Forced-flow TLC

Figure 7.19 Chronpes 25 overpressure development chamber for forced-flow TLC. The configuration shown is for on-line detection. 

FF-TLC Forced-flow TLC GD-OES Glow-discharge-optical emission... 

The separation characteristics of a considerable variety of other TLC supports were also tested using different dye mixtures (magnesia, polyamide, silylated silica, octadecyl-bonded silica, carboxymethyl cellulose, zeolite, etc.) however, these supports have not been frequently applied in practical TLC of this class of compounds. Optimization procedures such as the prisma and the simplex methods have also found application in the TLC analysis of synthetic dyes. It was established that six red synthetic dyes (C.I. 15580 C.I. 15585 C.I. 15630 C.I. 15800 C.I. 15880 C.I. 15865) can be fully separated on silica high-performance TLC (HPTLC) layers in a three-solvent system calculated by the optimization models. The theoretical plate number and the consequent separation capacity of traditional TLC can be considerably enhanced by using supports of lower particle size (about 5 fim) and a narrower particle size distribution. The application of these HPTLC layers for the analysis of basic and cationic synthetic dyes has also been reviewed. The advantages of overpressured (or forced flow) TLC include improved separation efficiency, lower detection limit, and lower solvent consumption, and they have also been exploited in the analysis of synthetic dyes. 

In this chapter, nonpressurized planar chromatography, where the mobile phase migrates solely by capillary action, is discussed. Aspects of forced flow TLC are considered occasionally when appropriate. 

During the AMD procedure fractions are focused into narrow bands with a typical peak width of about 1 mm, so that separation numbers around 80 over the useable separation distance of 80 mm can be achieved. This makes AMD an attractive alternative to forced flow TLC (OPLC) (1), the main merit of which has to be seen in the extension of the useful separation distance of the layer, in order... 

L. Botz, Sz. Nyiredy, E. Wehrli and O. Shelter, Applicability of Empore TLC sheets for forced-flow planar cltromatography. I. Characterization of the silica sheets , 7. Liq. Chromatogr. 13 2809-2828 (1990). 

Development in TLC is the process by idiich the mobile phase moves through the sorbent layer, thereby inducing differential migration of the suple components. The principal development modes used in TLC are linear, circular and anticircular with the velocity of the mobile phase controlled by capillary forces or forced-flow conditions. In any of these modes the development process can be extended by using continuous development or multiple development. 

For two-dimensional TLC under capillary flow controlled. conditions it should be possible to achieve a spot capacity, in theory, on the order of 100 to 250, but difficult to reach 400 and nearly impossible to exceed 500 [52,140]. Theoretical calculations indicate that by forced-flow development it should be relatively > easy to generate spot capacities well in excess of 500 with an upper bound of several thousand, depending on the choice of operating conditions. -fE... 

In TLC the stationary phase is pre-wet by volatile components in the mobile phase present in the vapour phase of the chromatographic chamber. The mobile phase is at the bottom of the developing chamber and advances on the stationary phase its movement depends on capillary forces. The stationary phase is equilibrated by the mobile phase front during its movement. Separations obtained under capillary flow controlled conditions are limited to a maximum of about 5000 theoretical plates. Forced-flow development requires an external force to move the mobile phase through the layer. 

LC uses mostly packed columns, as the use of open tubular columns in this method is not practical because of the extremely small column diameters required for good separation. In gas chromatography, both packed and open tubular columns can be used, but the latter are far more popular because of their vastly superior properties. The mobile phase is usually forced through the stationary phase at elevated pressure, although other approaches are also possible (e.g., electrically driven flow in electrochromatography (EC), gravity driven flow in classical LC or flow driven by capillary forces in TLC). 

These limitations have led to the development of forced flow development systems and to the technique of overpressured thin layer chromatography. The special feature of this method is that the adsorbent layer is in a completely sealed unit and the solvent is delivered under pressure at a controlled oniform flow-rate by a pump module as in HPLC. Thus, overpressured TLC (OPTLC) takes place in the absence of a vapour pressure and the migration of the solvent front is free from both evaporation and adsorption effects. As the eluant is delivered under controlled conditions it is possible to optimise the separation conditions by adjusting the flow-rate of the eluant and also to undertake continuous development proeedures. 

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