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Thiocarbamate herbicide mixtures containing eptam, pebulate, and cycloate; thiobencarb, molinate, pebulate, and vemolate; or pebulate and vemolate were separated by concomitant adsorption and partition TLC because of the close /fy-values. Silufol UV254 plates were developed by hexane-ether (3:1) for adsorption and an acetone-water (6:4) mixture for partition TLC where the plates were impregnated with vaseline oil (91).
2,6-dichloro- and 2,6-dibromobenzoquinone-/V-chloro-imine (Gibbs reagent) were introduced for the detection of thiocarbamate herbicides, e.g., EPTC, butylate, cycloate, molinate, and pebulate. The sensitivity of visual evaluation was about 0.05 jig/spot. In situ densitometry of the chromatograms was performed at 450 nm (92). The colored compounds formed in the detection procedure were investigated by overpressure TLC using xylene or mesitylene as the mobile phase. A charge-transfer complex formation was assumed to be responsible for the separation of the colored derivatives. The reagent was used in the extension of the AOAC method for the determination of thiocarbamate herbicides in food of plant origin (21).
Structurally related compounds, dithiocarbamates (XIV), have fungicidal activity. Reversed-phase chromatography of copper dithiocarbamates was studied, and the Rf values were found to be linearly dependent on the chain length of the chelate, the water content of the mobile phase, and the number of carbon atoms of the chemically bonded stationary phase (93).
5. Growth Regulators
In the quantitative determination of phytohormones there is an ever-increasing role for TLC among other chromatographic techniques. Much attention should be paid to breakdown products in order to obtain reproducible spectrodensitometric in situ results. Rapid breakdown of indole-3-ylacetic acid (IAA) was observed on silica gel plates. This destruction takes place under application of ,4C IAA solution to the origin of the silica gel layer during the period prior to development (94), and is accelerated in the presence of light. Furthermore, the light and silica gel act syngeristically in this process. Other supports such as cellulose or polyamide provide better recoveries of IAA (95).
LAA was separated from phenols in Diospyros kaki leaf by reversed-phase TLC on octadecylsil-ane layers. Alcohol or alcoholic buffer systems, e.g., isopropyl alcohol, isobutyl alcohol, or 60% methanol in citrate buffer, were applied as mobile phases (96).
Abscisic acid (ABA) and IAA were analyzed by in situ spectrodensitometry on Silufol and Merck (N 5721) plates. The best separation of ABA and IAA was performed with chloroform-acetic acid (17:3) (97). In algae culture media, the released ABA was studied after extraction. Silica gel 60 F254 HPTLC plates were developed with benzene-ethyl acetate-water (5:4:1), and the concentration was quantified by scanning densitometry at 270 nm (97a).
According to another method, ABA was determined by HPTLC on silica gel plates with fluorescent labeling. The mobile phase was composed of toluene-ethyl acetate-acetic acid (25:15:2) (97b). Sumilarv (pyriproxyfen), a new insecticide growth regulator, was studied by TLC. The enantiomers were separated on a cellulose coated layer. The best separation was achieved when cellulose /m(4-methylbenzoate) coated silica was the stationary phase and hexane-hexanol (9:1) the mobile phase (97c).
Several organic acids known as plant growth regulatorsócinnamic, ^-naphtaleneacetic, (3-naph-toxy acetic, phenoxyacetic, salicylic, tartaric acids, and IAAówere separated on barium sulfate-cal-cium sulfate coatings. Several solvents were applied in order to investigate their separation potential, and the sequence chloroform > ethyl acetate > distilled water > propanol > carbon tetrachloride was observed.
TLC methods developed for studying fungicides can be divided into three groups: (a) TLC detection methods for residue analysis, (b) TLC methods used in searching for new fungicides, and (c) TLC as a tool in the investigation of biological modes of action and activity.
The first group of methods is for residue analysis. Some phtalimide-type compounds, the broad-spectrum protective fungicides captan, folpet, and captafol, are widely used for the control of diseases of fruits and vegetables. Only captan and folpet can be determined simultaneously in apples and potatoes on homemade silica gel H adsorbent mixed with 0.1 M aluminum chloride. Fungicide spots appear as fluorescent zones after spraying the plates with sodium chlorate solution and heating. These spots can be quantitated by densitometry (99). But all three fungicides can be determined on the basis of their chlorine content. Prewashed plates using a dichloromethane-methanol (1:1) system were developed (Table 8) and dipped into silver nitrate solution after drying. After exposure to UV light the spot areas were evaluated densitometrically. Impregnated plates could be stored for several hours without further darkening. Stronger darkening of the background was observed on the plates containing organic binders than on the silica gel plates with gypsum binders (Table 8) (99).