Preconcentration procedure for determining trace amounts of Ni, Cd, Pb and Cu in high-salinity waters after cloud-point extraction

A procedure for determination of Cd, Pb, Cu and Ni in high-salinity waters by inductively coupled plasma optical emission spectrometry has been developed. It is based on cloud-point extraction of these metals as complexes of diethyldithiocarbamate (NaDDTC) in micellar media of non ionic surfactant octylphenoxypolyethoxyethanol (Triton X-114). Multivariate optimisation techniques have been applied to optimise the experimental variables. A full two-level factorial design was used to evaluate the influence of variables and Doehlert design was performed to find the optimum values. The effect of interference from residual salinity in surfactant-rich phase was also investigated. The developed procedure allows to achieve enhancement factors of 20.0, 20.4, 19.5 and 20.6, along with limits of detection (3B) of 0.030, 2.1, 0.62 and 0.27 g L-1, and precision expressed as relative standard deviation (%RSD, n = 10) of 3.7 (40.0 g L-1), 5.7 (20.0 g L-1), 6.6 (20.0 g L-1) and 3.1% (10.0 g L-1) for Cd, Pb, Cu and Ni, respectively. The accuracy was evaluated by spike tests on the seawater (salinity of 35) and petroleum produced formation waters (salinity between 15 and 75). It was obtained by recoveries between 79% and 105%.