Stationary Phase Gradients

From the aforementioned optimization strategies it can be seen that the solubility of the feed and the target compound in a suitable solvent is of utmost importance 

The optimization procedure follows quite a simple strategy  

1) Setting of boundary conditions, especially in the choice of appropriate solvents. 

3) Optimization of the combination of stationary phases using the PRISMA model. 

After the choice of the solvent has been optimized the screening of different stationary phases can start. The phases should be of similar particle size and show similar pore diffusivities to exhibit similar chromatographic properties. The surface modifications should cover different molecular-interaction principles but the 

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Conventioal development procedures of two or more component planar mobile-phase systems form phase gradients. The most polar phase stays near the bottom of the development plate [29]. Besides these mobile-phase gradients, stationary-phase gradients based on having a sorbent of varying composition and medium gradients such as temperature have been studied [27,47],... 

Reverse phase systems, such as squalane/water/alcohol, require the eluant to be saturated with the squalane stationary phase to avoid solvent stripping however the binding forces are so weak that shearing occurs at high pressures. The other main disadvantages associated with LLC are that as the eluant must be saturated with the stationary phase, gradient elution techniques cannot be used and to maintain the required stationary phase loading the temperature must be controlled to 0.5°. Some sample types with their respective mobile and stationary phases are shown in Table 6.5. 

The prejudices faced for preparative chromatography are often related to the mobile phase, for example, they are expensive, toxic, explosive, difficult to remove, yielding only low concentrations. By choosing an optimized mobile phase and later developing the appropriate stationary-phase gradient a lot of simplification in the phase system can be achieved. The concept of stationary phase gradients is thus a so far underutilized tool to improve preparative chromatography economics. 

Preparative separation of multicomponent mixtures using stationary phase gradients./. Chromatogr. A, 1215, 133-144. 

Gradient development. Gradient developments have been used to achieve separation of complex mixtures such as plant extracts, dyes, etc. However, their application to lipid separation is not very common. Golkiewicz (1996) discusses in detail stationary phase gradients and mobile phase gradients, the theory behind solvent selection, automated techniques and applications of gradient elution in TLC. 

The three types of gradients that have been used the most in TLC are mobile phase, temperature, and stationary phase gradients. Planned mobile phase gradients must be distinguished from the natural, uncontrolled gradients that result from solvent demixing during development. 

Stahl (33,34) described an apparatus for obtaining continuous stationary-phase gradients that maintained the basic construction principle of the normal spreader. A rectangular case divided diagonally into two compartments by a partition wall is filled with two different adsorbent suspensions. After opening the sliding bottom of the case, the suspensions fall into the spreader cylinder, which is divided into several small compartments, and mix in different proportions. After mixing of the compartments contents, the plates are coated in the usual way (for details see Refs. 3,6,31,33, and 34). 

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