Extração e pré-concentração no ponto nuvem para a determinação de metais em águas salinas por espectrometria de emissão ótica com fonte de plasma indutivamente acoplado

Abstract

This work presents the development of analytical methodologies for the simultaneous determination of cadmium, lead, copper and nickel in saline waters by inductively coupled plasma optical emission spectrometry, using ultrasonic nebulization. The procedures are based on cloud point extraction of metal complexes with sodium diethyldithiocarbamate DDTC-Na, using octylphenoxypolyethoxyethanol Triton X -114 as nonionic surfactant. A formal multidimensional optimization technique has been carried out in this study to optimize the experimental variables. Four variables were examined: pH, chelating reagent, surfactant concentration and centrifugation time. A two-level full factorial design was used to determine the influence of the variables to be optimized on the response. In a second stage, a surface-response methodology using Doehlert designs and Lagrange criterium was performed to find out the optimum values of those factors. The presence of matrix interferences in the final results was investigated. At pH 8.1, preconcentration of 50 mL of sample in the presence of 0.08% Triton X-114 and 0.015% DDTC-Na achieved the limits of detection 0.03 μgL-1; 2.1 μgL-1; 0.62 μgL-1 and 0.27 μgL-1 and the limits of quantification 0.11 μgL-1; 7.1 μgL-1; 2.10 μgL-1; 0.89 μgL-1 for cadmium, lead, copper and nickel, respectively. The enhancement factors were 20.0 (Cd), 20.4 (Pb), 19.5 (Cu) and 20.6 (Ni), with 2 mL final solution. The relative standard deviations were 3.7% (Cd), 5.7% (Pb), 6.6% (Cu) and 3.1 % (Ni) in samples containing 40 μgL-1 of lead, 20 μgL-1 of copper, 20 μgL-1 of nickel and 10 μgL-1 of cadmium. The proposed method has been applied in sea water and produced waters from petroleum exploration.