3. Results and discussion
3.1. SDME procedure optimization
The optimized parameters of the SDME/GC–MS were: extraction time — 30 min, solvent — toluene, drop volume — 1 μL, stirring speed — 300 rpm [7]. Additionally, multivariate optimization was performed by a 22 full factorial design to identify the influence of salt addition (% NaCl) and pH on the SDME procedure. Two replicates were performed at the central point to estimate the experimental error [38] and [39], which were minimal in the six experiments. The dependent variable (response) was the sum of the analyte peak areas in each experiment. Table 1 shows the studied parameters and their sampling levels.