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- A sequential injection system for the in-line digestion and colorimetric determination of phosphorus in milkPublication . Lima, M.J. Reis; Fernandes, S.M.V.; Rangel, António O. S. S.A sequential injection analysis (SIA) system involving in-line digestion for the determination of total phosphorus in milk is presented. The developed manifold consists mainly of 2 phases: conversion of all forms of phosphorus into orthophosphates by hydrolysis and oxidation catalyzed by UV radiation (necessary for the efficient oxidation of organic phosphates) and spectrophotometric determination of the phosphomolybdenum blue formed. The proposed method was applied to several cow milk samples and the results were in good agreement with those provided by the reference method, with a maximum relative deviation of 5.2%. The detection limit of the method was around 2 mg P/L with a sampling rate of approximately 17 h−1.
- 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.
- Synthesis and characterization of a 3-hydroxy-4-pyridinone chelator functionalized with a polyethylene glycol (PEG) chain aimed at sequential injection determination of iron in natural watersPublication . Mesquita, Raquel; Moniz, Tânia; Miranda, Joana L. A.; Gomes, Vânia; Silva, André M. N.; Rodriguez-Borges, J. E.; Rangel, António O. S. S.; Rangel, MariaThe synthesis of a highly water soluble 3-hydroxy-4-pyridinone ligand, functionalized with a hydrophilic ethylene glycol chain (PEG-HPO), was successfully achieved and is reported together with that of its iron (III) complex. The improved hydrophilicity of both the PEGylated 3,4-HPO ligand and its iron(III) complex were fully investigated in an analytical application and, the new chelator is proposed for the spectrophotometric sequential injection determination of iron in waters. The new ligand provided better sensitivity and a lower LOD for iron determination than that obtained for N-alkyl-3-hydroxy-4-pyridinone ligands. The developed sequential injection method presents a dynamic working range of 0.10-1.00 mg Fell. with a LOD of 48 mu g/L. Due to the use of a sequential injection system, the overall effluent production was <2 mL corresponding to the consumption of 44 mu g of PEG-HPO, 0.71 mg of NaHCO3, 0.92 mg of HNO3 and 500 mu L of sample. Two reference samples were assessed for accuracy studies and a relative deviation <5% was obtained; eight waters samples were analyzed and the results compared with the reference procedure, and no statistical difference was observed for the two sets of results.
- 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.
- A gas diffusion sequential injection system for the determination of sulphur dioxide in winesPublication . Segundo, Marcela A.; Rangel, António O. S. S.In the present work, a sequential injection system with spectrophotometric detection was developed for the determination of free and total sulphur dioxide in wines. It was based on the formation of a coloured product from the reaction among SO2, formaldehyde and pararosaniline. A gas diffusion unit (GDU) was incorporated into the manifold to prevent the wine matrix interference in the spectrophotometric measurement. An acid solution was added to the sample prior to its passage through the donor channel of the GDU to promote gaseous SO2 formation. For the free SO2 determination, the sample was directly aspirated into the holding coil; for the total SO2 determination, the sample was processed after previous in-line hydrolysis of bound SO2 with an alkali solution. Two second-order calibration curves were established, defining two concentration ranges: 2–40 mg l−1 for the free SO2 determination and 25–250 mg l−1 for the total SO2 determination. Relative standard deviations (n=10) were lower than 1.2% for the determination of free SO2 and lower than 2.3% for the determination of total SO2. The sample frequency was about 16 h−1. This methodology was applied to the determination of free and total sulphur dioxide in 10 table wines and the results were statistically comparable with those furnished by the recommended procedure.
- A new approach for the sequential injection spectrophotometric determination of the total antioxidant activityPublication . Lima, M. J. Reis; Tóth, Ildikó V.; Rangel, António O. S. S.A sequential injection system based on the ABTS (2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulphonic-acid) methodology was developed. The proposed method, incorporating a mixing chamber in the side port of the selection valve, was evaluated to measure the total antioxidant activity of several beverages and foods. The ABTS•+ is generated by oxidation of ABTS with potassium persulfate and is reduced in presence of hydrogen-donating antioxidants converting into a colourless product. The applicability of the developed method was tested by measurement of the antioxidant activity of pure compounds as well as by analysing complex food and beverage samples. The antioxidant activity was presented as l(+) ascorbic acid equivalence. The values obtained by this methodology were not significantly different from the results obtained by the original spectrophotometric ABTS assay. For most of the studied antioxidants, antioxidant activity varied with pH and dilution. The proposed SIA system is suitable for screening direct or diluted total antioxidant activity of pure compounds or food samples.
- Gas diffusion sequential injection system for the spectrophotometric determination of free chlorine with o-dianisidinePublication . Mesquita, Raquel B. R.; Rangel, António O. S. S.A gas diffusion sequential injection system for spectrophotometric determination of free chlorine is described. The detection is based in the colorimetric reaction between free chlorine and a low toxicity reagent o-dianisidine. A gas diffusion unit is used to isolate free chlorine from the sample in order to avoid possible interferences. This feature results from the conversion of free chlorine to molecular chlorine (gaseous) with sample acidification. With minor changes in the operating conditions, two different dynamic ranges were obtained enhancing the application both to water samples and bleaches. The results obtained with the developed system were compared to the reference method, iodometric titration and proved not to be statistically different. A detection limit of 0.6 mg ClO−/L was achieved. Repeatability was evaluated from 10 consecutive determinations being the results better than 2%. The two dynamic ranges presented different determination rates: 15 h−1 for 0.6–4.8 mg ClO−/L (water samples) and 30 h−1 for 0.047–0.188 g ClO−/L (bleaches).
- Iodine speciation in coastal and inland bathing waters and seaweeds extracts using a sequential injection standard addition flow-batch methodPublication . Santos, Inês C.; Mesquita, Raquel B. R.; Bordalo, Adriano A.; Rangel, António O. S. S.The present work describes the development of a sequential injection standard addition method for iodine speciation in bathing waters and seaweeds extracts without prior sample treatment. Iodine speciation was obtained by assessing the iodide and iodate content, the two inorganic forms of iodine in waters. For the determination of iodide, an iodide ion selective electrode (ISE) was used. The indirect determination of iodate was based on the spectrophotometric determination of nitrite (Griess reaction). For the iodate measurement, a mixing chamber was employed (flow batch approach) to explore the inherent efficient mixing, essential for the indirect determination of iodate. The application of the standard addition method enabled detection limits of 0.14 µM for iodide and 0.02 µM for iodate, together with the direct introduction of the target water samples, coastal and inland bathing waters. The results obtained were in agreement with those obtained by ICP-MS and a colorimetric reference procedure. Recovery tests also confirmed the accuracy of the developed method which was effectively applied to bathing waters and seaweed extracts.
- Use of a Mixing Chamber for Sample Preparation and Multiple Collection in Sequential Injection Analysis: Determination of Sulfate in WinesPublication . Silva, Helena R.; Segundo, Marcela A.; Rangel, António O. S. S.A sequential injection system for determination of sulfate in wines was developed. It was based on the formation of BaSO4, measured turbidimetrically at 420 nm. As sample dilution and acidification were required, both sample and acid were sent to a mixing chamber placed in a lateral port of the selection valve; after thorough mixing, up to six aliquots could be drawn and analyzed. A linear calibration curve was established using 10% v/v ethanol standards with concentrations between 300 and 1500 mg L-1 of K2SO4. The results obtained by the present methodology were statistically comparable with those furnished by the reference procedure, with relative deviations < 3.5%. The sample frequency was about 5 samples per h with relative standard deviations (RSD) < 10%.