In order to check the stability of the substances being studied, the calibration curves were initially assessed with the standards at room temperature and after refrigeration. Sample stability was evaluated by implementing cycles of freezing/defreezing at two concentration levels (250 and 1500 mg/L for Mixture A, 100 and 800 mg/mL for Mixture B and 100 and 750 mg/L for the chloroform). For this purpose, samples of blood, urine and vitreous humor were fortified at these concentrations. The samples were frozen for one day and eight days at -15 8C, then thawed, and analyzed according to the procedure already described. The concentrations after the freeze/thaw phase were compared withcontrol samples prepared and analyzed immediately