Percorrer por autor "Lima, M. J. Reis"
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- Colorimetric determination of phosphorus in milk by flow injection analysis using a thermal/uv induced digestionPublication . Lima, M. J. Reis; Fernandes, S.M.V.; Rangel, António O. S. S.Two flow injection systems for the determination of total phosphorus in milk were developed: one for the determination of phosphorus using offline digestion of milk and another with in-line digestion. The first procedure involves a classical Kjeldahl digestion of milk prior to introducing it into the flow system; the second uses acid, heat and UV-catalysed peroxodisulphate oxidation to convert in-line all forms of phosphorus compounds into orthophosphate, allowing a considerable amount of time to be saved. Concentrations of phosphorus in milk withinthe range of 706 to 984 mg P/L were found and FIA results were in good agreement with those provided by the reference method.
- Determination of nitrate and nitrite in dairy samples by sequential injection using an in-line cadmium-reducing columnPublication . Lima, M. J. Reis; Fernandes, Sílvia M.V.; Rangel, António O.S.S.A sequential injection analysis (SIA) system for the spectrophotometric determination of nitrate in dairy samples was developed. A test portion was aspirated into a carrier solution containing ethylendiaminetetracetic acid (EDTA) and ammonium buffer, which flowed into a copperized cadmium reduction column installed in-line for the determination of the nitrate plus nitrite contents of samples. For the nitrite determination, another test portion of sample was aspirated and directly sent to the detector without reduction. Nitrate content was calculated from the difference between nitrate plus nitrite (expressed as nitrite) and nitrite content. The spectrophotometric determination is based on the Griess reaction. The proposed method was used to test several dairy samples (ultrapasteurized milk with 1.7% milk fat, whey, raw bovine milk and several cheese varieties). Results were statistically in good agreement with those provided by the reference procedure, with a detection limit of 0.15 mgL 1. A sampling rate of 21 determinations per hour can be achieved with this procedure.
- Enzymatic determination of urea in milk by sequential injection with spectrophotometric and conductometric detectionPublication . Lima, M. J. Reis; Fernandes, Sílvia M. V.; Rangel, António O. S. S.In this work, an analytical system based on the coupling of gas diffusion separation and sequential injection analysis for urea determination in milk is presented. A versatile manifold that could simultaneously be used for either spectrophotometric or conductometric detection was constructed. The sample and urease solution are sequentially aspirated into the holding coil and sent to a thermoreactor, where urea is enzymatically hydrolyzed by urease and converted into ammonium. This stream merges an alkaline solution at a confluence point where ammonia is formed. Ammonia diffuses through a hydrophobic membrane and modifies the bromothymol blue indicator color, when spectrophotometric detection is used, or changes the conductance of a boric acid solution acceptor stream, when conductometric detection is used.This methodology was applied to the determination of urea in 18 milk samples and the results were statistically comparable with those furnished by the enzymatic recommended procedure. The detection limits were 2.6 10-4 and 2.8 10-5 mol L-1 for conductometric and spectrophotometric detection, respectively. Repeatability (relative standard deviation, RSD) was better than 3.7% and 2.6% for conductometric and spectrophotometric detection, respectively.
- 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.