2.3. Emulsion preparation
The multivariate optimization was carried out with emulsions prepared by mixing, in a plastic capped tube of 6 mL capacity, 4 mL of standard naphtha (free of metals) spiked with known concentrations of Cu, Fe and Pb with a defined volume of Triton X-100/Triton X-114 solutions prepared in HNO3 medium. The concentrations of the surfactants and HNO3, as well as the volume of aqueous solution used for the emulsificationwere in accordance with the Doehlert design drawn for the multivariate optimization. The mixture was vigorously shaken until the emulsion formation. After that, the emulsion was directly
transferred to the auto sampler cup and injected into the graphite tube. All the elements under study (Cu, Fe and Pb) were measured at the same emulsion.