PREPARATION, HOMOGENEITY AND STABILITY STUDIES OF FREEZE-DRIED WATER CONTAINING PESTICIDES

The preparation, the homogeneity and the stability of freeze-dried water samples containing pesticides of various chemical groups (Atrazine, Simazine, Fenitrothion, Fenamiphos, Parathion-ethyl, Carbaryl, Permethrin, Linuron and Propanil) was studied. The concentration of pesticides in various batches of water varied from 0.02 to 90 mug/1. All freeze-dried water samples were stored at -20-degrees-C, + 20-degrees-C and +40-degrees-C in amber bottles and analyzed after successive storage periods up to one year. Liquid chromatography with diode array and gas chromatography with nitrogen-phosphorus and electron-capture detection were used for the final determination of the pesticides after reconstitution of the water samples and extraction. The results indicate that the stability of the pesticides is dependent on the compound, its concentration in the freeze-dried residue and the storage temperature. All pesticides exhibited a sufficient stability during storage at -20-degrees-C. The interlaboratory exercise conducted with these samples gave data for certain compounds (e.g. carbaryl) indicating that the applied freeze-drying procedure of spiked water can be applied for the preparation of a candidate reference materials to be certified for some of the studied pesticides.