Forced-flow mobile phase velocity

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... 

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... 

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

In capillary flow conditions, there is an inadequate range of mobile-phase velocities, which does not allow working at Mopt values the role of the binder remains not completely clear. The zone-focusing mechanism causes an increase of separation performance of the system in the most simple way. Forced flow offers a modest increase in performance with a reduction in separation time. 

Each particle in a bed of porous particles is surroimded by a laminar sublayer (Figure 5.4), through which mass transfer takes place only by molecular diffusion. On one side, this layer is exposed to the flowing mobile phase and is entirely accessible. On the other side, it wraps the particle wall and is accessible from the particle inside only at the pore openings. The thickness of this layer, hence the mass transfer coefficient, is determined by hydrodynamic conditions and depends on the flow velocity. The mass transfer rates can be correlated in terms of the effective mass transfer coefficient, fcy, defined according to a linear driving force equation ... 

An alternative approach to forced flow is to seal the layer with a flexible membrane or an optically flat, rigid surface under hydraulic pressure, and to deliver the mobile phase to the layer by a pump [9,41,43-46]. Adjusting the solvent volume delivered to the layer optimizes the mobile phase velocity. In the linear development mode, the mobile phase velocity (uf) will be constant and the position of the solvent front (Zf) at any time (t) after the start of development is described by Zf = Uft. The mobile phase velocity no longer depends on the contact angle and solvent selection is unrestricted for reversed-phase layers in forced flow, unlike capillary flow systems. 

Figure 6.3. Variation of the average plate height as a function of the solvent-front migration distance for conventional and high performance layers using capillary flow and forced flow at the optimum mobile phase velocity. (From ref. [61] Elsevier).

Resolution in forced flow thin-layer chromatography is not restricted by the same factors that apply to capillary flow. Resolution increases almost linearly with the solvent-front migration distance and is highest for separations at the optimum mobile phase velocity. Resolution has no theoretical limit for forced flow the upper bounds are established by practical constraints (plate length, separation time and inlet pressure). 

Forced flow separations overcome the principal deficiencies of capillary flow separations by establishing a constant and optimum mobile phase velocity. Forced flow separations require specially designed developing chambers exploiting either centrifugal or pneumatic forces to drive the mobile phase through the layer. Centrifugal methods are more popular for preparative-scale separations and have been little used for analysis. The preferred approach for analytical separations is to seal the open face of the layer by contact with a flexible membrane, under hydraulic pressure, and deliver the mobile phase to... 

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. 

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