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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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III. THIN-LAYER CHROMATOGRAPHIC PROCEDURES FOR PESTICIDES
A. Organochlorine Insecticides
DDT (Fig. 1, structure La), its analogs, and its metabolites cause serious problems because of their toxicity and persistence (46), which led to their suppression 20 or 30 years ago by the authorities of various countries. In spite of this, DDT and similar compounds are even today applied against the tsetse fly in Africa in antimalaria programs. Very sensitive methods have been developed for pesticide determination in food, feed, and environmental samples. In this section the analysis of chlorinated insecticides will be considered.
The insecticides, hexachlorobenzene (HCB), HCH, DDE, DDT, and methoxychlor (Fig. 1, structure I.b) were determined in soil and plant samples after extraction with -hexane, n-pentane, or cyclohexane. Benzo[a]pyrene and other aromatic hydrocarbons were also investigated (47) (Table 2).
In the control of tsetse flies, large forest areas are sprayed with DDT and other insecticides.
Cl
II Aldrin
I.a R1, R2: Cl (DDT)
I.b Ri, R2: OCH3 methoxychlor
Figure 1 Organochlorine insecticides.
762
Fodor-Csorba
A new TLC spray reagent, tetraalkylbenzidine, was introduced for their detection (48) (Table 2). Chlordane and 14 other insecticides were analyzed in water samples by TLC using a hexane-ethyl acetate mobile phase. The detection limit was 1-2 |ig/spot obtained with o-tolidine (49) (Table 2). The degradation products of insecticides are chlorinated phenol derivatives. A procedure was developed for their determination on silufol (silica gel) plates eluted with hexane-benzene-ethyl acetate (6:4:1) (50) (Table 2). The results ofTLC were further confirmed by mass spectrometry (MS) and GLC.
Extraction and cleanup methods were developed for determination of insecticides in orange honey samples. GLC determination at the 0.05-ppb level was confirmed by TLC (10) in 25 samples.
Methoxychlor (I.b) in water samples was estimated by TLC in the presence of DDT isomers, Y-HCH, toxaphene, dieldrin, aldrin (), endrin, etc. After separation with hexane-acetone (90:10), the pesticides were detected with AgN03-NH40H reagent (51) (Table 2). Toxic components of toxaphene were investigated in Lake Michigan trout. Toxaphene is composed of more than 177 components, some of which are extremely toxic. Toxicant A can be characterized by the structure 2,2,5-endo,6-exo,8,9,9,10-octachlorobomane and 2,2,5-endo,6-exo,8,8,9,10-octachlorobomane and toxicant by 2,2,5-endo,6-exo,8,9,10-heptachlorobomane. Toxicant was quantitated in the original extract before TLC separation. TLC was used only as the separation technique for toxicant A. The quantitation of the compounds was made with GLC, MS, and nuclear magnetic resonance (NMR) spectrometry. Toxicant A was isolated by TLC on silica gel F254 plates after four developments with -heptane. The region of /0.30-0.43 was scraped off and eluted with 25% ether in hexane into a tube. This procedure allowed separation of toxicant A from pp-DDT (52).
A method has been published for the routine analysis of DDT and DDE using in situ fluorimetry. HPTLC plates developed with n-hexane-ether (85:15) were dipped into an ethanolic solution of diphenylamine and exposed to longwave (366-nm) ultraviolet (UV) light for 30 min. Rf values of DDT and its major metabolite DDE were 0.73 and 0.53, respectively. The results of HPTLC in situ fluorimetry and that of the GLC analysis using electron-capture detectors were compared (53) (Table 2).
Some general and specific procedures were reviewed for detection and determination of insecticides in the past four years (25a-e).
An individual method was described for the determination of 2,2-bis(p-methoxyphenyl)-l,l,l-trichloroethane isomer in the insecticide methoxychlor by TLC. The presence of this isomer in technical grade and emulsifiable concentrate of the methoxychlor could be established by this method, in which silica gel plates were developed with n-pentane-ether (9:1) (53a).
Tea samples were analyzed by a very simple method: n-hexane-acetone (9:1) mixture was applied for the extraction of the residues and an aliquot of the extract was washed with conc. H2SO4. A medical syringe was patched with alumina to serve as a chromatographic column for cleanup of the extract and as a spotting device as well. The column effluent was spotted directly on the TLC plate. Its temperature was maintained at 60C with a magnetic agitator and a hair dryer. When the spotting was complete, the TLC plates were developed. BHC, malathion, dimethoate, and quinalphos gave recoveries as high as insecticides (90%) and dichlorvos was recovered at a level of 30% because of its volatility (53b) (Table 2).
HPTLC plates with gradient multiple development were used for the separation of insecticides. The -heptane-dichloromethane mixtures were changed gradually (Table 2) with the length of the development. The plates were dryed between the runs. These factors caused a good resolution and lowered considerably the limit of detection. Visualization of the zones was carried out either by dipping the plates into AgN03 solution and exposure to UV light or with 3,5,3',5'-tetramethylbenzid-ine (TMB) (Table 2). The sensitivities of these reagents were comparable. The method allows an improved screening analysis of insecticides (53c).
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