associated to the determination of Cu, Fe and Pb in naphtha samples
by GF AAS, using an approach based on the injection of the samples as
detergent emulsions. The employment of a global response (obtained
by combining the individual responses) made possible the establishment
of optimum conditions for the preparation of only one emulsion
that could yield adequate sensitivity for the determination of the
three analytes.
As only three variables (concentration of surfactant, concentration
of nitric acid and volume of the aqueous phase) were chosen for the
optimization, a Doehlert design was directly employed for this
purpose. Two designs were drawn, one using Triton X-100 as
emulsifying agent and other one using Triton X-114 as emulsifying
agent. All the three variables under study presented significant effect
on the global response and important interactions among them was
only verified when Triton X-114 was used. The performance of the
method was not significantly different by employing either one of the
surfactants, once the global responses for the two cases showed
similar magnitudes. The experimental regions where the critical
points were situated were close to each other, indicating that the use
of Triton X-100 or Triton X-114 made possible the formation of
suitable detergent emulsions for GF AAS determination of the
considered metals. Triton X-100 was chosen in this present work
because of the high availability of this reagent in the most analytical
laboratories and also due to the easier formation of the emulsions
when it is employed. The stability of the emulsion prepared in the
optimized conditions was also tested and no significant variation of
the analytical signal measured for the three analytes was observed in
an interval of 250 min.
The optimized method was successfully applied in the determination
of Cu, Fe and Pb in five samples of naphtha. A recovery test was
performed to evaluate the accuracy of the procedure and recovery
rates in the range of 88–105%, 94–118% and 95–120% for Cu, Fe and Pb
were verified, respectively.