Forced-flow overpressure

Nyiredy, S. Thin-layer (planar)/modes of development forced flow, overpressured layer chromatography and centrifugal. In Chromatography Cook, M., Poole, C.F., Eds. Academic Press New York, 2000 Vol. 2. 

Figure 8.19 illustrates another example of the versatility of multidimensional OPLC, namely the use of different stationary phases and multiple development ("D) modes in combination with circular and anticircular development and both off-line and on-line detection (37). Two different stationary phases are used in this configuration. The lower plate is square (e.g. 20 cm X 20 cm), while the upper plate (grey in Figure 8.19) is circular with a diameter of, e.g. 10 cm. The sample must be applied on-line to the middle of the upper plate. In the OPLC chamber the plates are covered with a Teflon sheet and pressed together under an overpressure of 5 MPa. As the mobile phase transporting a particular compound reaches the edge of the first plate it must-because of the forced-flow technique-flow over to the second (lower) stationary phase, which is of lower polarity. 

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

In the case of circular development, the simplest one-point supply may be used. The samples are spotted all around a circle the mobile phase supply is in the centre. Equipment based on overpressured (forced-flow) chromatography has also been introduced commercially, such as the Chrompres 10. Chrompres 25 and Chrompres 50 instruments of Labor MIM (today Laberte), Budapest, Flungary. as well as the Model P-OPLC BS 50 of the Engineering Company Ltd. (Budapest, Hungary). 

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. 

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. 

Forced-flow planar chromatography (FFPC) with the use of external pressure (overpressured layer chromatography, OPLC) or centrifugal force (rotation planar chromatography, RPC) can be used for off-line or on-line PLC (Mincsovics et al., 1988 Nyiredy, 1990). In off-line FFPC, the procedures after development... 

Empore silica sheet, because of its physical characteristics, cannot used in a conventional chamber system over 5 cm development distance (62). Due to the forced flow OPLC makes a longer development distance and rapid separation possible on this sorbent. This is promoted also by the higher density of sheet caused by overpressure (63). 

---