Choosing RP Systems

Reversed phase silicas are called brush type, because of their structure. Numerous alkyl groups point from the surface into the mobile phase and thus the surface is similar to a brush, with bristles of alkyl groups. To describe the adsorption process a simple two-layer model of the system has been proposed (Galushko, 1991), which means that the surface layer of the alkyl groups is assumed to be quasi-liquid. The retained solutes penetrate into the surface layer and retention can be regarded as partitioning between hydrophobic stationary phase and mobile phase, similar to liquid-liquid chromatography. 

RP chromatography is mainly used to separate substances that differ in their lipophilic part or in the number of C atoms, for example, the separation of homo-logues. However, due to the anisotropic nature of the interaction between surface 

Most analytical separations are performed by reversed-phase systems while most preparative separations are performed using an NP system. For example, 84% of the analytical separations at the former Schering AG now part of Bayer Healthcare are done in RP mode while 85% of the preparative separations are performed on normal phase silica gels (Brandt and Kueppers, 2002). 

RP systems dominate analytical applications because of their robustness and reduced equilibration time. This fast column equilibration makes RP phases applicable for gradient operation. For preparative chromatography, normal phase systems are preferred because of some disadvantages of the RP systems. Although water is a very cheap eluent it is not an ideal mobile phase for chromatography because it evaporates at higher temperatures, the enthalpy for evaporation is quite 

Since the polarity range of adsorbents is bordered by silica on the polar side and RP-18 or hydrophobic polymeric phases on the nonpolar side it is easy to assign these absorbents to normal or reversed-phase systems. Medium polar packings (Section 3.1.1) possess polar properties because of the functional group as vell as hydrophobic properties contributed by the spacer (Unger and Weber, 1999). Owning to this mixed nature, these phases can not be directly assigned to a certain type of phase system. 

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Strategy for an Industrial Preparative Chromatography Laboratory 145 4.3.3 Criteria for Choosing RP Systems 150... 

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