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l-pyrene carboxal-dehyde mansyl-Cl
Rhodemin 6 G
Insecticides, acaricides, and fungicides were determined according to a method developed for the rapid TLC determination of residues in fresh and processed fruits. The best separation was obtained with two dimensional TLC on silica gel GF254 with cyclohexane-acetone (10:1) and light petroleum-benzene-ethanol (65:30:5) mobile phases. Detection of these pesticides by bromophenol blue (BPB), UV, Dragendorff reagent, and iodine vapor was compared (131a) (Table 10).
Carbofuran, atrazine, metolachlor, and their byproducts were separated on HPTLC plates containing fluorescent indicator. Several single and dual solvent systems were investigated for resolution by one-dimensional development. The quantification of the compounds was carried out by densitometric scanning, l-pyrene carboxaldehyde detected chlorpyrifos and its byproducts with a sensitivity of 0.5-0.05 |ig, but dansyl chloride, NBD-C1, DPH, and Rhodamine 6G were also investigated and compared to fluorescence quench detection (131b) (Table 10).
An optimized screening system for the analysis of 170 pesticides was reported recently which could be useful in forensic and toxicological determination of pesticides with TLC detection combined with GC and UV spectroscopy (131c).
Another review summarized analytical procedures for about 150 pesticides on 45 different supports, developed with 87 mobile phases and detected by 84 different methods. The review contained 290 references (13Id).
The method of the Association of Official Analytical Chemists (2) for the determination of ő— and OP insecticides in nonfatty foods was extended for the analysis of thiocarbamate herbicides in deep frozen food samples of plant origin. The sensitivity and selectivity of the following TLC detection systems were studied on various TLC sorbents: silver nitrate-2-phenoxyethanol for OCs, 4-[4'-nitrobenzyl]pyridine-tetraethylene-pentamine for OPs, and 2,6-dibromo- (DBBC) or 2,6-di-chlorobenzoquinone-N-chloroimine (DCBC) for thiocarbamate herbicides. The disturbing interferences of coextractives and of the above-mentioned pesticides were investigated. Although the sulfur-containing OP insecticides gave positive reactions with DCBC and DBBC, they did not interfere with the densitometric evaluation because of their column and TLC separation (21) (Table 10). Fifty-one pesticides, such as ő—, OP, and carbamate insecticides, fungicides, botanical insecticides, and herbicides, were analyzed by GLC, HPLC, and TLC. Two different solvent systems were applied in the TLC studies (Table 10). Spots were visualized either by UV light or by two selective detections: diphenylamine-zinc chloride reacted with ő— insecticides; OP and carbamate insecticides gave positive reaction when the plates were brominated and later sprayed with fluorescein and AgN03-2-phenoxyethanol. Some /fy-values are given in Table 10 (132).
Some papers have dealt with the reversed phase TLC of pesticides. The retention behavior of two pyrolyzed and two chemically modified silicas were investigated in adsorptive and reversed-phase modes in the analysis of triphenylmethane type fungicides and 2-nitro-4-cyanophenyl esters. The modified silicas and the untreated controls were compared (Table 10). The retention parameters of the compounds studied are given for all conditions (132a).
The lipophilities of thirty-one commercial pesticides were investigated by reversed phase TLC using water-methanol mixtures. The R,Ą values of the compounds decreased linearly with increasing concentration of the methanol (132b) (Table 10).
Thirteen 2-nitro-4-cyanophenyl esters and ten tri-substituted s-triazine derivatives were investigated on silica gel layers impregnated with silicone oil of variable vinyl content. The retention of the solutes increased with the increasing vinyl content (132c) (Table 10).
Pesticides in drinking water were analyzed by automated multiple development (AMD). This gradient TLC development consisted of 20 steps starting with 30% methanol in dichloromethane. The solvent was changed in 5 steps to 100% dichloromethane and that was altered to dichlorometh-ane-n-hexane (1:1) in the following 10 steps, and finally to Ž-hexane (100%) in the last 5 steps. Linuron, atrazine, and parathionmethyl were analyzed in the presence of other pollutants. Computer-controlled evaluation was carried out with a Camag TLC scanner II using software 86. The
multiwavelength scanning analyzed atrazine at 223 nm, linuron at 245 nm, and parathionmethyl at 273 nm (133).
The direct coupling of HPLC and HPTLC seems to be a very powerful method in the multiresidue analysis of pesticides. An instrument was developed for the direct connection of these chromatographic methods. The effluent obtained from a HPLC column was transferred directly to a TLC plate with this device. According to the Camag AMD method, the plates were developed by a 20-step universal elution gradient from methanol-dichloromethane to Ž-hexane. The compounds investigated in this system were benomyl, 2,4-D, etrimfos, atrazine, phenylmercury acetate, and linuron. Densitometric evaluation was carried out with a computer-controlled Camag TLC scanner II, with HP 9816 S and TLC evaluation software 86. This method opens a new way in the automated multiresidue analysis of pesticides (134).