Optimal design and experimental validation of synchronous, asynchronous and flow-modulated, simulated moving-bed processes using a single-column setup

The performance of asynchronous (Varicol) and flow-modulated (PowerFeed) simulated moving-bed processes, as well as their combination into a single hybrid scheme, are studied both experimentally and by numerical simulation. A recently developed single-column experimental setup is employed to demonstrate the feasibility of the various schemes, explore the effect of their major operating parameters, and illustrate the performance enhancements that are obtained when these schemes are properly optimized. The experimental feasibility and effectiveness of the various schemes are assessed by running and comparing optimized configurations for the linear separation of two nucleosides on a high-performance reversed-phase stationary phase.     

Determination of several pesticides in water by solid-phase extraction, liquid chromatography and electrospray tandem mass spectrometry

The analysis of pesticides in water samples is a problem of primary concern for quality control laboratories due to the toxicity level of these compounds and their public health risk. In order to evaluate the impact of pesticides in the Lisbon drinking water supply system, following the requirements of the European Union Directive 98/83/EC, we developed and validated an analytical method based on the combination of solid-phase extraction with liquid chromatography and tandem mass spectrometry. In this work, several pesticides were studied: imidacloprid, dimethoate, cymoxanil, carbendazime, phosmet, carbofuran, isoproturon, diuron, methidathion, linuron, pyrimethanil, methiocarbe, tebuconazole and chlorpyrifos. Several parameters of the electrospray source were optimized in order to get the best formation conditions of the precursor ion for each pesticide, namely capillary and extractor voltage, cone voltage, cone gas flow rate and desolvation gas flow rate. After optimization of the collision cell energy of the triple quadrupole, two different precursor ion-product ion transitions were selected for each pesticide, one for quantification and one for qualification, and these ions were monitored under time-scheduled multiple reaction monitoring (MRM) conditions. The selection of specific fragment ions for each pesticide guarantees a high degree of selectivity as well as additional sensitivity to quantify trace levels of these pesticides in water samples. This method showed excellent linearity ranges for all pesticides, with correlation coefficients greater than 0.9989. Determination limits (between 0.0041 and 0.0480 microg/L), precision (RSD <9.18%), accuracy and recovery studies in several water samples using solid-phase extraction were also performed.     

A local search using solution fragments for the 2-machine bicriteria scheduling problem

In this work, we introduce a local search strategy for combinatorial optimization problems which explores neighborhoods obtained using fragments of current solutions. We apply the approach to the well-known -hard 2-machine bicriteria flowshop scheduling problem. Computational experiments using benchmark data show the approach to be effective when compared to other algorithms available for the problem.     

On positive solutions for a class of singular quasilinear elliptic systems

We study through the lower and upper-solution method, the existence of positive weak solution to the quasilinear elliptic system with weights

     

Analysis of tricyclic antidepressant drugs in plasma by means of solid-phase microextraction-liquid chromatography-mass spectrometry

Solid-phase microextraction coupled to liquid chromatography and mass spectrometry (SPME-LC-MS) was used to analyze tricyclic antidepressant drugs desipramine, imipramine, nortriptyline, amitriptyline, and clomipramine (internal standard) in plasma samples. SPME was performed by direct extraction on a PDMS/DVB (60 microm) coated fiber, employing a stirring rate of 1200 rpm for 30 min, pH 11.0, and temperature of 30 degrees C. Drug desorption was carried out by exposing the fiber to the liquid chromatography mobile phase for 20 min, using a labmade SPME-LC interface at 50 degrees C. The main variables experimentally influencing LC-MS response were evaluated and mathematically modeled. A rational optimization with fewer experiments was achieved using a factorial design approach. The constructed empirical models were adjusted with 96-98% of explained deviation allowing an adequate data set comprehension. The chromatographic separation was realized using an RP-18 column (150 mm x 2.1 mm, 5 microm particles) and ammonium acetate buffer (0.01 mol/l, pH 5.50) : acetonitrile (50 : 50 v/v) as mobile phase. Low detection levels were achieved with electrospray interface (0.1 ng/ml). The developed method showed specificity, linearity, precision, and limit of quantification adequate to assay tricyclic antidepressant drugs in plasma.     

Immobilization of uricase in layer-by-layer films used in amperometric biosensors for uric acid

The layer-by-layer technique was exploited to immobilize the enzyme uricase onto indium tin oxide substrates coated with a layer of Prussian Blue. Uricase layers were alternated with either poly(ethylene imine) or poly(diallyldimethylammoniumchloride), and the resulting films were used as amperometric biosensors for uric acid. Biosensors with optimum performance had a limit of detection of 0.15 μA μmol l⁻¹ cm⁻² with a linear response between 0.1 and 0.6 μM of uric acid, which is sufficient for use in clinical tests. Bioactivity was preserved for weeks, and there was negligible influence from interferents, as detection was carried out at 0.0 V vs saturated calomel electrode. This paper is dedicated to the memory of Francisco C. Nart.     

