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- Sequential injection determination of nitrate in vegetables by spectrophotometry with inline cadmium reductionPublication . Oliveira, Sara M.; Lopes, Teresa I. M. S.; Rangel, António O. S. S.A sequential injection system for the determination of nitrate (NO3 2) in vegetables was developed to automate this determination, allowing for substantially reduced reagent consumption and generated waste using low-cost equipment. After extraction with water and filtration, the extracted nitrate is reduced inline to nitrite in a copperized cadmium (Cd) column and determined as nitrite. According to the Griess–Ilosvay reaction, nitrate is diazotized with sulfanilamide and coupled with N-(1-naphtyl)-ethylenediamine dihydrochloride to form a purple-red azo dye monitored at 538 nm. Nitrate can be determined within a range of 1.35–50.0 mg L21 of NO3 2 (corresponding to 0.270–10.0 g of NO3 2 per kg of vegetable), with a conversion rate of nitrate to nitrite of 99.1+0.8%. The results obtained for 15 vegetable extracts compare well with those provided by the classical procedure, with a sampling throughput of 24 determinations per hour and relative standard deviations better than 1.2%.
- Use of tetramethylbenzidine for the spectrophotometric sequential injection determination of free chlorine in watersPublication . Mesquita, Raquel B. R.; Noronha, M. Lúcia F.O. B.; Pereira, Ana I. L.; Santos, Arménia C. F.; Torres, André F.; Víctor, Cerdà; Rangel, António O. S. S.A sequential injection (SI) method was developed for the spectrophotometric determination of chlorine based on the reaction between tetramethylbenzidine (TMB) and free chlorine. The advantages resulting from the use of TMB are considerable: TMB is highly selective for chlorine, it enables a fairly low quantification limit and represents a less toxic alternative to reagents such as tolidine. The use of this reaction in SI adds other advantages as it enhances the degree of automation, minimisation of reagent consumption (6.8 g TMB/assay) and low effluent production (2.5 mL/determination). The developed method allowed a quantification limit of 90 g/L with a working range of 0.09–1.30 mg OCl−/L and a determination rate of 60 det./h. Based on these features, the system was applied to tap-water and surface water samples with no previous treatment required. The results obtained with the developed system were compared to the reference method, diethyl-p-phenylelediamine (DPD) colorimetric method, and proved not to be statistically different.
- Development of a flow method for the determination of phosphate in estuarine and freshwaters - Comparison of flow cells in spectrophotometric sequential injection analysisPublication . Mesquita, Raquel B. R.; Ferreira, M. Teresa S. O. B.; Tóth, Ildikó V.; Bordalo, Adriano A.; McKelvied, Ian D.; Rangel, António O. S. S.A sequential injection system with dual analytical line was developed and applied in the comparison of two different detection systems viz; a conventional spectrophotometer with a commercial flow cell, and a multi-reflective flow cell coupled with a photometric detector under the same experimental conditions. The study was based on the spectrophotometric determination of phosphate using the molybdenum-blue chemistry. The two alternative flow cells were compared in terms of their response to variation of sample salinity, susceptibility to interferences and to refractive index changes. The developed method was applied to the determination of phosphate in natural waters (estuarine, river, well and ground waters). The achieved detection limit (0.007 mu M PO43-) is consistent with the requirement of the target water samples, and a wide quantification range (0.024-9.5 mu M) was achieved using both detection systems.
- A flow system for the spectrophotometric determination of lead in different types of waters using ion-exchange for pre-concentration and elimination of interferencesPublication . Mesquita, Raquel B. R.; Fernandes, Sílvia M.V.; Rangel, António O.S.S.A flow system for the spectrophotometric determination of lead in natural and waste waters is proposed. The determination is based on the colorimetric reaction between malachite green and iodide, followed by the formation of a ternary complex between those reagents and lead cations. The developed flow system includes a lead pre-concentration step in a column packed with a cationic resin (Chelex 100) operating in a sequential injection mode. To improve the mixture of sample and reagents, a flow injection approach was adopted for the colorimetric determination. This way a hybrid flow system, involving both sequential and flow injection concepts was designed. Another feature of the proposed system is the efficient elimination of major interferent species, such as cadmium and copper. The elimination of cadmium interference is obtained by complexing Cd2+ with chloride and retaining the formed negatively charged complexes in an anionic resin, AG1 X-8. As for copper, with the presence of both ionic resins as well as the conditions for cadmium elimination, it no longer acts as an interferent. Different ranges of lead concentration (50–300 and 300–1000 g l−1) can be determined with minor changes in the controlling software, useful for application to both natural and waste waters. Therefore, a detection limit of 25 g l−1 was achieved. Repeatability was evaluated from 10 consecutive determinations being the results better than 4%. The recoveries of lead spikes added to the samples ranged from 93 to 102%. The sampling frequency was 17 and 24 determinations per hour, for 50–300 and 300–1000 g l−1 ranges, respectively.
- Sequential injection system for the spectrophotometric determination of reducing sugars in winesPublication . Araújo, Alberto N.; Lima, José L. F. C.; Rangel, António O. S. S.A sequential injection system for the spectrophotometric determination of reducing sugars in wines is described. The methodology is based on the formation of a coloured complex produced by the reaction of copper (I) with 2,9-dimethyl-1,10-phenanthroline (neocuproine), after reduction of copper (II) to copper (I) by reducing sugars. In the present SI system, a dialysis unit was incorporated not only to allow the sample dilution, but also to minimise the interference of some coloured compounds. In this way, direct sample introduction into the system was possible, without any previous treatment of the wine samples. The proposed method can be applied to the determination of reducing sugars in two concentration ranges: from 2 to 25 g l 1 (table wines) and from 20 to 140 g l 1 (Port wines). To perform determinations in these two ranges, the manifold configuration remained the same; just a few operational parameters were changed in the controlling software. A sampling-rate of 14–18 samples per hour was obtained with good repeatability for ten consecutive injections of wine samples (relative standard deviations (RSD) B2.1%). The results obtained from 19 wine samples were comparable to those obtained by the reference method.
