Spraying of TLC Plates

The spraying of TLC plates is a simple and rapid method of derivatizing substances. However, the coating produced is usually more inhomogeneous than that obtained by dipping. Moreover, it is more difficult to apply controlled amounts of the reagent. 

Analyses for the Saxitoxins. Early methods for analysis of the saxitoxins evolved from those used for toxin isolation and purification. The principal landmarks in the development of preparative separation techniques for the saxitoxins were 1) the employment of carboxylate cation exchange resins by Schantz et al. (82) 2) the use of the polyacrylamide gel Bio-Gel P2 by Buckley and by Shimizu (5,78) and 3) the development by Buckley of an effective TLC system, including a new solvent mixture and a new visualization technique (83). The solvent mixture, designated by Buckley as "E", remains the best for general resolution of the saxitoxins. The visualization method, oxidation of the saxitoxins on silica gel TLC plates to fluorescent degradation products with hydrogen peroxide and heat, is an adaptation of the Bates and Rapoport fluorescence assay for saxitoxin in solution. Curiously, while peroxide oxidation in solution provides little or no response for the N-l-hydroxy saxitoxins, peroxide spray on TLC plates is a sensitive test for all saxitoxin derivatives with the C-12 gemdiol intact. 

It is well known that flnorescence from an RP-18 phase is much brighter than from a silica gel plate, because the coating of RP-18 material blocks nomadiative deactivation of the activated sample molecules. By spraying a TLC plate with a viscous liquid, e g., paraffin oil dissolved in hexane (20 to 67%), the fluorescence of a sample can be tremendously enhanced. The mechanism behind fluorescence enhancement is to keep molecules at a distance either from the stationary layer or from other sample molecules [14]. Therefore, not only paraffin oil, but a number of different molecules show this enhancement effect. 

Thin-layer chromatography (TLC) is of fundamental importance for both the identification and determination of saponins. Both pure products and crude extracts can be handled, the equipment is simple and inexpensive, and results can be obtained rapidly. A number of visualization reagents are available for spraying the TLC plates and give color reactions for the different classes of saponins (Table 1). For quantitative spectrophotometry, the required constituents are separated on the TLC plate, scraped off, and then extracted with a suitable solvent, such as ethanol. After treatment with a reagent, the colored solution is measured at a specified wavelength. 

If in need of any of these plates, it is also possible to dip or spray these chemicals onto a plain silica gel plate (use the W-type mentioned above), dry, activate, and use. This could be done initially to see how the method works before purchasing a box of them. Often, most manufacturers of TLC plates can be called for sample plates. 

Such an apparatus is most time-saving when more than 100 TLC-plates are required daily. The experience gained in the film industry will have to be turned to account for mass preparation of TLC-plates or sheets, and preference given to a combination of conveyor belt and spraying procedure. 

Carbaryl is of great importance in pest control in India and in tropical countries. Its selective characterization is important to forensic authorities because in criminal cases this insecticide was used frequently as a poison. A selective detection was described with phenylhydrazine hydrochloride that was mixed with 10% sodium hydroxide solution in (1 1) ratio before spraying the TLC plates. This method is for the detection of carbaryl and its breakdown product 1-naphtol in biological and nonbiological materials (72a) (Table 4). 

The thickness of the absorbent on the TLC plates could be between 0.2mm to 2mm or more. In preparative work, the thicker plates are used and hundreds of milligrams of mixtures can be purified conveniently and quickly. The spots or areas are easily scraped off the plates and the desired substances extracted from the absorbent with the required solvent. For preparative TLC, non destructive methods for visualising spots and fractions are required. As such, the use of UV light is very useful. If substances are not UV active, then a small section of the plate (usually the right or left edge of the plate) is sprayed with a visualising agent while the remainder of the plate is kept covered. 

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