Surfactant to dye binding degree based approach for the selective determination of l-glutamate in foodstuffs

A selective method for the determination of l-glutamate in foodstuffs has been developed. It was based on the competition established between the analyte and the dye Coomassie brilliant blue G (CBBG) to interact with the surfactant didodecyldimethylammonium bromide (DDABr). The measurement parameter was the amount of DDABr required to reach a given dye-to-surfactant binding degree. It was obtained by photometric titration on the basis of the changes observed in the spectral characteristics of the dye when CBBG–DDABr aggregates were formed. The calibration graph obtained was linear in the l-glutamate concentration interval 0.2–5 mM (detection limit 0.05 mM). The high selectivity of the proposed method (other amino acids and food additives did not interfere at the concentrations present in foodstuffs) permitted the direct analysis of food samples after dissolution of the analyte in hot water. The accuracy of the surfactant to the dye binding degree method was demonstrated by determining l-glutamate in different kinds of foodstuffs (liquid and dried soups, seasonings, pasta sauces and dried mushroom creams) and comparing the results obtained with those provided by the commercial Boehringer Mannheim essay.     

Usefulness of the direct coupling headspace-mass spectrometry for sensory quality characterization of virgin olive oil samples

The applicability of the headspace coupled to mass spectrometry for evaluation of the sensory quality of virgin olive oil samples is presented. The volatiles of the oil are directly transferred from the sample vial to the detector without chromatographic separation. The mass spectrum obtained can be considered as a fingerprint of the oil sample and can be used for classification purposes. After a training step with samples previously qualified following the official method, a classification model was created using the supervised technique soft independent modeling of class analogy (SIMCA). Eight negative (rancid, winey-vinegary, muddy sediment, hay-wood, vegetable water, earthy, fusty and musty-humidity) and three principal positive attributes (fruity, bitter and pungent) have been included in this study. With them, a classification model consisting of two main classes (extra- and lampante-virgin olive oil) was constructed. In addition, the unsupervised technique cluster analysis permited the discrimination among oils with different negative attributes. The proposed procedure has been applied to the classification of commercial samples (as extra- or lampante-virgin olive oils) and the results were compared with those provided by the expert's panel with acceptable correlation.     

Assessment of the surfactant-dye binding degree method as an alternative to the methylene blue method for the determination of anionic surfactants in aqueous environmental samples

The surfactant to dye binding degree (SDBD) method is proposed for the routine monitoring of anionic surfactants in aqueous environmental samples and their analytical features compared with those provided by the standard methylene blue (MB) method. This new analytical approach is based on the effect that anionic surfactants exert on the binding degree of the cationic surfactant didodecyldimethylammonium bromide (DDABr) to the anionic dye Coomassie Brilliant Blue G (CBBG). The formation of DDABr-CBBG aggregates is monitored photometrically. The analytical applicability of the proposed method was demonstrated by determining anionic surfactants in tap, river and swamp water, and raw and treated sewage. The mean recoveries obtained ranged between 99 and 101%. The SDBD method offers important advantages over the classical MB method: it is more sensitive, selective, precise, simple and rapid; the analytical response is independent of the molecular structure of the anionic surfactants, and the volume of sample required for analysis and the consumption of organic solvents are significantly reduced.     

Continuous flow configuration for total hydrocarbons index determination in soils by evaporative light scattering detection

A continuous flow configuration is proposed for the estimation of the hydrocarbon index in soils using for the first time evaporative light scattering detection (ELSD). The method is based on a membrane-enrichment of the hydrocarbons of interest (C10-C40), which are previously extracted from the soil matrix with a water:hexane mixture using a household microwave oven. The organic supernatant is cleaned-up through silica, evaporated and redissolved in a sodium dodecyl sulphate aqueous solution which is introduced in the loop of an injection valve. The sample, carried by an aqueous stream, passes through a continuous filtration unit fitted with a 1 microm pore size PTFE membrane, where the hydrocarbons are retained while the potential coextracted compounds are wasted. Quantitative elution of the C10-C40 fraction is accomplished by passing an acetone stream through the filter which drives the analytes to the ELSD system for analytical measurement. The detection limit was 1.8 microg ml(-1), the linear range 5-25 microg ml(-1) and the precision 4.3%. The recoveries were between 93 and 97%. The proposed method was also evaluated by attenuated total reflexion-Fourier transform (ATR-FT-IR) analysis of the extracts.     

Computational and experimental structure–reactivity relationships: evidence for a side reaction in Alpine-Borane reductions of d-benzaldehydes

Extraordinary stereoselectivity, approaching 100%, has been reported in the reductions of d-benzaldehydes by B-isopinocampheyl-9-borabicyclo[3.3.1]nonane (Alpine-Borane). This is likely because of the extreme size disparity of groups on either side of the carbonyl. Here, we present a structure–reactivity study whereby the reductions of variably substituted d-benzaldehydes are explored using highly sensitive measures for enantiomeric excess and relative reactivity. These results are compared to the relative rates predicted from density functional calculations. The results indicate that 2,6-disubstitution adversely affects the stereoselectivity by means of a non-selective reduction via the dehydroboration product of Alpine-Borane, 9-borabicyclo[3.3.1]nonane.     

