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Chromatographic scince series - Cazes J.

Cazes J. Chromatographic scince series - Marcel Dekker, 1996. - 1098 p.
ISBN 0-8247-9454-0
Download (direct link): сhromatography1996.pdf
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Some antioxidants, e.g., tocopherol, ethylprotocatechuate, and propylgallate, were determined in peanut butter, potato chips, salami sausage, Vienna sausage, etc. by TLC and HPLC (141). In another study, seven food preservatives were determined by HPLC and the results were compared with those from TLC using a polyamide FM stationary phase with hexane-benzene-acetic acid (15:7:2) as the mobile phase (142).
Soil thin-layer chromatography is very useful in the investigation of the translocation and degradation of pesticides in soil. These layers combined with silica layers allow study of the movement of the pesticides and their breakdown products as well. Photochemical oxidation of aryldialkylthiourea herbicides on the soil surface can be investigated when these plates are exposed to sunlight (143). The sorption coefficients and the mobility of 14C-metalaxyl was investigated on three Brazilian soil
thin layers having different organic matter contents (144). The mobility of five pesticides having carbamoyl groupsdimecron, aldicarb, bavistin, carbofuran, and oxamylwas investigated by soil thin-layer chromatography as a model of agricultural conditions. In these experiments silt loam and sandy loam thin layers were prepared. Distilled water and 0.01 and 0.0S N solutions of inorganic salts such as calcium sulfate, magnesium sulfate, potassium carbonate, sodium bicarbonate, sodium sulfate; fertilizers such as ammonium nitrate, potassium chloride, sodium nitrate; and cetyl trimethyl ammonium bromide (cationic), sodium dodecyl sulfate (anionic) and manaxol Ҕ (nonionic) surfactants were applied as mobile phases. The Rf, Rm, and RB values of these pesticides were investigated for their dependence on pH as well (145).
A. Color Reactions
1. Chemical Color Reactions
The visualization of the developed chromatogram is as important as the problems of separation. Some possibilities are summarized below with regard to specific group reagents. For example, the amino part of a pesticide can be detected with ninhydrin (109,125) or by dimethylaminobenzaldehyde (8,55,69,129). Some pesticides give this amine-specific reaction only after hydrolysis, as in the case of chlorpropham, metobromuron, and chlorbromuron (69). These carbamate compounds can be detected by anisaldehyde (128) as well. Tolidine alone (22,49) or in combination with KI detects herbicides (76,146) and insecticides (55), reacting also with the amino part of the molecules.
Twelve carbamate, urea, and anilide herbicides investigated in water and soil samples were detected with Bratton-Marshall reagent after separation on KC18 reversed-phase TLC plates. These pesticides produce aromatic amines after acidic hydrolysis. These primary aromatic amines could be diazotized and detected by azo-coupling reactions giving azo-dyes. After development plates were sprayed with 6 N ethanolic HC1 and heated at 18 O' for 10 min. After cooling they were sprayed with NaN02 solution and later with N-1 -naphthy 1-ethylenediamine dihydrochloride solution. The colored spots were evaluated by densitometry at nanogram levels (147).
According to the method mentioned above, the hydrolyzed carbamates or anilines were transformed to diazo compounds and coupled with the naphthylamine derivative. There is the possibility of the opposite reaction, where primary aromatic amines of pesticides are coupled with the diazo reagent, e.g., 4-nitrobenzene-diazonium-fluoborate. The sensitivity was at the same level (23,129). The chlorine content affords the TLC detection of insecticides (11), triazine herbicides (14), and pyrethroids (121) with AgN03 after UV irradiation. The binders of the layers may disturb the densitometric measurements. According to a modified method, a AgNOj-NH^OH system is used for the detection of these insecticides (51,60), fungicides (19), or pyrethroids (125). The AgN03-2-phenoxy-ethanol system is applied in some multiresidue methods (21,132), and AgN03 with tetra-bromosulfophthalein is used for detection of OP insecticides (148).
The sulfar-containing OP insecticides can be detected either with iodine-azide (149) or with potassium iodate (150) reagents. 4-(4'-nitrobenzyl)pyridine-tetraethylenepentamine reagent is also applied for their detection (21).
2,6-dichIorobenzoquinone-Ar-chIoroimine detects OP insecticides (21,151) and reacts with the systematic insecticide and acaricide vamidothion (152) and thiocarbamate herbicides (21,153) with a sensitivity of 0.06 (ig/spot. 2,6-dihalobenzoquinone-S-alkyl sulfenylimine formation is responsible for the developed color in the detection of thiocarbamate herbicides.
In case of 2,6-dibromobenzoquinone-Ar-chIoroimine, the sensitivity of thiocarbamate detection is as good or higher than that of 2,6-dichlorobenzoquinone-/vr-chloroimine, but some side reactions took place during the chemical detection process (153a).
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