Gradients continuous solvent

Horizontal and descending TLC are other forms of chromatography which require special equipment. The advantages of these systems are not clear. They are usually more expensive than ascending equipment, but often have other features for stepwise gradient elution or continuous solvent programming. 

Section 4.3 described the possibilities of producing continuous solvent gradients. The change in solvent composition may be linear, composed linear, concave or convex (Figure 18.5). 

Reciprocating-piston pumps deliver a constant flow at a fixed backpressure. At high pressures some minor flow variations may arise due to the compressibility of the mobile phase. Some instruments incorporate a flow controller which provides a fixed backpressure for the pump to work against, independent of the column backpressure. The influence of pressure fluctuations, solvent compressibility, and solvent viscosity on the volumetric output of the pump is thereby eliminated. Reciproeating-piston pumps can provide continuous solvent delivery, fast solvent change over, gradient elution compatibility, and have low maintenance requirements. 

Let there be a dilute solution in which molecules of the solute do not collide with each other, and suppose that the molecules of the solute move under viscous drag in a homogeneous and continuous solvent. Let us consider a solution of two elements, i.e., one solute and one solvent, and the radius of each molecule of a spherical shape be Rq. Then by Stokes law of viscosity, the viscous drag is /vi = (6rr rj Ro) vi where r/ is the viscosity of the solvent, and / is the viscous coefficient. We can set this force equivalent to the negative gradient of the chemical potential of the solute ... 

In situ loading during the chromatographic development. By this method a continuous gradient of the hquid stationary phase is created on the PLC layer in the direction of the mobile phase flow. This is achieved by developing the plate with a suitable multieomponent solvent system. 

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