Application of Doehlert matrix and factorial designs in optimization of experimental variables associated with preconcentration and determination of molybdenum in sea-water by inductively coupled plasma optical emission spectrometry

A simple and efficient procedure for the preconcentration and determination of molybdenum in sea-water was developed using solid-phase extraction (SPE) of molybdenum(V) ion as the thiocyanate complex by polyurethane foam and inductively coupled plasma optical emission spectrometry (ICP-OES). The optimization process was carried out using two-level full factorial and Doehlert matrix designs. Four variables (solution volume, shaking time, thiocyanate concentration and hydrochloric acid concentration) were regarded as factors in the optimization. Results of the two-level full factorial design, 24 with 16 runs based on the variance analysis (ANOVA), demonstrated that all four factors, as well as the interactions between thiocyanate concentration and time and hydrochloric acid concentration and time, are statistically significant. Doehlert designs were applied in order to determine the optimum conditions for molybdenum preconcentration. The proposed procedure allowed molybdenum determination with a detection limit of 1.5 µg L−1 and a precision, calculated as relative standard deviation (RSD), of 1.8% for a set of 10 measurements for a molybdenum concentration of 10.0 µg L−1 in synthetic sea-water solution. The recovery of molybdenum in the presence of several cations achieved demonstrated that this procedure improved the required selectivity for sea-water analysis. The studied procedure was applied to molybdenum determination in surface sea-water samples collected in Salvador City, Brazil. Results showed good agreement with other reported data from the literature, including data obtained for samples from this same city by another procedure.