Optimisation of stir bar sorptive extraction applied to the determination of volatile compounds in vinegars

Stir bar sorptive extraction (SBSE) was evaluated for analysing volatile compounds in vinegar. The extraction and desorption analytical conditions have been optimised using a two-level factorial design expanded further to a central composite design. This chemometric tool is very appropriate in screening experiments where the aim is to investigate several possibly influential and/or interacting factors. For the extraction step, the optimum analytical conditions were: sample volume 25 ml without dilution, sampling time 120 min, NaCl content 5.85 g, and stirring speed 1250 rpm. For the desorption step, the optimised analytical conditions were: desorption temperature 300 degrees C, cryofocusing temperature -140 degrees C, flow of helium 75 ml min(-1), and desorption time 10 min. The SBSE procedure developed shows detection limits, and linear ranges adequate for analysing this type of compounds. The repeatability values obtained were lower than 10%. SBSE is a very simple, solvent-free, fast technique with better sensitivities, in general, than SPME. However, a disadvantage of this technique is that, up to now, the stir bar offers a limited enrichment capability for polar compounds because is only available with PDMS coating.     

Assembly of an allenylidene ligand, a terminal alkyne, and an acetonitrile molecule: formation of osmacyclopentapyrrole derivatives

Treatment in acetonitrile at -30 C of the hydride-alkenylcarbyne complex [OsH([triple bond]CCH=CPh2)(CH3CN)2(P(i)Pr3)2][BF4]2 (1) with (t)BuOK produces the selective deprotonation of the alkenyl substituent of the carbyne and the formation of the bis-solvento hydride-allenylidene derivative [OsH(=C=C=CPh2)(CH3CN)2(P(i)Pr3)2]BF4 (2), which under carbon monoxide atmosphere is converted into [Os(CH=C=CPh2)(CO)(CH3CN)2(P(i)Pr3)2]BF4 (3). When the treatment of 1 with (t)BuOK is carried out in dichloromethane at room temperature, the fluoro-alkenylcarbyne [OsHF([triple bond]CCH=CPh2)(CH3CN)(P(i)Pr3)2]BF4 (4) is isolated. Complex 2 reacts with terminal alkynes. The reactions with phenylacetylene and cyclohexylacetylene afford [Os[(E)-CH=CHR](=C=C=CPh2)(CH3CN)2(P(i)Pr3)2]BF4 (R = Ph (5), Cy (6)), containing an alkenyl ligand beside the allenylidene, while the reaction with acetylene in dichloromethane at -20 degrees C gives the hydride-allenylidene-pi-alkyne [OsH(=C=C=CPh2)(eta2-HC[triple bond]CH)(P(i)Pr3)2]BF4 (7), with the alkyne acting as a four-electron donor ligand. In acetonitrile under reflux, complexes 5 and 6 are transformed into the osmacyclopentapyrrole compounds [Os[C=C(CPh2CR=CH)CMe=NH](CH3CN)2]BF4 (R = Ph (8), Cy (9)), as a result of the assembly of the allenylidene ligand, the alkenyl group, and an acetonitrile molecule. The X-ray structures of 2, 5, and 8 are also reported.     

A reversible reaction inside a self-assembled capsule

Reversible encapsulation allows the direct observation of the isolated molecules under ambient conditions, at equilibrium and in the liquid phase. Here we show that capsules can amplify and stabilize molecules that are present in only trace concentrations in solution. Evidence is given that reversible chemical reactions take place within the capsule. Stabilization of reaction intermediates is a characteristic property of enzymes and is widely regarded as an essential feature of catalytic activity. Reactive molecules can also be stabilized by encapsulation, a process that involves completely surrounding the reactive species within synthetic receptors. Here, we show that self-assembled capsules can isolate and stabilize molecules that are present in only trace amounts in solution. The system amplifies the concentrations of high-energy species with reduced entropies.     

Dendrimer-functionalized single-wall carbon nanotubes: synthesis, characterization, and photoinduced electron transfer

We describe the synthesis and characterization of a series of single-wall carbon nanotubes (SWNTs) functionalized with polyamidoamine dendrimers. Importantly, the dendrimers are linked directly to the SWNT surface using a divergent methodology. This approach allows the number of functional groups on the nanotubes to be increased without provoking significant damage to the conjugated pi-system of the SWNTs. Several tetraphenylporphyrin moieties can be linked to the periphery of the dendrimers, and the photophysical properties of the resulting nanoconjugates have been investigated with a series of steady-state and time-resolved spectroscopy. The fluorescence kinetics provide evidence for two transient decays, one very short-lived (i.e., 0.04 +/- 0.01 ns) and one long-lived (i.e., 8.6 +/- 1.2 ns). A possible explanation is that some porphyrin units do not interact with the nanotubes, thus exhibiting a fluorescence lifetime similar to that of the free porphyrin. Complementary transient absorption measurements not only corroborate the fast decay of the photoexcited tetraphenylporphyrin but also confirm that intraconjugate charge separation evolves from the excited porphyrin to the SWNTs.     

