Ionic strength, an increase in the ionic strength weakens the interaction between the analytes and the sample matrix, thus facilitating their extraction by fiber. The influence of ionic strength was evaluated by adding different amounts of NaCl and HCl to the extraction solution. The effect was evaluated through additions of 10, 20 and 30% (w/w) of NaCl and HCl. The 30%(w/w) concentration yielded of NaCl was the best results for this study. However, this concentration impaired the repeatability of the results and also the stability of the solution due to precipitation of NaCl while the sample was at rest. On the other hand, 10% of HCl yielded the best results for some pesticides and did not affect the stability of the solutions, and was therefore chosen.