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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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The lipophilicity of triazine herbicides was investigated by reversed phase TLC using acetone, methanol, or acetonitrile as organic modifier of the mobile phase. Rm values were calculated (79b). A review published recently summarized analytical methods for the determination of triazine herbicides, among others (79c).
Atrazine and simazine residues in water samples were resolved on AgNC^-NR^OH-impregnated plates (13) (Table 5). Atrazine and its metabolites have been extracted from Nordica strains isolated from soil samples (80) and analyzed by TLC. Metribuzin and its metabolites were also examined in chernozem sandy soil, potatoes, and water samples. The recovery error was about 8-10% (15) (Table 5).
Atrazine, cyanazine, prometryne, simazine, and terbutryn were determined by TLC and HPLC methods in milk by Hill reaction inhibition, where a chronometric method allowed quantitative measurements. Recovery ranged from 65% to 83%. Comparing the detection limits of HPLC (0.01 mg/kg) and TLC (0.002 mg/kg), the latter showed one order of magnitude higher sensitivity. This method is an appropriate procedure for monitoring triazines in milk samples (4). Hill reaction inhibition is also used in another procedure offering the analysis of atrazine, chloridazone, lenacil, phenmedipham, and desmedipham in sugarbeet samples. This TLC method was combined with GLC analysis. There was no significant difference between the detection limits obtained by TLC and GLC (3).
2. Diphenyl Ether Herbicides
Diphenyl ethers represent a newer class of herbicides. These compounds can be depicted by the structures XVII and XVIII shown in Fig. 5.
Residue analysis of nitrofen, oxyfluorfen, bifenox, fluorodifen, acifluorfen as nitrobenzene derivatives (XVII), and chloroxuron and diphenoxuron as aniline-type compounds (XVIII) were investigated (81). The best separation of the compounds was achieved on silica gel plates developed with n-hexane-ether (3:1) and after air drying with n-hexane-acetone-acetic acid (35:25:0.05) in the same direction (Table 6). Densitometry was carried out at 280 nm for the two groups of compounds

NH>-C-NH

OJ TO
R
XVIII
Figure 5 Diphenyl ether herbicides, XVII-XVIII.
Table 6 Diphenyl Ether Herbicides
Compound
Stationary phase
Mobile phase
Detection
Rf x 100
Ref.
(1) (1.2)
Nitrogen
Oxyfluorfen
Bifenox
Fluorodifen
Acifluorfen
Fomesafen
Chloroxuron
Diphenoxuron
Nitrofen
2.4-dichloro-3' -(carboxymethyl)-4-hydroxy-DPE
2.4-dichloro-31 (carboxymethyl)-4-amino-DPE
4-Nitrophenol
Silica gel (Merck)
(1) n-hexane-ether (3: 1)
(2) Hexane-acetone-acelic
acid (35:25:0.05)
Silica gel 60 F254 (Merck) Alumina F254 (Type T)
RPSF reversed-phase (Analtech) Silica gel HF254
CCI4 ether (3:1) Hexane-ether (9:1) Me0H-H20 (3:1) Benzene-CHCI,
CHCl,-MeOH-acetic acid
Spectrodensitometry
300 nm
245nm
Pentacyano-amino-ferrate
70
53
43
34
91
82
72.5
65
49
43 28 11 81 41 30
44
37
81
82
83
(90:5 :5)
Other photoproducts
Pesticides
781
without any loss of sensitivity, although the analytical wavelength was found to be 300 nm for the nitrobenzene and 245 nm for the aniline derivatives in the diffuse reflection electronic spectra (81).
Nitrofen and other diphenyl ether herbicides require light to disrupt membrane permeability. In this light-induced photoreaction nitrofen itself undergoes some changes. Reduction of the nitro group takes place, where nitroso hydroxylamino > and aminonitrofen derivatives form. These alterations were studied by (among other methods) TLC (82) (Table 6). In solutions of diphenyl ether herbicides, the predominant route of the photodegradation was attacked by isopropanol, water, and other nucleophiles to produce phenolic degradation products, which were observed and analyzed by TLC and compared with authentic samples (83) (Table 6).
3. Chlorophenoxyalkylcarboxylic Acid Herbicides
Chlorophenoxyalkylcarboxylic acids, salts, and esters (XIX, Fig. 6) are widely used herbicides for control of broadleaf weeds in cereal crops, sugarcane, lawns, woody plants, etc. TLC methods make use of the chlorine content of these herbicides for both detection and separation.
In one method, which further retards darkening of the background, silica gel layers were impregnated with AgN03 and sensitized by 2-phenoxyethanol. These plates may be stored in the dark before use but not for a long time. This method was used for the separation of chlorophenoxyalkylcarboxylic acids in natural water and tapwater. Only these impregnated plates, which were developed with solvent mixtures containing acetic acid, were suitable for densitometric determination. This method gave well-defined but not extremely dark or compact spots. The /fy values are given in Table 7 (84).
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