Download (direct link):
(116). The metabolic fate of ,4C-fenvalerate was studied on wheat plants after foliar treatment, and its half-life was established as being approximately 3 weeks (117). The TLC separation of diastereomers of fenvalerate and decarboxyfenvalerate by six consecutive developments in hexane-ether (20:1) was reported (Table 9) (117). Cotton callus tissues were used as test materials for the metabolism study of fenvalerate in plants (118) (Table 9). The fate of 14C-fenvalerate in animals, e.g., in rats (119,120) and in Japanese quails (120), was also investigated. The TLC separation of the recovered fenvalerate and its metabolites was performed (Table 9) (118) by adsorption and reversed-phase TLC in various solvent systems (120).
An effective TLC separation and a simple but sensitive detection of the halogenated synthetic pyrethroids was developed (121). By this method permethrin, cypermethrin, decamethrin, and fenvalerate were detected and distinguished from DDT, DDT metabolites, and HCH isomers. Twenty different solvents and solvent mixtures were checked for the chromatographic separation of the above insecticides. The best resolutions were achieved by ë-hexane: benzene (45:55) and ë-hexane-chloro-form (60:40), which gave distinct separation of cis and trans isomers of permethrin and cypermethrin (121) (Table 9) as well.
Although much is already known about the metabolic pathways of fenvalerate in mammals, plants, and soils, its fate in insects had not been much investigated. Colorado potato beetle was the test organism of a metabolism study in which about 20 metabolites were separated and characterized. Seven different elution systems, with one- or two-dimensional TLC or several developments in the same direction using the same mobile phase, were applied to the separation of the metabolites. Some of them are listed in Table 9 (122). Degradation of cis- and frans-permethrin in flooded soil was studied using carbonyl-14C-ci‘j-, carbonyl-uC-trans-, and methylene-14C-c«-permethrin. The TLC analysis (123) (Table 9) of soil and water extracts showed a faster degradation of franj-permethrin than of ds-derivative.
Resmethrin possesses excellent insecticidal activity and low toxicity to mammals. It is applied as space spray for fly control in and around dairy facilities. The persistence and metabolism of its stereoisomers were studied in tissues and excreta of lactating cows (Table 9) (124).
Fenpropathrin exhibits a great potential for the control of various insects and mites in fruit plants, vegetables, etc. Ten different mobile phase systems were presented, allowing the separation of the parent compound and its major metabolites. The fy values of fenpropathrin in three mobile phases are given in Table 9 (125).
Pyrethroid insecticides were separated by TLC and detected with a new reagent. The compounds were oxidized with bromine vapor and then sprayed with o-tolidine solution. The residues appeared as blue colored spots on a white background (125a).
G. Multiresidue Methods
Multiresidue methods are the most powerful procedures for the analysis of pesticides in environmental, food, and/or feed samples. The maximum residue limits of the pesticides prescribed by health authorities include not only the residues of the parent compounds but their toxic metabolites as well. There is also a trend toward inclusion of residues of highly polar and conjugated metabolites (126). The ever-increasing demands on the quality of food, feed, and environmental samples, and the lowering of the maximum residue limits, require the development of new and more sensitive methods. To distinguish the pesticide signals from the interfering coextractives of samples demands more selective detection. TLC methods alone are not sufficient and should be combined with GLC, HPLC, mass spectrometry, etc.
A multiresidue method was developed for the determination of 16 pesticides in water samples. The ÎÑ insecticides were extracted with ÿ-hexane at pH 6-7, alkaline or neutral compounds with chloroform at pH 6-7.5, and the remaining acidic compounds with chloroform at pH 2. The ÎÑ pesticides were separated by one-dimensional TLC in an ë-hexane-acetone system (Table 10). Two-dimensional TLC was necessary for the separation of the pesticides present in the first and second chloroform extracts (Table 10). The recovery was about 80% (22).
According to another method, 45 compounds such as aromatic amines, chlorophenols, and herbicides were analyzed in water samples. Neutral, alkaline, and acidic pesticides were separated by TLC after their extractive enrichment (127).
ÎÑ, polychlorinated biphenyls (PCBs), OP insecticides, and carbamates were extracted by various methods and their TLC analysis was carried out either by adsorption or by reversed-phase TLC. Relative fy values and selective detection methods were also described (Table 10) (128).
Soil samples were extracted first with a 0.05 N CaCl2-acetone mixture, and later with ë-hexane and benzene or chloroform. The presence of the CaCl2 prevented the appearance of disturbing coextractives. This procedure resulted in 92-98% recovery. HCH and dilor were separated on Silufol plates in è-hexane, the a-thiodane and ji-isomer in petroleum ether, and pyrethroids, OP, and ÎÑ insecticides in various ë-hexane-acetone mixtures (20) (Table 10).