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This work showed a flow system designed with solenoid valves for preconcentration of fluoride using SPADNS method in water samples. The analyte was preconcentrated in a mini-column coated with Layered Double Hydroxides (LDH) used as adsorbent. Then, the fluoride ions were eluted with 0.5molL-1 sodium hydroxide and determined by spectrophotometry. The variables that affect the system such adsorbent mass, type of eluent, solutions flow rate, reagent concentration and pH effect were critically evaluated. Under optimized conditions, the detection limit, coefficient of variation, linear range and preconcentration factor were estimated at 15µgL-1 (99.7% confidence level), 0.8% (500µgL-1, n = 10), 50-500µgL-1 and 10, respectively. The accuracy of the method was evaluated by analysis of ALPHA APS 1076 (Simulated Rain Water) certified material, the values were not significantly different at a 95% level of confidence. The method was applied for fluoride determination in water samples and the levels found were below the maximum values established by Brazilian environmental and health legislations.

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An environmentally-friendly flow system was developed for the determination of fluoride in natural waters, based on its reaction with zirconium ions and 2-(parasulfophenylazo)-1,8-dihydroxy-3,6-naphthalene-disulfonate (SPADNS). The flow system was designed with solenoid valves in order to increase the versatility and improve the analytical performance, since each device handles the solution independently leading to a more efficient use of reagents. The minimization of reagent consumption and waste generation favors the development of a noteworthy system. Under optimized conditions, a linear response was observed within the range of 0.1-2.2 mg L(-1), with the detection limit, coefficient of variation and sampling rate estimated as 0.02 mg L(-1), 4.1% and 60 determinations per hour, respectively. In order to analyze samples containing high fluoride content a wider linear range (0.3-6.6 mg L(-1)) can be obtained by using a low sample flow rate and low sample volume. In comparison with other methods, the proposed system offers advantages such as wider analytical range (1.7-2.2 times), higher sample throughput (1.5-4.0 times) and lower quantification limit (by a factor of 5-68). Along with the consumption of reagents being 20-55 times lower, the generation of wastes is 1.2-3.0 times lower, which contributes to the development of a 'greener' system. This method is fast, amenable to automation, environmentally-friendly and of low-cost. In addition, it could be successfully applied to the determination of fluoride in water samples, the results obtained being in agreement with those of the ISE method.

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Metal contaminants are generally removed from effluents by chemical and physical processes which are often associated with disadvantages such as the use of toxic reagents, generation of toxic waste and high costs. Hence, new techniques have been developed, among them the study of natural adsorbents, for instance, the use of Moringa oleifera seeds. The potential of M. oleifera seeds for nickel removal in aqueous systems was investigated. The seeds utilized were obtained from plants grown in Uberlândia/Brazil. After being dried and pulverized, the seeds were treated with 0.1 mol/L NaOH. Fourier transform infrared spectroscopy, scanning electron microscopy and thermogravimetric analyses were used for the characterization of the material. Using the optimized methodology (50 mL of 4.0 mg/L Ni(II), pH range of 4.0-6.0, agitation time of 5 min and adsorption mass of 2.0 g) more than 90% of Ni(II) could be removed from water samples. The sorption data were fitted satisfactorily by the Langmuir adsorption model. Evaluation applying the Langmuir equation gave the monolayer sorption capacity as 29.6 mg/g. The results indicate that this material could be employed in the extraction of nickel, considering its ease of use, low cost and environmental viability, which make it highly attractive for application in developing countries.

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In this study, a method for the determination of low concentrations of lead in beer samples using solid-phase extraction with a flow injection analysis system and detection by flame atomic absorption spectrometry (FAAS) was developed. Moringa oleifera seeds were used as a biosorbent material. Chemical and flow variables of the online preconcentration system, such as sample pH, preconcentration flow rate, eluent flow rate, eluent concentration, particle size, and sorbent mass, were studied. The optimum extraction conditions were obtained using a sample pH of 6.0, sample flow rate of 6.0 mL min(-1), 63.0 mg of sorbent mass, and 2.0 mol L(-1) HNO(3) at a flow rate of 2.0 mL min(-1) as the eluent. With the optimized conditions, the preconcentration factor, precision, detection limit, consumption index, and sample throughput were estimated as 93, 0.3% (10.0 µg L(-1), n = 7), 7.5 µg L(-1), 0.11 mL, and 23 samples per hour, respectively. The method developed was successfully applied to beer samples and recovery tests, with recovery ranging from 80% to 100%.

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This paper reports a method for the determination of zinc in Brazilian sugar cane spirit, (cachaça in Portuguese), using solid-phase extraction with a flow injection analysis system and detection by FAAS. The sorbent material used was activated carbon obtained from Moringa oleifera husks. Flow and chemical variables of the proposed system were optimized through multivariate designs. The factors selected were sorbent mass, sample pH, sample flow rate, and eluent concentration. The optimum extraction conditions were obtained using a sample pH of 4.0, a sample flow rate of 6.0 mL min(-1), 30.0 mg of sorbent mass, and 1.0 mol L(-1) HNO(3) as the eluent at a flow rate of 4.0 mL min(-1). The limit of detection for zinc was 1.9 µg L(-1), and the precision was below 0.82% (20.0 µg L(-1), n = 7). The analytical curve was linear from 2 to 50 µg L(-1), with a correlation coefficient of 0.9996. The method developed was successfully applied to spiked Brazilian sugar cane spirit, and accuracy was assessed through recovery tests, with results ranging from 83% to 100%.

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The development, evaluation and application of a simple and low-cost graphite carbon electrode for the direct determination of citrate in food samples are described here. The electrode exhibits a linear response with a slope of -29.0 ± 1.0 mV decade(-1) in a concentration range of 0.07-7.0 mmol L(-1) in 0.1 mol L(-1) KCl/1.0 mmol L(-1) phosphate buffer solution with a limit of detection of 3.0 µmol L(-1). The electrode is easily constructed at a relatively low cost and has a fast time response (within 120 s) with no significant changes in its performance characteristics. The performance of the graphite sensor was tested to determine citrate in beverage samples (juices and an isotonic drink), and the results were validated against a reference procedure. The proposed method is quick, inexpensive, selective and sensitive, and is based entirely on conventional instrumentation.

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A method using bi-directional electrostacking (BDES) in a flow system is presented for As preconcentration and speciation analysis. Some parameters such as electrostacking time and applied voltage, support buffers and their concentrations were investigated. Boric acid plus sodium hydroxide at 0.1mol/l concentration was selected as support buffer to improve the pre-concentration factor (PF) for As(V). An analytical range from 2.0 to 50.0mugl(-1), and 0.35mugl(-1) as limit of detection, when applied 750V for 20min, were achieved. Under these conditions, a pre-concentration factor of 4.8 was obtained. The proposed method was applied to determine As(V) in mineral water and natural water samples (river, fountain and gold mine) from Ouro Preto city. Recoveries from 93.5 to 106.4% were achieved at 10mugl(-1) added As level (R.S.D.s between 3 and 7%). Potassium permanganate (10mgl(-1)) was used for oxidising As species in order to determine total As, being established the concentration of As(III) from the difference between total As and As(V).