Ultrasonic solid-liquid extraction (USLE) of trace metals from biological and environmental samples and its subsequent quantification by electrothermal atomic absorption spectrometry (ET-AAS) is nowadays an emerging methodology in the analytical laboratory. However, this methodology is far from maturity as can be readily demonstrated from the controversial data reported by different workers. In the light of our own experience and a survey of published data, a general approach for USLE-ET-AAS is discussed, taking into account the different variables which affect ultrasonic solid-liquid extraction, namely, the ultrasonic device chosen for ultrasonic extraction (e.g. ultrasonic bath or ultrasonic probe), particle size, acid concentration, sonication time and sonication amplitude, sample mass and analyte-matrix binding.
The interaction between Cu(II) and pectin extracted from citrus fruit was studied in KNO 3 0.10 mol dm À3 at 25 8C and pH 5.5, using ion selective electrode potentiometry and voltammetry, namely differential pulse polarography and square-wave voltammetry. Although many independent variables may affect Cu(II)-polymer interactions such as charge density, polymer concentration and copper to polymer concentration ratio, a good fitting was observed for the model with ML and ML 2 complex species, when M:L total concentration (mol dm À3 ) ratio varies from 0.2 to 2.7 and the ligand concentration is in the range (0.2 to 1) g dm À3 , i.e., (0.4 to 2) Â 10 À3 mol COO À dm À3 . The complex parameters found in these conditions were log b CuL ¼ 3.5 AE 0.1 and log b CuL2 ¼ 8.0 AE 0.2. For lower total ligand and total metal ion concentrations, used in voltammetry, the interaction Cu(II)-pectin is affected by a cooperative mode (increase of metal ion-ligand affinity) when the total metal ion concentration increases and by an anti-cooperative mode when the total ligand concentration increases, possibly due to different conformations of the polymer.
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