Determination of competitive isotherms of enantiomers by a hybrid inverse method using overloaded band profiles and the periodic state of the simulated moving-bed process

A procedure for determination of adsorption isotherms in simulated moving-bed (SMB) chromatography is presented. The parameters of a prescribed adsorption isotherm model and rate constants are derived using a hybrid inverse method, which incorporates overloaded band profiles of the racemic mixture and breakthrough data from a single frontal experiment. The latter are included to reduce the uncertainty on the estimated saturation capacity, due to the dilution of the chromatograms with respect to the injected concentrations. The adsorption isotherm model is coupled with an axially dispersed flow model with finite mass-transfer rate to describe the experimental band profiles. The numerical constants of the isotherm model are tuned so that the calculated and measured band profiles match as much as possible. The accuracy of the isotherm model is then checked against the cyclic steady state (CSS) of the target SMB process, which is readily and cheaply obtained experimentally on a single-column set-up. This experiment is as expensive and time consuming as just a few breakthrough experiments. If necessary, the isotherm parameters are adjusted by applying the inverse method to the experimental CSS concentration profile. The method is successfully applied to determine the adsorption isotherms of Tr?gers base enantiomers on Chiralpak AD/methanol system. The results indicate that the proposed inverse method offers a reliable and quick approach to determine the competitive adsorption isotherms for a specific SMB separation.     

Regioselective monohydrolysis of per-O-acetylated-1-substituted-β-glucopyranosides catalyzed by immobilized lipases

The regioselective monohydrolysis of different peracetylated-β-glucopyranosides in aqueous media using immobilized preparations of three different lipases—those from Aspergillus niger (ANL), Candida rugose (CRL) and Candida antarctica B (CAL-B)—has been studied. Three very different immobilization strategies—covalent attachment, anionic exchange and interfacial activation on a hydrophobic support—were employed for each lipase. The role of the immobilization strategy and the effect of the presence of different moieties in the anomeric position of the substrate on the hydrolytic activities, specificities and regioselectivities of the lipases were investigated. For example, the PEI-ANL preparation exhibited 800 times higher activity than the octyl-ANL in the hydrolysis of 2-acetamido-2-deoxy-1-(4-nitrophenyl)-3,4,6-tri-O-acetyl-β-d-glucopyranoside—producing 4-OH derivative in 18% yield—whereas the octyl-ANL was five times more active than the PEI-ANL in the hydrolysis of 1-(4-nitrophenyl)-2,3,4-tri-O-acetyl-β-d-xylopyranoside, producing 4-OH monohydroxy product in >99% yield.The octyl-CRL preparation hydrolyzed regioselectively 3,4,6-tri-O-acetyl-glucal in position 6 in 68% yield while the PEI-CRL produced the 3-OH product in 11% yield, although with moderate specificity. The CNBr-CAL-B hydrolyzed specifically and regioselectively the glucal producing the 3-OH product in >99% yield.     

Gas-phase toluene oxidation over platinum-containing mixed oxides

The gas-phase toluene oxidation was studied over platinum-containing mixed oxides derived from hydrotalcite-like compounds with high specific surface area. Both the temperature of the reaction and toluene and oxygen initial concentrations proved to influence toluene oxidation rate. Activation energy of 107 and 248 kJ/mol was found for toluene and oxygen, respectively, with an oxygen dissociative model.     

Hydroalcoholic Extract of Encholirium spectabile Mart. (Bromeliaceae) in O/W Emulsions as an Additive against the UVB Radiation

The use of sunscreen is one of the photoprotection measures most used by the population, so these products must offer broad-spectrum protection against UVA and UVB radiation. Encholirium spectabile, popularly known as “macambira-de-flecha,” is a species characterized by its rocky outcrops and found in the Caatinga. This biome is known for extreme droughts and dry periods, and for this reason, its vegetation needed to develop resistance mechanisms. The aim of this work was to evaluate the use of E. spectabile incorporated in O/W emulsion as a potential photoprotective agent and their antioxidant activity. Four chemical constituents (ferulic acid, caffeic acid, p-coumaric acid and apigenin) were identified by HPLC-DAD analysis, and emulsions containing different concentrations (1%, 2.5% and 5%) of the extract without and with the addition of chemical filters (octyl methoxycinnamate and benzophenone-3) were prepared and submitted to the test of preliminary stability. The Q formulation demonstrated little variation in the preliminary stability test and was selected for estimated your protection against UVB and determination of in vitro protection factor UVA. The formulations remained stable during the freeze–thaw cycle; the extract despite maintaining the UVA-PF and decreasing the wavelength showed an increase in FPS from 14.4 (control) to 18.8 (control+ Es-HA80).