- Spectrophotometric determination of zinc and copper in a multisyringe injection analysis system using a liquid waveguide capillary cell: Application to natural watersPublication . Páscoa, Ricardo N. M. J.; Tóth, Ildikó V.; Rangel, António O. S. S.This work exploits a multisyringe injection analysis (MSFIA) system coupled with a long liquid waveguide capillary cell for the spectrophotometric determination of zinc and copper in waters. A liquid waveguide capillary cell (1.0mpathlength, 550mmi.d. and 250mL internal volume) was used to enhance the sensitivity of the detection. The determination for both ions is based on a colorimetric reaction with zincon at different pH values. The developed methodology compares favourably with other previously described procedures, as it allows to reach low detection limits for both cations (LODs of 0.1 and 2mg L−1, for copper and zinc, respectively), without the need for any preconcentration step. The system also provided a linear response up to 100mg L−1 with a high throughput (43 h−1) and low reagent consumption and effluent production. The developed work was applied to natural waters and three certified reference water samples.
- Sequential injection lab-on-valve system for the for the determination of the activity of peroxidase in vegetablesPublication . Vidigal, Susana S. M. P.; Toth, Ildiko V.; Rangel, António O. S. S.Horseradish peroxidase (HRP) has been broadly used and investigated for many analytical purposes; it is an enzyme that catalyzes the reduction of hydrogen peroxide in the presence of a reducing compound. The objective of this work was to develop a methodology for the spectrophotometric determination of the activity of peroxidase in vegetable extracts using a flow method with a sequential injection lab-on-valve format. The developed system is based on the reaction between hydrogen peroxide (H2O2) and 2,2-azinobis(3-ethylbenzothiazoline-6)sulfonic acid (ABTS) catalyzed by the enzyme (HRP). The method presented a sample consumption of 15 μL per assay and a consumption of ABTS and H2O2 of 24 μg and 12 μg per assay, respectively. It was also possible to monitor online the thermal inactivation of peroxidase at different temperature ranges.
- Sequential injection system for the enzymatic determination of ethanol in alcoholic beverages with in-line dilutionPublication . Pais, Teresa F. M.; Vidigal, Susana S. M. P.; Tóth, Ildikó V.; Rangel, António O. S. S.A sequential injection system for the enzymatic determination of ethanol in wines with spectrophotometric detection was developed. The determination was based on the enzymatic reaction involving ethanol and NAD(+) in the presence of alcohol dehydrogenase. A gas-diffusion unit was introduced in the system to perform an in-line dilution. It was possible to operate the system with a reagent consumption of 1.3 mg of NAD(+), 0.45 U of ADH, and 32 mu L of sample, per assay with a determination rate of 21 h(-1). The results obtained from the analysis of wine samples were statistically equivalent to those obtained by the reference method, with good repeatability; the relative standard deviation (n = 10) was <1% with a LOQ of 0.3% (v/v). (C) 2012 Elsevier Ltd. All rights reserved.
- A multi-commuted flow injection system with a multi-channel propulsion unit placed before detection: Spectrophotometric determination of ammoniumPublication . Oliveira, Sara M.; Lopes, Teresa I. M. S.; Tóth, Ildikó V.; Rangel, António O. S. S.A flow system with a multi-channel peristaltic pump placed before the solenoid valves is proposedto overcome some limitations attributed to multi-commuted flow injection systems:the negative pressure can lead to the formation of unwanted air bubbles and limits the useof devices for separation processes (gas diffusion,dialysis or ion-exchange). The proposed approach was applied to the colorimetric determination of ammonium nitrogen. In alkaline medium, ammonium is converted into ammonia, which diffuses over the membrane,causing a pH change and subsequently a colour change in the acceptor stream (bromothymol blue solution). The system allowed the re-circulation of the acceptor solution and was applied to ammonium determination in surface and tap water, providing relative standard deviations lower than 1.5%. A stopped flow approach in the acceptor stream was adopted to attain a low quantification limit(42ugL−1) and a linear dynamic range of 50–1000ugL−1 with a determination rate of 20h−1.
- Development of a gas diffusion multicommuted flow injection system for the determination of sulfur dioxide in wines, comparing malachite green and pararosaniline chemistriesPublication . Oliveira, Sara M.; Lopes, Teresa I. M. S.; Tóth, Ildikó V.; Rangel, António O. S. S.A flow system based on the multicommutation concept was developed for the determination of free and total sulfur dioxide in table wines, exploiting gas diffusion separation and spectrophotometric detection. The system allowed the comparison of malachite green and pararosaniline chemistries, using the same manifold configuration. Free and total SO2 were determined within the ranges 1.00-40.0 and 25.0-250 mg L-1, at determination throughputs of 25 and 23 h-1, respectively. Employing the malachite green reaction, detection limits of 0.3 and 0.8mgL-1 were attained for free and totalSO2, respectively. Pararosaniline chemistry provided detection limits of 0.6 mg L-1 for free SO2 and 0.8 mg L-1 for total SO2. Relative standard deviations better than 1.8 and 1.4% were obtained by the malachite green and pararosaniline reactions, respectively. With regard to the two tested chemistries, 18 wines were analyzed and the results achieved by the pararosaniline reaction compared better with those furnished by the recommended procedure.