyrrolidines and cyclization of the corresponding imino-octitols. The new indolizidines and their precursors (imino-octitols 20, 25, 26) and indolizidinones 28a and 28b have been tested for inhibitory activities toward 26 glycosidases. The enzymatic inhibition of trans-7-hydroxy-d-(-)-swainsonine (44) toward alpha-mannosidases is similar to that described for trans-7-hydroxy-l-(+)-swainsonine (11b) toward naringinase (alpha-l-rhamnosidase from Penicillium decumbens).     

Carbocyclic analogues of nucleosides from bis-(Hydroxymethyl)-cyclopentane: synthesis,antiviral and cytostatic activities of adenosine,inosine and uridine analogues

Six new carbocyclic nucleosides were prepared by constructing a purine base (in compounds 9-11) or pyrimidine base (in 6-8) on the amino groups of (+/-)-(1 beta,2 alpha,4 beta)-4-amino-1,2-cyclopentanedimethanol (4) and (+/-)-(1 beta,3 alpha,4 beta)-4-amino-1,3-cyclopentanedimethanol (5), and their activities against a variety of viruses and tumour cell lines were determined.     

Confirmation method for the identification and determination of some organophosphorus and organochlorine pesticides in cocoa beans by gas chromatography-mass spectrometry

The main objective of this work is to develop a routine quality control method for pesticide residues in cocoa beans, using gas chromatography-mass spectrometry. The investigated pesticides, which are used to control pests in the growing of cacao, are: Acephate, Propoxur, HCH, Heptachlor, Fenitrothion, Pirimiphos-methyl, Aldrin, Dieldrin, pp'-DDE, op-DDE and DDT. Two extraction methods were tested. The first was based on strong attack by concentrated sulphuric acid and later extraction with n-hexane: the investigated residues were Acephate, HCH, Fenitrothion and DDT; recoveries were 68-95% and the detection limits 0.5-10 ppb. The second extraction method was based on the Universal Trace Residue Extractor (UNITREX), which consists of a distillation system for organophosphorus and organochlorine pesticides in fatty samples. The investigated residues were Heptachlor, Pirimiphos-methyl, Aldrin, Propoxur, Dieldrin, op-DDE and pp'-DDE; recoveries were 67-88% and the detection limits 1-10 ppb.     

Concentrated reverse micelles in a random graft block copolymer system: structure and in-situ synthesis of silver nanoparticles

The structure and rheological properties of a poly(dimethylsiloxane)-graft-poly(oxyethylene) copolymer at high concentrations in block-selective solvents were studied by small-angle X-ray scattering (SAXS) and rheometry. Analysis of SAXS data indicates that quasispherical, reverse micellar aggregates (with no ordered packing) are present in concentrated solutions of the copolymer in nonpolar solvents, and that upon addition of water, the size of such aggregates increases due to the solubilization inside the micellar cores. The viscosity of concentrated polymer solutions increases exponentially as water is added, and finally, viscoelastic, gel-like behavior is found in the vicinity of the phase separation limit. It was found that small silver nanoparticles with an average diameter of ≈3 nm can be synthesized inside the copolymer aggregates without the need of a reducing agent; namely, particles embedded in a viscoelastic matrix are obtained. The synthesis seems to follow first-order kinetics. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.