PREPARATION, HOMOGENEITY AND STABILITY OF POLAR PESTICIDES IN FREEZE-DRIED WATER INTERLABORATORY EXERCISE

An interlaboratory exercise was carried out to improve the state of th e art of some polar pesticide determination in water (atrazine, simazi ne, carbaryl, propanil, linuron, fenamiphos and permethrin). The prepa ration, the homogeneity and the stability of freeze-dried water sample s containing the above pesticides was studied. The final concentration of each pesticide in water was in the 50-80 mu g.l(-1) range with a s alt content of 2.5 g.l(-1) of NaCl. After the lyophilization the resid ue was rehomogenised, filled into amber glass bottles and stored at -2 0 degrees C and +20 degrees C. Every three months, one sample was anal ysed to verify the stability of the residue. All pesticides were deter mined by high performance liquid chromatography with diode-array detec tion (HPLC-DAD) except permethrin which was determined by gas chromato graphy with electron capture detection (GC-ECD). The results obtained show that the atrazine, carbaryl, propanil, linuron and fenamiphos sam ples were homogeneously distributed, whereas simazine and permethrin w ere not. With respect to the stability over three months, all pesticid es were stable at -20 degrees C. Atrazine, simazine, carbaryl, propani l and linuron are also stable for the maximum storage time at +20 degr ees C but the concentration of fenamiphos decreased by about 70% after one month. The results obtained in the interlaboratory study by the p articipants were in good agreement for many of the pesticides.