Construction and evaluation of a flow injection micro-analyser based on urethane-acrylate resin

A flow injection micro-analyser with an integrated injection device and photometric detection is described. Channels measuring 205-295 μm depth by 265-290 μm maximum width were manufactured by deep UV lithography on two layers of urethane-acrylate oligomers-based photoresist. Hypodermic syringe needles (450 μm diameter) were connected to the channels for introduction of solutions into the system. Plastic optical fibres were connected to the ends of a 5.0 mm long channel, in order to conduct the light from and to a homemade photometer. The device has a total volume of 7.0 μL and three different sample volumes (0.09, 0.22 and 0.30 μL) can be inserted into the system by choosing the appropriate loop of the hydrodynamic injection approach. The micro-analyser, designed as a single line manifold, was evaluated by determining chloride in waters (mercuric thiocyanate method), and chromium (VI) in wastewater and total chromium in metallic alloys (diphenylcarbazide method). For chloride determination two micro-pumps were employed to impel the solutions, while for chromium determination this task was performed by a conventional peristaltic pump. The results obtained in all determinations did not differ significantly from the reference methods at a confidence level of 95%. In the chloride determination, a flow rate of 50 μL min⁻¹ was used, providing a sample frequency of 45 injection h⁻¹, generating ca. 0.7 mg of Hg(II) after an 8-h working day (ca. 20 mL of solution). This result suggests the potential of the micro-analyser towards the reduction of waste, following the philosophy of Green Chemistry.     

Automatic Electronic Tongue for On-Line Detection and Quantification of Organophosphorus and Carbamate Pesticides Using Enzymatic Screen Printed Biosensors

This work presents an automatic system, based on an electronic tongue, for resolution of mixtures of three pesticides. Inhibition detections were performed during the steady state of biosensors response. Three biosensors were built using two enzymes, electric eel (EE), genetically-modified Drosophila melanogaster (B131), and electric eel co-immobilized with drosophila melanogaster (BH). Calibrations curves for paraoxon, dichorlvos, and carbofuran were performed in the ranges 0.4–50.4 µM, 0.01–1.01 µM, 0.01–0.41 µM with LOD of 3.91 × 10^?8, 6.30 × 10^?11, and 5.84 × 10^?10, respectively. An artificial neural network (ANN) was used to model the combined response of three pesticides. A set of 19 mixtures were prepared in order to train the artificial neural network, the modeling was validated with a set of 6 spiked samples of river water. The error and recovery yields were found in consistent with expected values.     

Spectroelectrochemical identity of Prussian blue films in various electrolytes: comparison of time-derivative voltabsorptometric responses with conventional cyclic voltammetry

Using Prussian blue (PB) electrodeposited on gold-covered foil as a model system, we have demonstrated the usefulness of the time-derivative measurements of absorbance versus potential (linear potential-scan voltabsorptometry) for spectroelectrochemical characterization of thin electrochromic films. The time-derivative signals were monitored for PB at 680 and 420 nm in potassium, sodium and lithium electrolytes. Information obtained from cyclic voltabsorptometry is equivalent or complementary to that from conventional cyclic voltammetry. In the case of PB films investigated in lithium electrolyte, the voltabsorptometric time-derivative peaks are better defined than the respective voltammetric peaks. The combination of voltabsorptometry with voltammetry enables molar absorptivity and/or film loading to be determined. Also, concentration changes of differently colored mixed-valence redox centers can be monitored as a function of applied potential. Received: 16 January 1997 / Accepted: 11 March 1997     

A comparative study of the non-isothermal degradation of natural rubber from Mangabeira (Hancornia speciosa Gomes) and Seringueira (Hevea brasiliensis)

Due to the increasing search for alternative sources of natural rubber (NR) whose properties are similar to Hevea brasiliesis, several sources have been studied in the past few years. Among them, Mangabeira (Hancornia speciosa Gomes), which is native to Amazon rainforest and other regions of Brazil, has a potential as another viable rubber source. As a continuation of a series of comparative studies between Hancornia and Hevea (clone RRIM 600) these two species by our research team, their thermal behavior was analyzed by thermogravimetry (TG) using Flynn–Wall–Ozawa’s approach in order to obtain kinetic parameters (reaction order, pre-exponential factor and activation energy) of the decomposition process. Results indicated that the thermal behavior of NR from Hancornia was comparable to Hevea with some differences observed as follows: reaction order for Hancornia was higher than for Hevea at the beginning of degradation and very close for temperatures over 350 °C; activation energy and pre-exponential factor had the same trend, i.e., increased with increasing degree of conversion remaining almost constant between 20 and 70% and then increasing for higher degrees, although Hevea was slightly more thermally stable than Hancornia. These major influences in the degradation process in the early stage are attributed to differences in non-rubber constituents present in these two species.     

Determination of the β-Glucosidase Activity in Different Soils by Pre Capillary Enzyme Assay Using Capillary Electrophoresis with Laser-Induced Fluorescence Detection

Enzyme activities can provide indication for quantitative changes in soil organic matter (SOM). It is known that the activities of most enzymes increase as native SOM content reflecting larger microbial communities and stabilization of enzymes on humic materials. β-Gucosidase (β-Glu) activities have been frequently used as indicators of changes in quantity and quality of SOM. In this study we propose a simple and very sensitive method, which has lower limit of detection compared with classic spectrophotometric method with the aim of determinate the β-Glu activity in soil samples using Fluorescein mono-β-D-glucopyranoside (FMGlc) as a substrate. The fluorescein released by the enzymatic reaction was quantified by capillary electrophoresis-laser induced fluorescence (CE-LIF) method. The background electrolyte (BGE) consisted in 40 mM phosphate buffer, pH 6. The LOD and LOQ for fluorescein were 1.3 10⁻⁷ mg mL⁻¹ and 6.4 10⁻⁶ mg mL⁻¹, respectively. This work deals with the minimization of the mixture for the enzymatic reaction and with the optimization conditions of CE separation. To the best of our knowledge, this is the first time that an enzymatic activity was detected in soil using CE-LIF system.