Temperature effects on platinum single-crystal electrodes

This work reviews three different approaches for the study of temperature effects on the platinum single-crystal | solution interphase. First, the method of analysis of temperature-dependent voltammetric data with the help of a generalized isotherm is described and illustrated for the case of adsorbed hydrogen and OH species on Pt(111) in 0.1 M HClO₄. This method of analysis allows a detailed evaluation of the thermodynamic data of charge-transfer adsorbed species, namely, the standard molar Gibbs energy, enthalpy and entropy of adsorption and the magnitude of the lateral interaction between adsorbed species. However, a number of assumptions are involved in the application of a generalized isotherm, namely, the assumption of a Langmuirian configurational term and an arbitrary separation of capacitive and faradaic processes. The second approach described here overcomes these assumptions, since it employs Gibbs thermodynamic equations for interphaces to describe temperature effects on electrosorption processes including those in which charge-transfer is allowed. This approach allows evaluation of the entropy of formation of the interphase, being defined as the difference in entropy of the components of the interphase when they are forming it and when they are in the bulk phases. This method of analysis is illustrated through the evaluation of the entropy of formation of the interphase for Pt(111) in 0.1 M HClO₄. Finally, it is shown that temperature effects on interfacial processes can also be studied by means of the application of a fast temperature perturbation. This approach opens the possibility to separate the effect of temperature on the different components of the interphase. The fast temperature perturbation is usually achieved with short laser pulses, and hence the method is called the laser-induced temperature jump method. This approach is illustrated here for the case of Pt(111) in 1 mM HClO₄ + 0.1 M KClO₄, and it is shown that the corresponding laser-induced transients provide direct evidence on the reorientation of the interfacial water network.     

Promotion of the oxidation of carbon monoxide at stepped platinum single-crystal electrodes in alkaline media by lithium and beryllium cations

The role of alkali cations (Li(+), Na(+), K(+), Cs(+), and Be(2+)) on the blank voltammetric response and the oxidative stripping of carbon monoxide from stepped Pt single-crystal electrodes in alkaline media has been investigated by cyclic voltammetry. A strong influence of the nature of the cation on both the blank voltammetric profile and the CO oxidation is observed and related to the influence of the cation on the specific adsorption of OH on the platinum surface. Especially Li(+) and Be(2+) cations markedly affect the adsorption of OH and thereby have a significant promoting effect on CO(ads) oxidation. The voltammetric experiments suggest that, on Pt(111), the influence of Li(+) (and Be(2+)) is primarily through a weakening of the repulsive interactions between the OH in the OH adlayer, whereas in the presence of steps also, the onset of OH adsorption is at a lower potential, both on steps and on terraces.     

Determination of bisphenol diglycidyl ether residues in canned foods by pressurized liquid extraction and liquid chromatography-tandem mass spectrometry

A new confirmatory method for simultaneous determination of bisphenol diglycidyl ether residues (BADGE, BADGE.H(2)O, BADGE.2H(2)O, BADGE.H(2)O.HCl, BADGE.HCl, BADGE.2HCl, BFDGE and BFDGE.2HCl) from canned food has been developed. The proposed method includes extraction by pressurized liquid extraction (PLE) followed by liquid-liquid partition and purification by solid phase extraction (SPE). Several solvent systems and different operating conditions (time, temperature) have been investigated for PLE optimization. A reversed-phase high-performance liquid chromatography (RP-HPLC) coupled to atmospheric pressure chemical ionisation tandem mass spectrometry (APCI-MS-MS) method was developed for the separation, quantification and confirmation. The ion source settings were optimized using a design of experiments (DOE). The optimized method was applied to the determination of these chemicals at very low levels in different samples with a quantification limit of 5 ng/g. Recoveries ranged between 82 and 101% and standard deviations were less than 10%.     