The mechanism of double proton transfer in dimers of uracil and 2-thiouracil--the reaction force perspective

The intermolecular double proton transfer in dimers of uracil and 2-thiouracil is studied through density functional theory calculations. The reaction force framework provides the basis for characterizing the mechanism that in all cases has been associated to a dynamic balance between polarization and charge transfer effects. It has been found that the barriers for proton transfer depend upon the nature of the acceptor atoms and its position within the seminal monomer. Actually, the change in the nature of the hydrogen bonds connecting the two monomers along the reaction coordinate may favor or disfavor the double-proton transfer.     

Potential of supramolecular solvents for the extraction of contaminants in liquid foods

Amphiphile-based supramolecular solvents (ASSs), which are water immiscible liquids consisting of supramolecular aggregates in the nano- and micro-scale regimes dispersed in a continuous phase, were assessed for the extraction of trace contaminants in liquid foods. The ASS selected was made up of reversed micelles of decanoic dispersed in tetrahydrofuran (THF)-water and the contaminants used as a model were bisphenol A (BPA), ochratoxin A (OTA) and benzo(a)pyrene (BaPy). The influence of matrix components on the extractant solvent production, extraction recoveries and actual concentration factors was investigated by using commercial foods such as wine and wine-based products, beer, soft drinks and tea and coffee brews, and/or aqueous synthetic solutions containing specific food matrix components. The method involved the addition of decanoic acid (80mg) and THF (0.8-1.7mL) to the food sample (15mL), stirring of the mixture for 5min, centrifugation for 10min and analysis of 10-20microL of the extract by liquid chromatography coupled to fluorimetry for OTA and BaPy or to mass spectrometry for BPA. No clean-up of the crude extracts was required for any of the samples analysed. The quantification limits for the contaminants (14-31ngL(-1), 0.37-0.39ngL(-1) and 562-602ngL(-1) for OTA, BaPy and BPA, respectively) were far below their respective European legislative threshold limits. Recoveries for food samples were in the ranges 79-93%, 90-96% and 78-82% for OTA, BaPy and BPA, respectively, with relative standard deviations ranging from 1 to 7%, and actual concentrations factors between 65 and 141. The methods developed were applied to the determination of the target compounds in a variety of commercial foods. OTA was found in vinegar, must and beer samples, the concentrations ranging from 92 to 177ngL(-1), BaPy was quantified in samples of tea and coffee at concentrations between 1.5 and 16.6ngL(-1) whereas BPA was detected in two canned soft drinks and quantified in one of them (tea beverage) at a level of 2.3microgL(-1).     

Separation of carbon nanotubes in aqueous medium by capillary electrophoresis

A robust and reproducible method for the dispersion of carbon nanotubes, either single-walled or multi-walled is presented. Dispersion of nanotubes was achieved as surfactant-coated species of sodium dodecyl sulphate. The addition of small amounts of hydroxypropyl methyl cellulose (HPMC) together with the surfactant, sodium dodecyl sulphate, was found critical to achieve reproducible nanotubes dispersion and to obtain an homogeneous and stable solution. This solution is further analyzed by capillary electrophoresis using a background electrolyte solution containing a polymer, 0.025% (w/v) HPMC solution prepared in 5 mM ammonium acetate at pH 8.03. This electrophoretic method presents a high reproducibility between runs, being an interesting alternative to study nanotube size distribution or characterization after synthesis. In addition, the methodology developed allowed the study of the interaction of the different types of carbon nanotubes with a molecular probe such as pentachlorophenol. This procedure was showed effective to detect small differences on the chemical/physical surface properties of the nanotubes. The different interaction behavior found within the two SWNTs selected was critically discussed.     

Continuous autoanalyzer for the evaluation of the exhaustion of industrial degreasing baths based on the determination of total grease and surfactant contents

A continuous flow autoanalyzer for at-line monitoring of total grease and surfactant contents in alkaline degreasing baths is proposed. For this purpose, a simple, robust, automated configuration has been on-line coupled to an universal response detector, such as the evaporative light scattering one. The proposed autoanalyzer constitutes an advantageous alternative to manual procedures and achieves the determination of both indices in ca. 15 min. The parameters thus obtained can be used to evaluate the degree of exhaustion of the bath as therefore allows the timely decision-making about reusability, maintenance or renewal. The fractionation model has been validated using attenuated total reflection-Fourier transform-infrared (ATR-FT-IR) spectroscopy. The potential of the method was realized by applying it to the analysis of 15 real samples obtained from precleaning and cleaning alkaline baths from a production line of a metal industry.     

Analysis of linear alkylbenzene sulfonate homologues in environmental water samples by mixed admicelle-based extraction and liquid chromatography/mass spectrometry

Hemimicelles and admicelles of cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC), adsorbed onto silica, were tested as sorbents for the solid phase extraction (SPE) of linear alkylbenzene sulfonate (LAS) homologues from environmental water samples. LASs were quantitatively retained on both surfactants due to high hydrophobic and ionic interactions, which led to the formation of analyte-extractant mixed aggregates. Parameters affecting the SPE of LASs were optimised. Recoveries of analytes from wastewater influent and effluent and river water samples ranged between 86 and 110%. Combination of SPE with liquid chromatography/mass spectrometry provided detection limits for the different LAS homologues of about 4 ng L(-1). The precision of the method, expressed as relative standard deviation, ranged from 5 to 9%. The method was applied to the analysis of LASs in wastewater and river samples using sample volumes between 10 and 25 mL. The LAS concentrations found ranged from 9 to 503 microg L(-1). No cleaning step was required to get accurate results.