CO adsorption and reaction on clean and oxygen-covered Au(211) surfaces

We have used primarily temperature-programmed desorption (TPD) and infrared reflection-absorption spectroscopy (IRAS) to investigate CO adsorption on a Au(211) stepped single-crystal surface. The Au(211) surface can be described as a step-terrace structure consisting of three-atom-wide terraces of (111) orientation and a monatomic step with a (100) orientation, or 3(111) x (100) in microfacet notation. CO was only weakly adsorbed but was more strongly bound at step sites (12 kcal mol(-1)) than at terrace sites (6.5-9 kcal mol(-1)). The sticking coefficient of CO on the Au(211) surface was also higher ( approximately 5x) during occupation of step sites compared to populating terrace sites at higher coverages. The nu(CO) stretching band energy in IRAS spectra indicated that CO was adsorbed at atop sites at all coverages and conditions. A small red shift of nu(CO) from 2126 to 2112 cm(-1) occurred with increasing CO coverage on the surface. We conclude that the presence of these particular step sites at the Au(211) surface imparts stronger CO bonding and a higher reactivity than on the flat Au(111) surface, but these changes are not remarkable compared to chemistry on other more reactive crystal planes or other stepped Au surfaces. Thus, it is unlikely that the presence or absence of this particular crystal plane alone at the surface of supported Au nanoparticles has much to do with the remarkable properties of highly active Au catalysts.     

The influence of poly(ethylene oxide) and illumination on the copper electrodeposition process onto n-Si(100)

In this study, we examined the influence of illumination and the presence of poly(ethylene oxide) (PEO) as an additive for the copper electrodeposition process onto n-Si(100). The study was carried out by means of cyclic voltammetry (CV) and the potential steps method, from which the corresponding nucleation and growth mechanism (NGM) were determined. Likewise, a morphologic analysis of the deposits obtained at different potential values by means of atomic force microscopy (AFM) was carried out. In a first stage, Mott-Schottky measurements so as to characterize the energetics of the semiconductor/electrolyte interface were made. Also, parallel capacity measurements were carried out in order to determine the surface state density of the substrate. It was found that when PEO concentration is increased, the number of these surface states decreases. The CV results indicated that the presence of PEO inhibits the photoelectrochemical reaction of oxide formation on the surface of the semiconductor. This allows a decrease in the overpotential associated with the electrodeposition process. The analysis of the j/t transients shows that the NGM corresponds to progressive three-dimensional (3D) diffusional controlled (PN3D(Diff)), which was confirmed by the AFM technique. Neither illumination nor the presence of PEO changes the mechanisms. Their influence is in that they diminish the size of the nuclei and the speed with which these are formed, which produces a more homogeneous electrodeposit.     

On the fold-Hopf bifurcation for continuous piecewise linear differential systems with symmetry

In this paper a partial unfolding for an analog to the fold-Hopf bifurcation in three-dimensional symmetric piecewise linear differential systems is obtained. A particular biparametric family of such systems is studied starting from a very degenerate configuration of nonhyperbolic periodic orbits and looking for the possible bifurcation of limit cycles. It is proved that four limit cycles can coexist after perturbation of the original configuration, and other two limit cycles are conjectured. It is shown that the described bifurcation scenario appears for appropriate values of parameters in the celebrated Chua's oscillator.     

A capillary zone electrophoresis for determination of thiolic peptides in biological samples

A new method to improve the analyses of thiolic peptides (cysteine, γGlu-Cys, glutathione, phytochelatins and desglycyl-phytochelatins) derivatized with monobromobimane (mBrB) in complex biological samples by CZE is described. The method involves a SPE using Sep-Pak Light C18 Cartridges after derivatization and a later CZE analysis. Elution of mBrB-thiols was achieved with 10 mM HCl + 70% methanol v/v in deionised water. Electrophoretic parameters, such as BGE pH and concentration, different organic additives (methanol and trifluoroethanol), applied voltage and capillary length were studied in order to establish suitable analytical conditions. Optimum separation of the mBrB-thiolic peptides was obtained with 100 mM sodium borate buffer at pH 7.60. The electrophoretic conditions were +15 kV, capillary length of 90 cm from inlet to detector (98 cm total length, 50 μm ID), samples were loaded into the capillary by hydrodynamic injection (50 mbar, 20 s) and detection was performed at 390 nm. The improved method showed good reproducibility, linearity and sensitivity. The LODs and LOQs estimated using a standard of GSH were 1.41 and 4.69 μM respectively.     

Temporal Talbot effect applied to determine dispersion parameters

The space–time duality theory and the temporal selfimaging phenomenon (or Talbot effect) are used to propose a method for determining dispersion parameters associated with an optic fiber link. From the space–time analogy, the actions of free-space propagation and phase curvatures taking place in the general spatial Talbot effect are implemented for time-varying wavefields. Using the temporal selfimaging conditions, a relationship is derived for determining the second-order dispersion coefficient of a given dispersive medium under test. As a particular application, we analyze the feasibility of the measuring approach using a linearly chirped fiber grating as the dispersive component under test. Some simulations are carried out in order to study the sensitivity and accuracy of the developed method.