Adélio A. S. C. Machado

Fluorescence quenching of anthropogenic fulvic acids by Cu(II), Fe(III) and UO22+

The quenching of the fluorescence of three anthropogenic fulvic acids (FA) provoked by Cu(II) (pH 6.0), Fe(III) (pH 4.0) and UO(2)(2+) (pH 3.5), was analyzed by a non-linear method and by Stern-Volmer plots. The FA samples were extracted from composted sewage sludges (csFA), composted municipal wastes (mwFA) and composted livestock wastes (lsFA). Synchronous-scan fluorescence (SyF) spectra were collected as a function of metal ion concentration. Spectral data were treated by a self-modeling mixture analysis method (SIMPLISMA) to detect the SyF spectral band with the strongest quenching and to calculate the corresponding quenching profile. The analysis of these profiles by a non-linear method allowed the estimation of conditional stability constants (K) and of the percentage of non-complexing fluorophores. The same quantitative information was obtained by the modified Stern-Volmer equation taking into account the existence of fluorophores that do not participate in the complexation. Good agreement was found between the results of the two procedures. The logK calculated by the non-linear method were (standard deviation in parenthesis): csFA, Cu(II), 4.22 (5); Fe(III), 5.0 (1); UO(2)(2+), 5.2 (2); mwFA, Cu(II), 4.21 (3); Fe(III), 5.6 (2); UO(2)(2+), 4.7 (3); lsFA, Cu(II), 4.51 (8); Fe(III), 5.5 (2); UO(2)(2+), 3.6 (2).

Flow-through tubular PVC matrix membrane electrode without inner reference solution for flow injection analysis

A simply constructed tubular PVC matrix membrane electrode without inner reference solution suitable for flow injection analysis is described. The nitrate-selective electrode developed allows simple replacement of an exhausted membrane, or change of function with a new sensor membrane. The compact design ensures mechanical stability and the tubular configuration minimizes distortion of the hydrodynamic flow in the detector. Nitrate can be determined in the range 10−1–10−5 M, under different flow conditions, with a sampling frequency of about 120 h−1.

UREA POTENTIOMETRIC BIOSENSOR BASED ON UREASE IMMOBILIZED ON CHITOSAN MEMBRANES

Potentiometric biosensors based on urease (E.C. 3.5.1.5.) immobilized on chitosan membranes coupled to all-solid-state nonactin ammonium ion selective electrodes are described. The enzyme was immobilized on the chitosan membranes by four procedures: (A) adsorption; (B) adsorption followed by reticulation with dilute aqueous glutaraldehyde solution; (C) activation with glutaraldehyde followed by contact with the enzyme solution; and (D) activation with glutaraldehyde, contact with the enzyme solution and reduction of the Schiff base with sodium borohydride. The response characteristics of the biosensors obtained with these enzymatic membranes were determined and compared. The biosensor with best response characteristics, obtained by procedure (B), showed the following characteristics of response to urea: (i) linearity in the 10(-4) to 10(-2) M range; (ii) slope of up to 56 mV per decade; (iii) response time between 30 s and 2 min; and (iv) lifetime of 2 months. This biosensor was tested in the determination of urea in blood serum samples.

Seasonal variations of heavy metals in sediments and aquatic mosses from the Cávado river basin (Portugal)

Abstract Concentrations of Cd, Cr, Cu, Ni, Pb and Zn in surface sediments and aquatic mosses from the Cavado river basin were determined in order to evaluate the overall metal contamination and trace the main pollution sources. The natural background levels were calculated for both plants and sediments (fraction μ m) collected at uncontaminated sites and concentrations were normalized to the natural levels. The maximum resulting contamination factors in the aquatic mosses ranged from 6 (Zn) to 101 (Cr). In the sediments the accumulation rates are lower, between 3 (Zn) and 18 (Pb). The degree of contamination was also evaluated by calculating a metal pollution index and the more polluted reaches were identified. Metal concentration variations in plants and sediments in two different surveys (1988 and 1989) were studied by factor analysis. Three factors are sufficient to characterize data variance. Factors corresponding to plants and sediments show different metal composition and provide evidence that industrial effluents and drainage waters from a mining area are the main causes of variations of element concentrations. The seasonal variations of metal concentrations both in plants and sediments are also discussed.

Flow-Through Sandwich PVC Matrix Membrane Electrode for Flow Injection Analysis

Abstract A flow-through sandwich potentiometric detector for FIA in which a PVC membrane sensor is applied on a conductive epoxy support is described. It allows simple replacement of the membrane when exhausted or substituition of the sensor system and shows good mechanical and electric stability. The device can be easily included in a block with a location for the reference electrode. The detector was activated with a commercial sensor selective to nitrate and its behaviour in FIA was studied. A comparison of its perfomance with that of a nitrate selective electrode of classic shape constructed by the same technique is presented.

