Assessment of accuracy and precision in speciation analysis by competitive ligand equilibration-cathodic stripping...

The analytical performances of Competitive Ligand Equilibration with Cathodic Stripping Voltammetric detection of the labile fraction (CLE-CSV) were assessed. 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. Literature data were discussed and general trends in the precision of the determined parameters identified: ligand concentrations were affected, on average, by a 10% relative percentage standard deviation (RSD%), whereas conditional stability constants showed much lower precision, with an average RSD% of 50%. New experimental data were collected to obtain a complete assessment of accuracy and precision attainable for the determination of strong ligands at the ultra trace level, enabling the whole protocol to be evaluated. Firstly, the side reaction coefficient alpha for the formation of the complex between the added ligand and the investigated metal (alpha(CuL)) was determined. The method was subsequently applied to the analysis of solution containing ligand at trace levels (5-50 nM) with known complexing characteristics. Copper was used as the model metal ion and ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) as the model ligands. Results evidenced that the CLE-CSV protocol is not affected by systematic errors in the determination of both ligand concentration and the conditional stability constants. Good precision is obtained for ligand concentrations, with an average relative standard deviation (RSD%) of 5%; an average RSD% of 23% was calculated for the conditional stability constants. Including the contribution of the uncertainty in the value of alpha(CuL) in the evaluation of the uncertainty in the latter parameter increased the RSD% up to 40%. The CLE-CSV protocol was subsequently applied to the detection of strong ligands in water samples collected in Antarctica: precision was shown to be comparable with literature data.