The samples are agitated for 60 minutes. To complete the extraction, the samples are centrifuged for 5 minutes at 1500 rpm and the organic phase is removed. 1 mL of the organic phase is mixed with 0.5 mL of a 100 μg / l OFN solution in DCM and prepared for gas chromatographic determination. The addition of OFN allows to eliminate device- and injection-related differences between individual measurements.