Reagent sequences

It is often possible to increase the selectivity of detection by carrying out a sequence of reactions on one and the same chromatographic plate - a technique that is only possible in thin-layer chromatography. In principle it is possible to distinguish between two sorts of reagent sequence ( TYPE A and TYPE B ), which are discussed in this and the next volume. 

Fig. 22 Scheme for type B reagent sequence (cf. the reagent sequence ninhydrin — iron(lll) chloride — Dragendorff — potassium iodoplatinate [6]). 

The specific detection of aromatic nitro compounds is a second example. These can be converted by reduction to primary amines, which are then diazotized and coupled to yield azo dyes (cf. reagent sequence Titanium(III) chloride — Bratton-Marshall reagent ). Sodium nitrite —naphthol reagent, diazotized sulfanilic acid and other reagents specific for amino groups (e.g. ninhydrin, fluorescamine, DOOB, NBD chloride [9]) can also be used in the second stage of the reaction (Fig. 21). 

Primary alcohols can be selectively detected using reagent sequences involving an initial oxidation to yield aldehydes that are then reacted in acid medium with electron-rich aromatics or heteroaromatics, according to the above scheme, to yield intensely colored triphenylmethane dyes. 

The following examples of reagent sequences, which include the reagent Ammonium monovanadate — p-anisidine described in the second part of the book can also be classified as redox reactions. 

Note The reagent sequence is a modification of the chlorine —potassium iodide-starch reagent. 

Note The reagent sequence can be employed on silica gel, kieselguhr and Si 50000 layers. 

Note This reagent sequence is a modiflcation of the reagent chlorine — potassium iodide — starch . Mobile phases containing ammonia must be removed completely before treatment with the reagent sequence, since otherwise the background will be colored too. Some secondary amines (e.g. diphenylamine) and some amides (e.g. 2,4-dinitrobenzamide) and methionine sulfoxide do not give reactions even in quantities of up to 1 to 2 (ig. 

The reagent sequence is specific for endosulfan and phosphamidon. Other insecticides, e.g. organochlorine insecticides, such as endrin, aldrin, dieldrin, DDT and BHC, organophosphorus insecticides, such as malathion, parathion, dimethoate, quinalphos, phorate and fenitrothion, or carbamate insecticides, such as baygon, car-baryl and carbofuran do not react. Neither is there interference from amino acids, peptides or proteins which might be extracted from the biological material together with the pesticides. 

Note Sulfldes, thiols and thioethers also react. The blue background of the chromatogram treated with the reagent sequence fades with time. 

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