Obtaining material for TLC

Various hydrated aluminum hydroxides serve as starting material for TLC alumina. By a series of nonuniform thermal dehydration processes, a variety of aluminas are obtained, and the ones most suitable for TLC are the crystalline modifications of x-ALOs and y-ALO (Rossler, 1969 Snyder, 1975). The physicochemical properties or the exact nature of adsorption sites of alumina are not well understood. Snyder (1975) has suggested that exposed A1 atoms, strained Al-O bonds, and perhaps other cationic sites serve as adsorption sites, whereas, unlike silica, surface hydroxyl groups are probably not important. Acids are probably retained by interaction with basic sites such as surface oxide ions. Gasparic and Churacek (1978) report that every A1 atom is surrounded by six atoms of... 

One of the major uses of TLC is as a separative or purification technique. Preparative TLC is very useful to isolate components in a mixture, either for further analytical studies or to obtain pure materials for use as standards. An analytical TLC plate is normally 100 to 250 xm thick, whilst preparative plates are from 0.5 to 2 mm tiiick. They cm accept from 10 to 100 mg of material depending on the number of components md tiieir chemical nature. As a general rule, the amoimt that can be applied to a TLC plate without overloading it (tiie capacity) increases as the square root of the thickness of the adsorbent layer. It is generally better to apply the sample as a band rather tiian a spot, so that the maximum amount of sample can be chromatographed at the same time. 

A multichannel detector for In-situ analysis of fluorescent materials on TLC plates was investigated (26). The optical system was designed to obtain a fluorescence spectrum from each position along the elution axis of a one-dimensional plate without the mechanical scanning. By use of tetraphenylporphlne and octaethylporphine it can be shown how overlapping spots can be resolved into their components. 

Reaction Conditions. Gently stir the reaction mixture at room temperature. After a period of 5 min, sample the solution (2-5 xL) for TLC analysis. Sample the reaction solution by inserting a 9-in. Pasteur pipet down the air condenser to obtain a drop of solution by capillary action. If starting material is present as shown by TLC analysis, add an additional 0.4 mL of the hypochlorite solution as before. Stir for 5 min and sample again. Continue this process until TLC analysis shows that the 9-fluorenol has been completely consumed. Approximately 1.2-1.4 mL of hypochlorite solution should be sufficient, if the reagent is fresh. 

Eurther improvements can be expected on each of the different steps of isolation, cleanup, chromatography, and detection systems for determination of VK in biological samples. TLC and direct-phase HPLC as chromatographic methods will be reduced to a minimum. Development of new materials for solid phase extraction (e.g., Oasis, a new polymeric SPE sorbent) and improved materials for reversed-phase HPLC, including smaller, narrow-bore or capillary columns, should bring advantage to RP-HPLC, especially in connection with MS detection, but also with fluorescence and electrochemical detection and help to obtain smaller sample volumes and/or shorter analysis times. However, the greatest advance, in our opinion, will be in the field of detection instruments—in particular, in mass selective detection instruments. 

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