Analysis and dissipation of dislodgable residues of acephate,dimethoate and formetanate hydrochloride on citrus foliage

Dislodgable residues of acephate and its hydrolysis product methamidophos, dimethoate and its oxygen analogue omethoate (dimethoxon), and formetanate hydrochloride on citrus foliage were determined by gas–liquid chromatography. The use of 5% OV-101 coated on a Carbowax 20M surface-modified support (Ultra-Bond 20M) allowed the reproducible analysis of dimethoate and omethoate residues. Formetanate hydrochloride residues were analysed as the free base, without preparation of a derivative, on a column packed with 5% Apiezon N coated on 80–100 mesh Ultra-Bond 20M. The dissipation of the residues of the three pesticides applied to citrus trees was studied. Acephate and formetanate hydrochloride dissipation curves followed first-order kinetics; dimethoate residues were best characterised by two first-order kinetic processes. The half-life values were 8.2 days for acephate, undeterminably long for formetanate hydrochloride, and 2.2 days (the 1–10 days portion) and 7.0 days (the 10–49 days portion) for dimethoate.