Validazione del protocollo analitico di equilibrazione competitiva dei leganti accoppiata a stripping catodico per la speciazione di elementi in traccia.

The analytical performances of Competitive Ligand Equilibration with Cathodic Stripping Voltammetric detection of the labile fraction (CLE-CSV) were assessed and optimized. This speciation method enables the concentration of natural ligand(s) and their conditional stability constants for the complexation of the investigated metal to be determined through thermodynamic considerations. The CLE-CSV procedure requires three working steps: firstly, total metal determination has to be performed with a good accuracy and precision; secondly, the calibration of the alfa coefficient of the added ligand with the metal considered is required; finally, titration of the samples with increasing metal additions provides the speciation results mentioned above. To allow an accurate and precise measurement of the total metal concentration at trace levels (nM), the entire analytical procedure was assessed in term of effect of acidification of the samples and/or reagents required for the analysis and in term of influence of the UV-photooxidation used as pretreatment step for the total metal quantification with voltammetric techniques. The purity grade of nitric and hydrochloric acids produced by subboiling distillation was evaluated with the aim to avoid metal contamination during acidification processes. UV-photooxidation is commonly accepted as a sample pretreatment for voltammetric measurements to remove organic matter. Starting from the critical evaluation of the performances and costs of the commercially available systems, we decided to design and develop a prototype UV-digester, using only commonly commercialised parts (e.g. low cost, standard connection UV lamps). As a result, the target of coupling low investment and low maintenance costs to high performances is completely fulfilled. Its suitability for the treatment of natural waters prior to cathodic stripping voltammetry was assessed: the total concentration of copper, uranium and platinum in natural waters could be reliably determined applying a 30 minute UV treatment. The final aim of this doctorate thesis is the complete evaluation of the speciation procedure CLE-CSV in term of accuracy and precision.