Selective spot treatments

An important feature of using these manual sprayers is that it is possible to do selective spot treatments, for example, confining the application of herbicides to patches of weeds. Also, where there is concern about downwind spray drift, placement of the nozzle nearer to the target and using a low pressure can enable treatments closer to a sensitive area. In conjunction with an attractant such as molasses or protein hydrolysate, insecticide bait sprays have been used in discrete spots for fruitfly control. Knapsack sprayers are often used in difficult terrain where access to vehicles is difficult or impossible. However, the lack of water in some situations, and the high requirement for labour, have created a demand for alternative application systems (see below). 

Chlorthiamid is a preemergence herbicide inhibiting germination, used at a rate of 20-30 kg active ingredient/ha for total weed control, at 4-14 kg active ingredient/ha for selective weed control. It is recommended as spot treatment to control docks (Rumex spp.) and thistles (Cirsium spp.) in the renovation of old pastures. Its other areas of application are the same as those of dichlobenil (Sandford, 1964 Jenner, 1977). 

This is an oversimplified treatment of the concentration effect that can occur on a thin layer plate when using mixed solvents. Nevertheless, despite the complex nature of the surface that is considered, the treatment is sufficiently representative to disclose that a concentration effect does, indeed, take place. The concentration effect arises from the frontal analysis of the mobile phase which not only provides unique and complex modes of solute interaction and, thus, enhanced selectivity, but also causes the solutes to be concentrated as they pass along the TLC plate. This concentration process will oppose the dilution that results from band dispersion and thus, provides greater sensitivity to the spots close to the solvent front. This concealed concentration process, often not recognized, is another property of TLC development that helps make it so practical and generally useful and often provides unexpected sensitivities. 

An interesting variation of the latter technique finds application in enzyme chemistry. In this procedure a tissue section is exposed to a relatively colorless derivative of j8-naphthol, such as sodium j8-naphthyl acid phosphate. A phos-photase enzyme reacts with this reagent (often called an enzyme substrate ), leaving free j8-naphthol behind. Subsequent treatment with a solution of a diazonium salt produces highly colored spots in the tissue section. Thus not only can the presence of phosphotase enzyme be demonstrated, but also the location of the enzyme in the tissue can be determined. The intensity and chroma of the color produced and the solubility of the azo dye in the cell materials can be varied by judicious selection of the reagents. 

There are many chemical procedures used for rendering the spots on the plate visible some procedures are general and they expose all the components in the mixture as visible derivatives, whereas others are very specific, and only provide colored derivatives with selected chemical types, e.g. amino acids. Examples of both classes of derivatization will be given but, irrespective of the type of product that is chosen, some pre-treatment will be necessary. After chromatographic development, the plate will be wet with the mobile... 

Afsar et al. (1987) have developed a method to differentiate mancozeb from a mixture of maneb and zinc salts or from a mixture of maneb and zineb. Compounds are distinguished on the basis of color differences after treatment of the saturated solutions of fungicides in n-propanol-acetone mixture first with dithizone and then with monosodium dihydrogen phosphate. Stevenson (1972) presented a similar earlier method that distinguished maneb, zineb, mancozeb, and selected fungicidal mixtures by successive application of acid dithizone, sodium hydroxide, and acid dithizone to the spot. 

Further methods for the determination of lithium have been reviewed by Birch (1999). During the late 1980 s, the lithium ion selective electrode (Li-ISE) (Xie and Christian 1986) has been developed in order to optimize lithium treatment (Phillips et al. 1989, Sim et al. 2001). This technique allows on-the-spot estimations of lithium in whole... 

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