Ureas herbicide mobility

The selectivity of TLC systems was compared by use of correlations between Rf(ll) and Rf(l) (by analogy with two-dimensional TLC). The greatest spread of points, indicative of individual selectivity, was obtained for nonaqueous mobile phases on silica and aqueous mobile phases on octadecyl silica adsorbent wettable with water (RP-18 W). The correlation of Rf values in normal- and reversed-phase systems was utilized in the practical separation of a mixture of 14 triazines and urea herbicides using 2-D TLC on a Multi-K CSS dual phase (3 cm strip of octadecyl silica parallel to silica layer). The plate was videoscanned showing the real picture of the plate <2002JCH277>. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

Figure 2 Effects of the concentration of 2-propanol,

The retention characteristics of nine phenylureas (e.g., phenuron, hydroxy-methoxuron, bis-A, A -(3-chloro-4-methylphenyl)urea, neburon, and 7V-phenylurea) were studied on a cyanopropyl column (A = 254 nm) under normal-phase conditions of hexane/IPA (90/10 to 10/90), and reversed-phase conditions of IPA/water (10/90 to 90/10). Complete resolution of all nine herbicides was not achieved, but six were well resolved and eluted within 15 min using an isocratic 90/10 hexane/IPA mobile phase [210]. Extensive tables of k values for a wide range of isocratic mobile phases for both the NP and RP separations were presented. 

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