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- Construction and use of a tubular picrate ion-selective electrode for reducing sugar determination in Port wine by flow-injection analysisPublication . Lopes, Teresa I. M. S.; Rangel, António O. S. S.; Lima, José L. F. C.; Montenegro, M. Conceição B. S. M.A picrate ion-selective electrode without inner reference solution and based on bis(triphenylphosphoranylidene)ammonium picrate dissolved in 2-nitrophenyloctyl ether was constructed and its characteristics assessed. With respect to reducing sugar determination in Port wine, a tubular electrode based on the same sensor system was developed and incorporated into a flow-injection manifold devised for this determination. The methodology involves a reaction between the reducing sugar and picric acid, the decrease in the picrate concentration being monitored with the tubular electrode. A linear dependence between peak width, at a fixed potential and the logarithm of sugar concentration in the range 25–200 g l−1 was obtained. The proposed system allows wine samples to be injected with no previous treatment and allows a sample throughput of 50 determinations per hour, a relative standard deviation less than 4%, and the results are comparable to the reference procedures.
- Merging zones standard addition technique for determination of copper in beer by flow injection atomic absorption spectrophotometry.Publication . Fernandes, Sílvia M. V.; Rangel, António O. S. S.; Lima, José L. F. C.A flow injection system for determination of copper in beer by atomic absorption spectrophotometry by the standard additions method is described. The manifold, based on the merging zone technique, prevents the burner head from clogging, as observed with the conventional reference method. With 5 standard additions, results are comparable with those of the reference method. Relative deviations were less than 5.8%, precision was better than 6.4%, and sampling rate was about 30 samples/ h. A less precise, less accurate, but faster procedure (75 samples/h) is possible with only 2 standard additions. The detection limit was 5 micrograms/L.
- Potentiometric determination of chloride in vegetables by flow injection analysisPublication . Lima, José L. F. C.; Rangel, António O. S. S.; Souto, M. Renata S.A flow injection system for the potentiometric determination of chloride (Cl) in vegetables is reported. A tubular chloride ion-selective electrode based on a homogeneous crystalline membrane without inner reference solution was used as indicator electrode. A methodology to minimize the influence of the sample matrix on the electrodes response is described. The results obtained with the developed flow injection manifold are in good agreement with those provided by the reference method and present good precision (relative standard deviations lower than 2%). Sample throughput ranges from 150 to 300 determinations per hour
- Enzymatic determination of L(+) lactic and L(-) malic acids in wines by flow-injection spectrophotometryPublication . Lima, José L. F. C.; Lopes, Teresa I. M. S.; Rangel, António O. S. S.A flow-injection system for the enzymatic determination of L( ) lactic acid and L(ÿ) malic acid in wines with spectrophotometric detection is described. The samples are dialysed in-line, and the enzymes in solution (malate dehydrogenase and lactate dehydrogenase) are injected as a train of plugs in the acceptor stream of the dialysis unit, yielding two peaks corresponding to the NADH formed for each determination. This methodology enables the determination of both acids with a single detector with a sampling rate of 20 h ÿ1 (0.4±3 g l ÿ1). The results are comparable to those obtained by the reference procedure, the repeatability is better than 5% (rsd), with low enzyme consumption (1.3 ml of suspension per sample).
- Spectrophotometric determination of bromate in drinking water using a multi-syringe flow injection systemPublication . Oliveira, Sara M.; Segundo, Marcela A.; Lima, José L. F. C.; Cerdà, Victor; Rangel, António O. S. S.
- 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.
