Forced-flow development

On the basis of theory and experimental observations it can be predicted that a zone capacity of ca. 1500 could be achieved by 2-D multiple development. Because the same result can be achieved by application of 2-D forced-flow development on HPTLC plates, it can be stated that the combination of stationary phases, FFPC and "D offers a fruitful future in modem, instmmental planar chromatography. 

Figure 7.2 Relation between the solvent front position and tine for (1) an enclosed layer with forced-flow development, (2) an exposed layer in a saturated chamber with capillary controlled flow, (3) a covered layer (sandwich chamber) with capillary controlled flow, and (4) an exposed layer in an unsaturated atmosphere with capillary controlled flow. (Reproduced with permission from ref. 30. Copyright Or Alfred Huethlg Publishers).

Forced-flow development enables the mobile phase velocity to be optimized without regard to the deficiencies of a capillary controlled flow system [34,35). In rotational planar chromatography, centrifugal force, generated by spinning the sorbent layer about a central axis, is used to drive the solvent... 

Figure 7.5 Left, variation of the average plate height of fine-and coeurse-particle layers as a function of the solvent aigration distance and method of developaent. Right, relationship between the optiauB plate height and solvent migration distance for forced-flow 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... 

Resolution in forced-flow development is not restricted by the same limitations that apply to capillary flow controlled systems. The maximum resolution achieved usually corresponds to the optimum mobile phase velocity and R, increases approximately linearly with the solven)t migration distance (48). Thus there is... 

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. 

Kalasz, H. Bathori, M. Ettre, L.S. Polyak, B. Displacement thin-layer chromatography of some plant ecdyste-roids with forced-flow development. J. Planar Chromatogr. 1993, 6, 481-486. 

The mobile-phase velocity is set by the system variables and cannot be independently optimized unless forced flow development conditions are used. 

Fernando, W.P.N. Poole, C.F. Determination of kinetic parameters for precoated silica gel thin layer chromatography plates by forced flow development. J. Planar Chromatogr. 1991, 4, 278-287. 

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. 

Advances in stationary phase technology have led to commercial availability of adsorbents such as high performance sihcas, aluminas, polyamides, celluloses and derivatised silicas [9,10], The development of automated method development (AMD) systems [127] now allow multi-step gradients of different elution strengths to be achieved in a relatively short time compared to earlier manual approaches. AMD systems are ideally suited for separation of complex mixtures with a wide range of polarities. Further improvements in sample resolution and reduced method development times in TLC include the use of two-dimensional development approaches [128] and forced-flow development by over-pressure liquid chromatography (OPLC) [129]. 

Figure 3 Relationship between the solvent-front migration distance for dichloromethane on an HPTLC silica gel layer as a function of time for different experimental conditions. Identification (1) forced flow development at Uopti (2) capillary flow in a saturated chamber (3) capillary flow in a saturated chamber with a covered layer (sandwich chamber) and (4) capillary flow in an unsaturated chamber.

Forced-flow development. Forced-flow planar chromatography is a development technique wherein pressure is used to aid the mobility of the developing solvent. Examples of this are over-pressure layer chromatography (OPLC) and over-pressure thin-layer chromatography (OPTLC). In the latter a forced-flow technique is used to decrease the development time and thus speed up the separations. A pump controls the speed of the mobile phase. Theoretically, this method is faster than when movement of the solvent is due to capillary action alone (normal TLC) and can be used to advantage if slow-moving viscous solvents are involved as developing solvents. 

Mobile-phase velocity is higher with forced-flow development than in capillary-flow TLC. The actual flow rate is influenced by the type of chamber (rectangular or sandwich, saturated or unsaturated), the pressure and solvent viscosity (OPLC), or the rotational speed (RPC) (Nyiredy et al., 1988a). Nyiredy (1992) discussed the relation among resolution, separation distance, and time for forced-flow planar chromatography compared to capillary flow. It was stated that for separation of nonpolar compounds by FFPC on silica gel, a separation time of 1—2.5 min over a separation distance of 18 cm can be used without great loss in resolution. By contrast, longer separation times are needed for separation of polar compounds. 

Harrison et al. also indicated that a uniform lubricating layer at the die wall interface must occur to eliminate the slip-stick phenomenon responsible for forced flow. Development of a lubricating layer was dependent on the length of the die (a minimum length required), wall shear stress and upstream pressure loss. They represent the frictional forces at the die wall interface and the estimated pressure loss at zero die length in the barrel of the ram extruder. The method for deriving these values is described in Ref. 27. These parameters allow for a quantitative comparison between formulations and process however, no specific values can be targeted since they vary with materials. 

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