Optimization of diffusion-filtered NMR experiments for selective suppression of residual nondeuterated solvent and water signals from 1H NMR spectra of organic compounds

Interpretation of 1H NMR spectra of organic compounds is sometimes hampered by the presence of strong peaks arising from residual nondeuterated solvent and water that obscure compound signals. Classical solvent suppression techniques such as presaturation or those based on pulsed field gradients are not effective in this regard because they also remove the compound resonances that overlap with the solvent signal being suppressed. Here, we propose an alternative scheme by using an optimized NMR diffusion filter that eliminates the nondesired peaks while retaining the signals of interest. This strategy has proved to be useful in three common deuterated solvents, namely, CDCl3, DMSO-d6, and CD3OD, resulting in clean spectra with no interference from solvent or water peaks.     

Microflow injection system based on a multicommutation technique for nitrite determination in wastewaters

In this work a microflow structure, suitable for micro-FIA (micro flow injection analysis), will be described, evaluated and applied to real samples. Microchannels, the detector flow cell and input/output ports have been micromachined in silicon and sealed with anodically bonded Pyrex glass. The channels are defined by etching approximately 200 microm depth in silicon using a dry reactive ion etching (RIE) process. Optical windows integrated in the chip structure allow simple absorbance/transmission measurements to be made. The optical measurements were made using an LED as emitter (lambda=525 nm) and a photodiode as a detector. A Visual-Basic program has been developed to control an automatic burette, three-way solenoid valves and the data acquisition system. The micro-FIA for nitrite determination using the Griess-Ilosvay reaction has been implemented for the on-line monitoring of wastewater treatment plants (WWTPs). The multicommutation concept has been applied in order to enhance the mixing process inside the microsystem. Tandem streams of reagent and sample were generated and evaluated at different commutation frequencies. Two optimal frequencies, 400/200 ms and 150/450 ms, were found to be the most suitable ones. The first commutation ratio gave rise to wide linear working range (0-250 ppm), in spite of a high detection limit (0.35 ppm) and a low sensitivity (0.0041+/-0.0004 AU ppm-1). With the second ratio, the working linear range was smaller (0-50 ppm) but the detection limit (0.17 ppm) and the sensitivity (0.0091+/-0.0003 AU ppm-1) improved remarkably. Finally, real samples with a high nitrite concentration (0-1500 ppm) coming from a study of kinetic inhibition in the nitrification process at a WWTP has been analysed with the proposed micro-FIA system. The obtained results have allowed the corroboration of the model of inhibition by the nitrite ion with great exactitude.     

Novel and efficient syntheses of (-)-methyl 4-epi-shikimate and 4,5-epoxy-quinic and -shikimic acid derivatives as key precursors to prepare new analogues

We have developed simple methods that provide a rapid entry into the synthesis of a series of quinate and shikimate analogues, including (-)-methyl 4-epi-shikimate and the 4,5-epoxy analogues of the parent acids. Epoxy derivatives of quinic and shikimic acids were converted into methyl scyllo-quinate and (+)-methyl 3-epi-shikimate, respectively, by processes involving a regio- and stereoselective epoxide ring opening. The strategies described take place through short, high-yield reaction sequences.     

Atomic force microscope studies on the interactions of Candida rugosa lipase and supported lipidic bilayers

Using the Langmuir–Blodgett technique we prepared substrate supported well-defined lipid/phospholipid (1-mono-palmitoyl-rac-glycerol (MPG)/l,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC)) bilayers in which the MPG lipid leaflet was exposed to the aqueous phase. Hydrolysis of MPG performed by Candida rugosa lipase (CRL) on the upper MPG layer of these supported bilayers on mica was imaged by real time atomic force microscope (AFM) using a liquid cell, so that the area increase of the initial structural defects could be followed over time. Our data strongly suggest that the edges of the initial structural defects are the preferred activation sites for CRL once the enzyme is adsorbed onto these interfaces. When a 2.5 nM bulk concentration of CRL was assayed on this planar lipid substrate, we found a long lag phase before a sharp increase of catalytic activity. The lag–burst kinetic behaviour was related to the interfacial activation phenomenon although we propose that it is also dependent on the gel-phase state of this interface.