- Determinação de anião nitrato em águas por FIA usando um detector potenciométrico tubularPublication . Lima, José L. F. C.; Rangel, António O. S. S.; Souto, M. E. Renata S.In this work, a flow injection system for the potentiometric determination of nitrate content in waters, is described. A tubular nitrate ion-selective electrode without inner reference solution and an ion-exchanger system composed of (4,7-diphenylphenanthroline) nickelII nitrate in o-nitrophenil octyl ether was used as indicator electrode. These units were evaluated in two ionic strength adjuster solutions and their operational characteristics compared with those of conventionally shaped electrodes with the more sensor systems. The developed methodology allows the determination at nitrate anion in waters without a high sample pretreatment and with a high sampling rate (ranging from 60 to 120 determinations per hour) and precision (relative standard deviations interior to 2%). Results are in good agreement with the reference procedure and the lower limit of linear response is 14.9 mg L1.
- Determination of iron in soils by flow injection atomic absorption spectrometryPublication . Ferreira, M. R. Alexandra; Rangel, António O. S. S.; Lima, José L. F. C.A single-channel flow injection system was optimized for the determination of available iron (Fe) in soil extracts by atomic absorption spectrophotometry. This method of introducing the samples in the spectrophotometer worked particularly well in preventing blockage of the burner head which was observed in the conventional introduction of Fe for its determination by atomic absorption spectrophotometry. The appropriate selection of the manifold parameters, such as injection volume, tube length and flow rate, allowed introduction of any soil extract without requiring any pre-treatment. This system allowed determinations at a detection limit of 0.36 mg L-1 to 5 mg L-1, with an output of 300 determinations per hour. The results obtained for analysis of 15 soil extracts were in good agreement with those provided by the colorimetric method, with average relative deviations of 1.6%. Relative standard deviations of 4.8, 2.5, and 2.3% were obtained for contents of 1.03, 1.85, and 3.99 mg Fe L-1, respectively.
- Automatic flow system for sequential determination of ABTS scavenging capacity and Folin-Ciocalteu index: A comparative study in food productsPublication . Magalhães, Luís M.; Segundo, Marcela A.; Reis, Salette; Lima, José L. F. C.; Tóth, Ildikó V.; Rangel, António O. S. S.In the present work, an automatic flow procedure for the sequential spectrophotometric determination of Folin-Ciocalteu reducing capacity (FC assay) and 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS•+) scavenging capacity expressed as the trolox equivalent (TEAC assay) is proposed for a comparative study of antioxidant properties in food products. Exploiting the flexibility of flow management associated to the computer control offered by multisyringe flow injection analysis, both methodologies were carried out in the same manifold using gallic acid and trolox as standard compounds. The proposed system configuration allowed the performance of each method separately or in tandem, providing 24 determinations per hour, which accounts for its application in routine analysis. The present methodology was applied to a large number of beverages (n = 72), namely red and white wines, herbal and tea infusions, juices and beers. The results obtained showed that FC reducing capacity and TEAC values of red wines were significantly different from those obtained for the other beverages, while tea infusions provided significantly higher TEAC values when compared to white wines, herbal infusions, juices and beers. A good correlation was found between TEAC and FC reducing capacity (R > 0.9) for red wines, herbal and tea infusions, and beers. For these beverages, similar slope values were found (106–140 mg L−1 of gallic acid per mM of Trolox), except for beers that showed a higher response for FC assay. These results provided evidence that the correlation between these assays vary according to the type of sample, reinforcing the idea that more than one method should be used for evaluation of antioxidant capacity.
- Flow injection titration of chloride in food products with a silver tubular electrode based on an homogeneous crystalline membranePublication . Ferreira, Isabel M. P. L. V. O.; Lima, José L. F. C.; Rangel, António O. S. S.A flow injection system for the pseudo-titration of chlorides in food products with potentiometric detection has been developed. For this purpose, a silver ion selective tubular electrode based on a homogeneous crystalline membrane, prepared by pressing silver sulphide at high pressure, was constructed. Its operational characteristics were evaluated and compared with the corresponding conventionally shaped electrodes obtained with the same sensor. The results obtained for chloride determination in wine, milk, beer and vinegar by using the FIA methodology were in good agreement with those provided by reference procedures. The sampling rate achieved varied between 120 and 360 samples per hour.