3.4. Evaluation of the emulsion stability
The main problem in direct measurement of trace elements in
naphtha is related to the high volatility of the samples, which yields
unstable absorbance signal for the standard solutions prepared with
naphtha free of metals (or even hexane) and commercial organometallic
standards. Also, the high volatility of the sample is inconvenient
because the vials must be filled continuously during analysis,
requiring constant supervision of the operator. These problems
were already reported in the literature and can be considered the
most important drawbacks in the trace metals determination in this
kind of sample. In order to solve this problem, the injection of the
sample, as detergent emulsions, has been chosen as sample
preparation strategy, since the current literature reports that good
stability of volatile organic liquids can be attained with the emulsion
or microemulsion formation. So, an experiment was run to evaluate
the emulsion stability in the optimized conditions. The experiment
comprised a continuous measurement of Cu, Fe and Pb in the
emulsion in intervals of 10 min during 250 min. The experiments
were performed with an emulsion prepared with naphtha free of
metals spiked with 25 μg L−1 of each analyte in the form of organic
standard. The results obtained in this experiment are shown in Fig. 4
and no noticeable variation in the analytical signals for any of the
studied metals was verified, indicating that the detergent emulsion
optimization was a correct strategy for the establishment of the
experimental conditions for the naphtha stabilization aiming metals
determination by GF AAS.