Stability of pesticides in plant extracts used as calibrants in the gas chromatographic analysis of residues

The stability of commonly used pesticides in plant sample extracts was evaluated. Matrices differing in the character of coextracts were represented by wheat, oranges and white cabbage. After homogenisation with ethyl acetate and anhydrous sodium sulphate, spiked filtrates were stored for 60 days at 20 degrees C or 40 degrees C. The decrease of concentrations was observed at 20 degrees C after 40 days for chlorothalonil and iprodione in cabbage extracts and some degradation was observed for most organophosphates, iprodione and pirimicarb in orange extracts. At increased temperature (40 degrees C), degradation of most pesticides in the orange and cabbage extracts was observed. No decomposition was noticed for synthetic pyrethroids in all tested extracts. The stability of pesticides in wheat extracts was distinctly higher than that in other extracts. Most pesticides are stable enough to store plant sample extracts several weeks prior to further handling, or to use them as calibrants to avoid matrix-induced enhanced GC response. Some degradation of pesticides, in "pure" ethyl acetate solutions was noticed only for some organophosphates (mevinphos, methamidophos, dichlorvos, heptenophos, pirimiphos-methyl) after 60 days at 40 degrees C. (C) 1998 Elsevier Science B.V.