Papers by Alexandre Fonseca
Química Nova, 2016
Classification of mineral waters based on digital images acquired by smartphones. Studies for app... more Classification of mineral waters based on digital images acquired by smartphones. Studies for application of digital images in rapidclassification of commercial waters using smartphone and K-NN model.
Analytical and Bioanalytical Chemistry, 2009
This work describes the development of flow analysis microsystems with integrated fluorimetric de... more This work describes the development of flow analysis microsystems with integrated fluorimetric detection cells. Channels (width of 300-540 µm and depth of 200-590 µm) were manufactured by deep-UV lithography in urethane-acrylate (UA) resin. Plastic optical fibers (diameter of 250 µm) were coupled to a 2.0-mm-long detection channel in order to guide the excitation radiation from an LED (470 nm) and collect the emitted radiation at a right angle towards a photomultiplier. A single-line miniaturized system, with a total internal volume of 10.4 μL, was evaluated by means of standard fluorescein solutions (0.53-2.66 µmol L −1 , pH 8.5). The analytical signals presented a linear relationship in the concentration range studied, with a relative standard deviation of 1.9% (n=5), providing a detection limit of 0.37 µmol L −1 and an analytical frequency of 60 samples/h, using a flow rate of 60 µL min −1. Optical microscopy images and videos acquired in real time for the hydrodynamic injection of 130 and 320 nL of sample solutions indicated the good performance of the proposed sampling strategy. Another microsystem with a total internal volume of 38 µL was developed, incorporating a confluence point for two solutions. This device was applied to the determination of the total concentration of Ca 2+ and Mg 2+ in commercial mineral waters using the calcein method. Microscopy images and videos demonstrated the mixing efficiency of the solutions in the microchannels. A linear relationship was observed for the analytical signal in the Ca 2+ concentration range from 25 to 125 µmol L −1 , with relative standard deviations of 3.5%. The analysis of mineral waters with the proposed system provided results that did not differ significantly from those obtained by the EDTA titration method at a confidence level of 95%. These results demonstrate the viability of developing micro flow injection systems with an integrated fluorimetric detection cell.
Analytica Chimica Acta, 2004
The development of a multichannel photometer based on an array of eight light emitting diodes (LE... more The development of a multichannel photometer based on an array of eight light emitting diodes (LED) as light sources is described. Optical fibre bundles were employed to guide the light radiation of the LED to the measuring cell and from the cell to the photodiode detector. The instrument was controlled by a microcomputer and was designed to perform absorbance measurements in a single wavelength continuously as well as in all wavelengths (470, 500, 562, 590, 612, 636 and 654 nm) sequentially, allowing its application in multivariate analysis methods. The photometer presented a drift lower than 2 V h −1 , allowing absorbance measurements with a precision better than 0.003 units. The instrument was applied for the simultaneous determination of Zn(II) and Cu(II) in pharmaceutical and metallic alloy samples, whose pH adjustment followed by the addition of xylenol orange complexing agent were accomplished by employing a monosegmented flow system. Solutions containing both metal ions in the concentration range from 1.0 to 4.0 mg L −1 were employed to constructed the calibration model based on multiple linear regression (MLR), providing root mean square errors of prediction of 0.06 and 0.12 mg L −1 for Zn(II) and Cu(II), respectively. The results obtained by employing the photometer were compared with those obtained by flame atomic absorption spectrometry, showing no significant differences at the 95 % confidence level.
Analytica Chimica Acta, 2007
This work describes the use of a multi-LED photometer for discrimination of mineral water samples... more This work describes the use of a multi-LED photometer for discrimination of mineral water samples, employing chromogenic reagents and chemometric techniques. Forty-five water samples (including 7 different brands of mineral water and samples of deionised, distilled and tap waters) were analysed in a monosegmented flow system, using three different chromogenic reagents (murexide, PAR and eriochrome black T) in a pH 10.0 NH 3 /NH 4 + buffer in separate injections. Measurements were performed at 470, 500, 525, 562, 590, 612, 636 and 654 nm. Analyses were carried out using PCA, employing data sets including absorbance values obtained with one, two or all three reagents, which comprise 8, 16 or 24 variables, respectively. The best result was obtained with the data set from murexide and eriochrome black T, providing a clear distinction between 9 groups (distilled and deionised waters were classified in the same group). Based on the loading values, it was possible to select four wavelengths (470, 500, 590 and 654 nm) that provided a similar discrimination. With the use of these four LED, an HCA was performed, providing discrimination between 8 groups at a similarity level of 0.88. A model based on SIMCA allowed correctly classifying 94% of the samples. The discrimination between different groups is due to the metal ion contents in the water samples, mainly calcium and magnesium. Therefore, the use of common complexing reagents, such as murexide and erichrome black T, a multi-LED photometer and chemometric techniques provide an easy and simple method for water discrimination.
Journal of the Brazilian Chemical Society, 2013
Vis spectrophotometry (NH 4 +). In addition, by using a flow rate of only 40 µL min −1 for donor ... more Vis spectrophotometry (NH 4 +). In addition, by using a flow rate of only 40 µL min −1 for donor and acceptor solutions, a reduction of approximately 75% in residues generation was estimated by comparing the micro-analyzer with a conventional flow analysis system. These results demonstrate the viability of developing microflow analyzers with an integrated gas diffusion unit and conductometric detection.
Analytica Chimica Acta, 2007
A flow injection micro-analyser with an integrated injection device and photometric detection is ... more A flow injection micro-analyser with an integrated injection device and photometric detection is described. Channels measuring 205-295 m depth by 265-290 m maximum width were manufactured by deep UV lithography on two layers of urethane-acrylate oligomersbased photoresist. Hypodermic syringe needles (450 m 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 L and three different sample volumes (0.09, 0.22 and 0.30 L) 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 L 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.
Uploads
Papers by Alexandre Fonseca