Internal quality controls, such as the use of a blank matrix extract to eliminate false positives due to a possible contamination with pesticides during the extraction procedure, in the instrument or in the chemicals, the extraction with the samples of a spiked blank sample at 10-fold LOQ concentration to check the extraction efficiency and a calibration curve to evaluate sensitivity as well as linearity in the working range of concentrations, were carried out. A reagent blank (acetonitrile) was also injected after every six sample injections to check for carry over and to perform simple cleaning of the chromatographic system.