Once the LC modeand the type of column packing (porous, superficially porous) are selected, the choice of the stationary phase is determined by the requirements and the nature of the sample. In general, each stationary phase can have a unique selectivity towards sample components.
Selection of the Liquid-Solid Chromatography (LSC) Stationary Phase
Silica and alumina are the two most popular stationary phases (adsorbents). Silica is the preferred stationary phase mainly because of its availability, known performance and low cost. Silica has a lower reactivity, yields columns of better efficiency, and offers a higher linear capacity than alumina.
In general, silica and alumina are both used for the separation of the same type of compounds, although certain compounds tend to favor one over the other.
The following semiempirical relationships have been found by comparing the two adsorbents with regard to their selectivity for various functional groups:
- Moderately strong bases (pKb < 5) are preferentially adsorbed on silica
- Base-sensitive compounds should be separated on silica
- Acidic compounds are preferentially adsorbed on alumina
- Unsaturated molecules (olefins, aromatic) are preferentially adsorbed on alumina
- Halogen groups are preferentially adsorbed on alumina
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