Single-cultivar extra virgin olive oil classification using a potentiometric electronic tongue

Label authentication of monovarietal extra virgin olive oils is of great importance. A novel approach based on a potentiometric electronic tongue is proposed to classify oils obtained from single olive cultivars (Portuguese cvs. Cobrançosa, Madural, Verdeal Transmontana; Spanish cvs. Arbequina, Hojiblanca, Picual). A meta-heuristic simulated annealing algorithm was applied to select the most informative sets of sensors to establish predictive linear discriminant models. Olive oils were correctly classified according to olive cultivar (sensitivities greater than 97%) and each Spanish olive oil was satisfactorily discriminated from the Portuguese ones with the exception of cv. Arbequina (sensitivities from 61% to 98%). Also, the discriminant ability was related to the polar compounds contents of olive oils and so, indirectly, with organoleptic properties like bitterness, astringency or pungency. Therefore the proposed E-tongue can be foreseen as a useful auxiliary tool for trained sensory panels for the classification of monovarietal extra virgin olive oils.

Array of potentiometric sensors for the analysis of creatinine in urine samples

The development of potentiometric biosensors for creatinine based on creatinine iminohydrolase (E.C. 3.5.4.21) immobilized on chitosan membranes coupled to a nonactin based ammonium ion selective electrode is described. The response characteristics of three types of biosensors with the enzyme immobilized by three different procedures were evaluated. The biosensors with better response characteristics were obtained by coupling the ammonium ion selective electrodes to chitosan membranes with the enzyme immobilized by adsorption. The linear response range of these biosensors to creatinine was 10(-4) to 10(-2) M, the response time was between 30 and 60 s, they showed an operational lifetime of 44 days and the slope of the response to creatinine in the first day varied between 50 and 52 mV decade-1. An array of six potentiometric sensors, constituted by two creatinine biosensors and four ion selective electrodes for potassium, sodium, ammonium and calcium was calibrated and a multivariate model based on PLS1 for the response to creatinine was obtained and validated. The array was used for the analysis of creatinine in urine samples and the results were compared with the results of a clinical analysis laboratory, based on the Jaffé reaction.

Sequential flow-injection determinations of calcium and magnesium in waters

Abstract A tubular PVC membrane electrode for calcium without inner reference solution and a device for location of the reference electrode are described. In the flow-injection system, calcium is determined potentiometrically and then magnesium is determined by atomic absorption spectrometry. The electrode provides linear response to calcium in the range 5 × 10 −5 /2-10 −1 M. On-line dilution of the sample allows magnesium determination in the range 0/2-10 mg l − . Flow rates between 3 and 6 ml min −1 are possible. The sampling frequency is 60/2-90 h −1 .

Characterisation of poly(vinyl chloride) barium ion-selective electrodes without an internal reference solution

Two types of poly(vinyl chloride)(PVC) barium ion-selective electrodes are compared. Each electrode is based on a sensor formed from the tetraphenylborate of the non-ionic surfactant Antarox CO-880 and its complex with barium (tetraphenylborate complex) and uses 2-nitrophenyl phenyl ether as solvent mediator. The first electrode is an original type, fabricated as a classic PVC model, whereas in the second, the PVC sensor material is applied to the outer surface of an epoxy resin and has no internal filling solution. Apart from the fact that the epoxy-based electrode has a longer lifetime, the two types of ISE possess virtually identical electrochemical properties. The electrodes were also assessed for measuring sulphate by analate subtraction.

Array of potentiometric sensors for simultaneous analysis of urea and potassium.

Urea biosensors based on urease immobilized by crosslinking with BSA and glutaraldehyde coupled to ammonium ion-selective electrodes were included in arrays together with potassium, sodium and ammonium PVC membrane ion-selective electrodes. Multivariate calibration models based on PCR and PLS2 were built and tested for the simultaneous determination of urea and potassium. The results show that it is possible to obtain PCR and PLS2 calibration models for simultaneous determination of these two species, based on a very small set of calibration samples (nine samples). Coupling of biosensors with ion-selective electrodes in arrays of sensors raises a few problems related to the limited stability of response and unidirectional cross-talk of the biosensors, and this matter was also subjected to investigation in this work. Up to three identical urea biosensors were included in the arrays, and the data analysis procedure allowed the assessment of the relative performance of the sensors. The results show that at least two urea biosensors should be included in the array to improve urea determination. The prediction errors of the concentration of urea and potassium in the blood serum samples analyzed with this array and a PLS2 calibration model, based on nine calibration samples, were lower than 10 and 5%, respectively.